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Article

Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors

by
Xiaoyu Liu
†,
Biao Zhang
*,†,
Wenqi Guo
,
Baiyu Jiang
,
Miao Du
,
Zhisheng Fu
* and
Zhiqiang Fan
*
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310058, China
*
Authors to whom correspondence should be addressed.
†
Current address: Zhejiang Sinopont Technology Co., Ltd., Hangzhou 311254, China.
Catalysts 2025, 15(4), 330; https://doi.org/10.3390/catal15040330
Submission received: 20 February 2025 / Revised: 22 March 2025 / Accepted: 23 March 2025 / Published: 31 March 2025
(This article belongs to the Section Catalysis in Organic and Polymer Chemistry)
Browse Figures
Versions Notes

Abstract

:
External donor (De) modification is an effective way to enhance the stereoselectivity of propylene polymerization with supported Ziegler–Natta catalysts. Aminosilane as a novel type of external donor has been found to have the excellent ability of enhancing the isoselectivity of propylene polymerization. In this work, dipiperidyldimethoxysilane (Donor-Py) was compared with cyclohexyl(methyl)dimethoxysilane (Donor-C) and dicyclopentyldimethoxysilane (Donor-D) for propylene polymerization in the presence or absence of hydrogen. By analyzing the effects of external donors on catalytic activity, polymer chain structures and their distributions, and the number and reactivity of three groups of active centers with different stereoselectivities, the performance of each De in enhancing stereoselectivity was compared. Propylene polymerization in the presence of RSi(OR’)3-type De (R = n-propyl or i-butyl, R’ = methyl or ethyl) and hydrogen was also studied. Donor-Py produced PP with higher molecular weight and was more sensitive to hydrogen than Donor-D. According to the fractionation results, Donor-Py produced PP with the lowest content of medium-isotactic PP and the highest content of highly isotactic PP, especially at high hydrogen concentrations. By raising hydrogen concentrations, the number of active centers was enhanced in systems with Donor-C and Donor-D while it was reduced when Donor-Py was added. When Donor-Py was used as De, the effects of H2 concentration on active center distributions and the reactivity of different active centers were evidently different from those of Donor-C and Donor-D. Donor-Py showed the best performance among the De used in this work in producing PP with both high isotacticity and good processability. The mechanism of De effects and hydrogen effects is discussed based on the results of polymerization kinetics and PP chain structures.

Graphical Abstract

1. Introduction

Polypropylene (PP), one of the most important general purpose synthetic resins, is characterized by a valuable combination of properties, such as excellent mechanical performances, high thermal and chemical stability, and optical transparency in some cases. Owing to this fact, PP has been extensively applied in various industries, particularly in automobiles, electric appliances, and packaging materials [1,2,3,4,5]. So far, PP is still the second-largest synthetic polymer with a global annual yield of more than 80 million tons. MgCl2-supported Ziegler–Natta (Z-N) catalysts play a predominant role in the industrial production of PP resin, accounting for more than 95% in recent decades [1,6]. The Z-N catalyst system with the general formula of TiCl4/Di/MgCl2-AlR3/De is most widely used in PP production, where Di (internal donor) is the donor introduced in the precatalyst during its preparation and De (external donor) is the donor added together with cocatalyst (AlR3) in the polymerization stage.
In propylene homopolymerization reactions, both internal and external donors play important roles in controlling the catalytic productivity, stereoselectivity, and hydrogen response of the catalyst. Consequently, the molecular weight distribution (MWD), chemical composition distribution (CCD), and other chain structure parameters of the PP product are regulated by Di and De [7]. It is commonly accepted that a suitable Di/De pair and De/Al molar ratio are required to achieve an ideal combination of polymerization activity and stereoselectivity [6]. Unlike the internal donor, the design and application of the external donor are more flexible and have attracted much more attention. Since the application of TiCl4/Di/MgCl2-AlR3/De-type Ziegler–Natta catalysts in propylene polymerization was realized, great efforts have been made in mechanistic studies on the effects of external donors [5,7,8,9,10,11,12,13,14,15,16,17,18]. Generally, the most important role of external donors in propylene polymerization is considered to be the enhancement of the isospecificity of catalytic systems. When TiCl4/Di/MgCl2-type catalysts were treated with an AlR3/De mixture, Di was extracted from the MgCl2 surface via complexation with alkylaluminum, and De occupied the positions left by Di [19,20,21]. The adsorption of De on the MgCl2 surface adjacent to an originally aspecific active center transforms it into an isospecific active center, suggesting that De plays more important roles than Di in enhancing isospecificity in propylene polymerization [7,12,16].
Up until now, hundreds of external donors have been reported in the literature, many of which have been commercialized, such as esters, alkoxysilanes, diethers, etc. Among them, alkoxysilanes, especially methoxysilanes containing relatively bulky alkyl groups, are most widely employed as external donors to achieve a large extent of isotacticity improvement in propylene polymerization in the industry [14,17,22,23,24,25]. For instance, cyclohexyl(methyl)dimethoxysilane (Donor-C) and dicyclopentyldimethoxysilane (Donor-D) are typical alkoxysilane De used in PP production. Alkoxysilane-type De with different alkyl or alkoxy substituents usually exhibits evidently different catalytic performances. It was reported by Zhang et al. that De carrying more or larger alkoxy substituents caused a reduction in the catalytic activity, isotacticity index (I.I.), and molecular weight of PP products; however, the molecular weight was enhanced when alkoxysilanes carrying larger alkyl substituents were applied as De [23]. Diethers were initially used as Di in MgCl2-supported catalysts to avoid De addition in propylene polymerization. In some cases, diethers were further extended as De to produce PP with good processability and a narrower MWD [9].
In recent years, various new types of external donors have been explored for developing Ziegler–Natta catalysts with improved performances [24,26,27,28,29,30]. Aminosilane, structurally similar to alkoxysilane, where the alkyl groups are replaced by dihydrocarbyl amino or cyclic amino groups, has attracted much more attention due to its great performance in enhancing the isoselectivity of propylene polymerization. The application of an aminosilane compound containing polycyclic amino groups carrying more than seven carbon atoms as an external donor was first reported in a patent authorized to Grand Polymer Co., Ltd. in 1998 [31]. Later, it was reported by H. Ikeuchi et al. that aminosilane-type De was conducive to producing PP with broad molecular weight distributions (MWDs) without reducing its isotacticity. Specifically, the aminosilane carrying two piperidinyl groups (Donor-Py) was found to be a highly efficient external donor [32]. Recently, a comparative study on De effects in propylene polymerization between alkoxysilane- and aminosilane-type external donors was conducted by Chang et al., which disclosed that Donor-Py exhibited much better performances than Donor-C and dicyclohexyldimethoxysilane (Donor-H), especially including higher isotacticity and good hydrogen responses [33]. In addition, among the external aminosilane donors carrying structurally similar substituents, Donor-Py also showed a particularly stronger ability to enhance PP’s isotacticity compared to the aminosilane external donors carrying dimorpholine, 1-methylpiperazine, or isopropylpiperazine groups [21]. However, the effects of amino structure on the distribution and activities of active centers have been scarcely studied in the literature.
It is widely accepted that adding hydrogen usually causes an evident reduction in PP’s isotacticity when conventional alkoxysilane type De is used. However, it is still unclear how the steric bulkiness of alkyl groups in alkoxysilane-type De will influence the structure and properties of PP synthesized in the presence of a relatively large amount of hydrogen. For instance, among various alkoxysilane-type De, Donor-D was particularly effective at enhancing PP’s isotacticity without sacrificing the catalyst’s activity [34,35]. However, the sensitivity of PP molecular weight to hydrogen concentration was worsened by adding Donor-D rather than the frequently used De like Donor-C. In other words, more hydrogen is required to increase the melt flow rate of the PP sample to the same level when Donor-D is used. Unfortunately, the combination of a large amount of H2 with Donor-D led to an evident increase in stereo-defects in the PP chains. This reveals that the production of PP with both low molecular weight (high MFR or good processibility) and high isotacticity (high stiffness) is more challenging. Therefore, finding the ideal external donor with both a strong ability of isotacticity enhancement and good hydrogen response is the key target in developing high-melting-point PP with high stiffness. In this work, propylene homopolymerization with supported Ziegler–Natta catalysts in the presence of different De and hydrogen concentrations was carried out. Comparative studies on three types of external donors (dialkyldialkoxysilanes (Donor-C and Donor-D), aminosilanes (Donor-Py), and alkoxysilanes carrying three alkoxy groups (n-propyltrimethoxysilane, Donor-NM; n-propyltriethoxysilane, Donor-NE; iso-butyltriethoxysilane, Donor-IE) were carried out to evaluate their effects on the polymerization activity, MWD, fraction distribution, distribution, and activities of active centers, thermal behaviors, and the chain structure of PP samples in the absence or presence of hydrogen. The mechanism of De effects in propylene polymerization was studied by determining changes in the number and distribution of active centers based on the method developed in our previous work [36]. The influences of hydrogen on the distribution and reactivity of active centers in propylene polymerization with Z-N catalysts were studied based on the same approach [37]. The knowledge obtained in this work will be very helpful in exploring better De or De combinations for the production of PP with both low MW and high stiffness.

2. Results and Discussion

2.1. Polymerization Activity and Chain Structure of the PP Products

Propylene slurry-phase polymerizations were carried out at 0.1 MPa propylene pressure with a MgCl2-supported Ziegler–Natta catalyst (Ti% = 2.87 wt%) containing a non-phthalate-type internal donor. Hydrogen, commonly used as an effective chain transfer agent to reduce the molecular weight of polyolefin, was also applied in the polymerization at different concentrations to study the hydrogen responses of the catalyst in combination with different external donors. The amount of H2 was expressed by its partial pressure in the propylene/H2 atmosphere over the polymerization slurry. A series of silane-type external donors, including two dialkyldialkoxysilanes (Donor-C and Donor-D), an aminosilane (Donor-Py), and three alkoxysilanes carrying three alkoxy groups (Donor-NM, Donor-NE, and Donor-IE) were applied to investigate their effects on polymerization performances. The molecular structures of the external donors are shown in Scheme 1.
The activities of propylene polymerization in the presence of different external donors are shown in Figure 1. Obviously, polymerization activity was enhanced at 35–60% via the addition of 2 mol% hydrogen in systems with Donor-C, Donor-D, and Donor-Py. However, when the hydrogen amount was further increased from 2 mol% to 5 mol%, activity was only slightly enhanced by about 10% in the systems with Donor-D and Donor-Py, and it was even slightly reduced with Donor-C. So, it can be summarized that hydrogen in a certain amount can greatly enhance polymerization activity. Dormant propagation centers (Ti–CH(CH3)–CH2–P, where P represents a polymer chain) have been found to exist in propylene homopolymerization due to the 2,1-insertion of a propylene monomer in active propagation centers (Ti–CH2–CH(CH3)–P) [38,39]. This kind of dormant species can be easily transformed into Ti–H by reacting with hydrogen, and then, it can be easily activated by a 1,2-insertion of propylene in Ti–H [38,39,40,41]. Among the three R1R2Si(OCH3)2 (R = alkyl or amino)-type external donors, activity decreased in the following order: Donor-D > Donor-Py > Donor-C. Generally, the activities of PP production with three alkoxysilanes carrying three alkoxy groups (RSi(OR′)3) were about 25–45% lower than those with R1R2Si(OCH3)2-type external donors. The lower activities of polymerization in the presence of Donor-NM, Donor-NE, and Donor-IE could be attributed to their stronger electron-donating ability for the existence of three alkoxy groups.
The molecular weight (MW) and its distribution of PP produced with different external donors in the presence or absence of H2 are shown in Figure 2 (the corresponding data are listed in Table S2). Among the three R1R2Si(OCH3)2-type external donors, Donor-Py produced PP with higher MW than Donor-C and Donor-D. The MW of PP produced with Donor-NM, NE, and IE is also lower than those produced with Donor-D and Donor-Py. Hydrogen as an efficient chain transfer agent can greatly reduce the molecular weight of all PP samples, but the efficiency tended to decline when its concentration rose from 2% to 5%. Donor-D as an excellent De is widely used in industrial PP production for its good ability to improve PP’s isotacticity with high activity. However, its poor hydrogen response and poor stereoregulating ability at high hydrogen concentrations, to some extent, greatly limited its applications [35].
Among the alkoxysilanes carrying three alkoxy groups, De with bulkier alkyl and alkoxy groups can produce PP with higher MW. The MW of PP was greatly enhanced when OCH3 was replaced by a bulkier alkoxy OCH2CH3 (PP-NM-2 vs. PP-NE-2). However, it was barely changed when n-propyl was replaced by i-butyl (PP-NE-2 vs. PP-IE-2), which indicated that the De carrying the bulkier alkoxy group is much more effective for enhancing MW.
Because different types of active centers coexist in supported Ziegler–Natta catalysts, polypropylene is actually composed of polymer chains of evidently different chain characteristics, e.g., different stereoregularity and molecular weight [42]. In order to determine the polymer chain structure and find its correlation with the external donor, each of the 12 samples in Figure 1 was fractionated into 3 parts via two-step solvent extractions: a fraction soluble in n-octane at room temperature (C8-sol), a fraction soluble in boiling n-heptane (C7-sol), and a fraction insoluble in boiling n-heptane (C7-insol). The sum of (C7-sol + C7-insol) percentages is close to the percentage of insoluble room-temperature xylene that is usually taken as the isotacticity index of the PP product in the industry. The fraction distributions of the PP samples are summarized in Figure 3 and Table 1.
As confirmed via DSC and 13C-NMR analyses, the C8-sol, C7-sol, and C7-insol fractions are composed of atactic PP (aPP), medium-isotactic PP (miPP), and iPP chains, respectively [16,35]. Figure 3 shows that adding Donor-D can reduce the percentage of aPP more efficiently than Donor-C and Donor-Py, irrespective of whether hydrogen is introduced or not. However, the iPP percentage was reduced by introducing H2 in the presence of Donor-D and Donor-C. In contrast, the iPP percentage increased when Donor-Py was used in the presence of H2. Comparing the fraction distributions of PP produced with Donor-D, Donor-C, and Donor-Py, it is clear that only the latter can reduce the percentage of miPP and produce PP with high C7-insol contents (>91 wt%) in the presence of hydrogen. Because the miPP fraction has much lower MW and lower melting temperatures than the iPP fraction [16,35], its effects on the mechanical properties should be negative. Therefore, only Donor-Py showed the capability to produce PP with the desired combination of high isotacticity and medium/low MW in the presence of H2. For example, the PP produced with Donor-Py at 5% H2 has a weight average MW of 10.9 × 10−4 and C7-insol content of 91.3%, in contrast to the PP with an MW of 11.8 × 10−4 and C7-insol content of 88.8% produced with Donor-D at 2% H2. It is highly probable that former PP samples could show better balances between mechanical properties and processability than the latter. Compared to Donor-D and Donor-Py, the extent of isotacticity improvements by alkoxysilanes carrying three alkoxy groups is much lower, as the C8-sol and C7-sol fractions of their PP products are much higher, especially in the system with Donor-NM.

2.2. Active Center Distribution and Reactivity of Different Active Centers

In the literature, investigation on the micro-kinetics of olefin polymerization, including the number and reactivity of multiple active centers in the catalyst, has contributed a lot in elucidating the mechanism of donor effects [14,43,44]. It was found that adding De in MgCl2-supported Ziegler–Natta catalysts causes a reduction in the number of active centers ([C*]/[Ti]) and an enhancement in the chain propagation rate constant of the isospecific active centers (kpi) [16,42]. In this work, the same methodology as used in our previous work has been adopted to elucidate the mechanism of De effects in propylene polymerization in the absence or presence of H2 [16]. Since the active centers producing the C8-sol, C7-sol, and C7-insol fractions have low, medium, and high stereoselectivities, respectively, their number fractions represent the number distribution of the three groups of active centers in the catalysis system. By correlating the active center’s distribution and reactivities of the three groups of active centers with the structure of alkoxysilane-type external donors, it is possible to establish the structure–performance relationship of De.
The number of active centers ([C*]/[Ti]) in each PP fraction was experimentally determined via quench-labeling the chain propagation centers with TPCC [16,36], and the corresponding apparent propagation rate constant (kp) of each group of active centers was calculated from the yield of PP fractions and their active center numbers (refer to the Experimental Section for details). The influence of De and H2 on the active center distribution and the kp values of three groups of active centers is shown in Figure 4. The total number of active centers (sum of [Ca*]/[Ti], [Cm*]/[Ti], and [Ci*]/[Ti], the number of active centers in C8-sol, C7-sol, and C7-insol fractions) in each PP sample is less than 10%, similarly to the level found in our previous results on propylene polymerization [7,16,45]. Among the three R1R2Si(OCH3)2-type external donors, the [C*]/[Ti] of the PP sample produced by Donor-Py is the lowest (around 4%), but the kp value is the largest in the presence or absence of H2. An increase in hydrogen concentration led to enhancements in [Cm*]/[Ti] in both Donor-C and Donor-D systems, but [Ci*]/[Ti] was nearly unchanged. In contrast, in the Donor-Py system, [Cm*]/[Ti] remained nearly unchanged; meanwhile, [Ci*]/[Ti] decreased with an increase in hydrogen concentration. The kp values of C7-sol and C7-insol fractions in the Donor-C and Donor-D systems were enhanced via the addition of H2. However, the kp value of the C7-insol fraction experienced a much larger extent of enhancement via H2 in the Donor-Py system. It is clear that hydrogen effects on the kinetic parameters show dramatic different trends between dialkyldimethoxysilane-type and diaminodimethoxysilane-type external donors. The mechanistic details of the phenomena will be discussed later.
Figure 4 also shows that using silanes with three alkoxy groups as external donors led to an increase in the number of active centers and a reduction in the kp value in comparison with the donors containing only two methoxy groups. The stronger tendency of the former to coordinate with the acidic Lewis AlEt3 could weaken its coordination with the Ti of the active centers, as the third alkoxy (methoxy or ethoxy) group can promote the coordination of De with AlEt3. When De has two alkoxy groups and two alkyl groups, it can adsorb on the catalyst’s surface through the two chelating alkoxy groups. Meanwhile, the two bulky alkyl groups work as a stereochemical shield that expels the approaching AlR3 [18]. Replacing one of the alkyl groups with an alkoxy could increase the probability of coordination between De and AlR3.
It is worth noticing that the kp value of Ci* in the Donor-Py system in the presence of 5% H2 was markedly greater (592 L/mol·s) than those in the Donor-C and Donor-D systems. Since the kp value is positively related to the stereoselectivity of the active centers in propylene polymerization [16,37], it is expected that Donor-Py can produce PP chains with the highest isotacticity among the external donors studied in this work. In our fundamental studies on the synthesis, characterization, and structure–property relationship of PP/EPR in reactor alloys, the key role of segmented ethylene–propylene copolymer (EPS) fractions in enhancing compatibility between the dispersion phase (random ethylene–propylene, EPR) and the continuous phase (iPP matrix) was confirmed, and evident improvements in the stiffness–toughness balance of PP/EPR alloys was achieved by increasing the content of iPP blocks in the EPS fraction [46,47,48]. In the previous research work for obtaining fundamental knowledge on ethylene–propylene (EP) copolymerization with supported Ziegler–Natta catalyst, the effects of external donors on the copolymer chain’s structure and the microkinetics of copolymerization were investigated. It was proposed that the EPS chains were produced by a portion of highly isospecific active centers. The enhancement of their stereoselectivity should be beneficial to the formation of longer iPP segments in the EPS chains [48]. Based on the previous analysis, it can be expected that Donor-Py should have the ability to produce copolymers containing longer iPP segments in EP copolymerization in contrast to the other donors because its Ci* showed a much larger kp value, especially in the presence of high H2 concentrations. This feature of Donor-Py should be beneficial to the production of PP/EPR alloys with both excellent toughness–stiffness properties and good processability (high MFR).
Looking into the DSC melting traces (second heating scan) of the C7-insol fractions of the PP samples produced with different external donors (see Figures S1 and S2), it is seen that the melting enthalpy of each C7-insol fraction was above 100 J/g, indicating a high crystallinity of around 50%, which is a characteristic of isotactic PP. In addition, there was not much difference in the melting enthalpy of each C7-insol fraction considering the integration error. However, the melting temperature of each sample shows an evident difference: The isotactic PP produced with Donor-D or Donor-Py has a rather high melting point at around 164 °C, but that produced with Donor-NM is lower than 162 °C. It is widely accepted that the melting point of isotactic PP is closely related to its lamellar thickness. Therefore, the PP sample produced with Donor-D or Donor-Py shows much higher isotacticity than the samples produced with other donors, which is in good agreement with the above analysis. It is worth noticing that there is not much difference in melting temperatures between Donor-Py and Donor-D, which may be attributed to the limitation of conventional DSC analysis methods in distinguishing the melting traces of almost perfectly isotactic PP chains. Therefore, successive self-nucleation and annealing (SSA) will be applied to carry out more detailed investigations of the PP’s chain structure.

2.3. Further Characterization of the Isotactic Fraction of PP Products

According to the fractionation results shown in Figure 3, the C7-insol content of PP produced with Donor-D was severely decreased under high H2 concentrations in comparison with Donor-Py, indicating a reduction in stereoselectivity in the former system. To more closely look into the effects of external donors and hydrogen on the chain structure of PP products, especially the isotactic fraction, which is the key contributor to mechanical properties, it is necessary to carry out a more precise characterization of the C7-insol fraction of PP.
Successive self-nucleation and annealing (SSA) technology was designed and first reported by Müller et al. in 1997 [49], in which self-nucleation and annealing steps were sequentially applied to a polymer sample to carry out annealing on unmolten crystals at each stage of the process; then, potential molecular fractionation was promoted during crystallization such that small differences in molecular structure can be distinguished [25]. SSA technology has been widely used to analyze the chain structures of semi-crystallized polymers, such as ethylene homopolymer and its copolymers with α-olefins, and evaluate the compatibility of polymer blends, etc. [50]. Isotactic PP produced by heterogeneous Ziegler–Natta catalysts can also be analyzed via SSA to provide very useful information on the stereo-defect distribution of PP chains [51]. In the SSA experiment, the most important parameter is the first self-nucleation temperature (Ts1), which is defined as the minimum temperature within Domain II of the SSA thermal conditioning. According to the self-nucleation (SN) technique, predicted Ts values from 169 °C to 162 °C were tested with the same sample to determine the Ts1 temperature. For the C7-insol fractions produced with Donor-C, Donor-D, Donor-Py, and DonorNM in the presence of hydrogen, several melting endotherms of the C7-insol fraction annealed at different self-nucleation temperatures for 5 min were recorded for each sample (see Figure S3). The lowest Ts at which the high-temperature shoulder peak disappeared was defined as Ts1. In this way, the Ts1 of 166 °C was determined for the C7-insol fractions of PP produced with Donor-Py and Donor-D, and 164 °C was determined for those produced with Donor-NM and Donor-C.
The SSA melting endotherms of the C7-insol fractions produced with different external donors are shown in Figure 5 (the melting temperature and melting enthalpy determined from the SSA endotherms are listed in Table S4). Obviously, the SSA endotherms all comprise a main peak relative to the high-temperature region and a shoulder peak relative to the low-temperature region, showing that each C7-insol fraction is a mixture of isotactic PP chains with different stereo-errors. The melting enthalpy of each C7-insol fraction appears at around 125 J/g after the SSA treatment, much higher than the DSC results from the conventional two-cycle heating–cooling experiments (see Figures S1 and S2). However, the melting temperature, which can be taken as an indicator of stereo-errors in the isotactic PP chain, shows evident differences among the samples from different external donors. The Tm values declined in the order of Donor-Py = Donor D > Donor-C > Donor-NM. According to SSA results, it is noticeable that Donor-Py and Donor-D showed a relatively stronger ability to enhance isotacticity than Donor-C and Donor-NM. Interestingly, the endotherms of the C7-insol fractions produced with Donor-D and Donor-Py can nearly overlapped with each other, meaning that they have very similar micro-isotacticity and distributions of stereo-errors.
To benefit discussions, the peaks in the SSA melting endotherm were classified into two groups: The main peak located in the high-temperature region is denoted as Peak 1, which is considered to correspond to the thickest crystalline lamellae formed by PP chains with the highest isotacticity. The shoulder peak in the low-temperature region is named as Peak 2, which should correspond to PP chains with relatively lower isotacticity. The relative contents of the two peaks were calculated via the deconvolution of the SSA melting endotherms and the measurement of the integral area of the peaks. The fitted curves for each SSA melting endotherm are shown in Figure 6, and the calculated results of the deconvolution treatment are shown in Table 2.
Obviously, the higher the relative content of Peak 1 in the SSA endotherm, the more the most isotactic PP chains the C7-insol fraction. As shown in Table 1, the relative content of Peak 1 is almost twice as much as that of Peak 2. According to the deconvolution results, there was not much difference in the relative content of Peak 1 or Peak 2 among the external donors, especially for Donor-Py and Donor-D; these things considered, the C7-insol fraction produced by Donor-NM contains a lower percentage of the most isotactic PP chains. Accordingly, in order to carry out a reasonable evaluation of the external donor effect on the isotacticity of propylene polymerization, the weight percentage of the C7-insol fraction in the PP sample should also be taken into consideration. The melting enthalpy of the most isotactic PP chains in the C7-sol per gram of the PP sample (ΔHP1) is calculated via the equation ΔHP1 = (ΔH)C7-insol × PP1 × (weight fraction of C7-insol in PP sample), and the data are also listed in Table 1. The ΔHP1 value can be taken as a quantitative indicator in evaluating the content of the most isotactic PP chains in the PP sample. A larger ΔHP1 value can be correlated with the better ability of an external donor to enhance stereoselectivity in propylene polymerization. The ΔHP1 value was gradually improved from 61.1 J/g to 76.2 J/g in the order of Donor-NM < Donor-C < Donor-D < Donor-Py, indicating that Donor-Py has the strongest ability to enhance the stereoselectivity among the four kinds of external donors. Meanwhile, the melting temperature of Peak 1 (Tm1) representing the PP lamellae thickness can be taken as another indicator of isotacticity enhancement for an external donor. It is commonly recognized that higher melting temperatures correspond to higher isotacticity and longer isotactic sequences in PP chains [25,33,51,52]. As seen in Table 2, the Tm1 and Tm2 values of Donor-Py and Donor-D show no differences with each other, and they are the highest among the four external donors, but those of Donor-NM are the lowest. This means that the PP chains produced by Donor-NM or Donor-C contain more stereo-defects and thus cannot form PP lamellae as thick as those produced with Donor-Py and Donor-D. In summary, Donor-Py appears to be the best candidate among the four kinds of De to produce isotactic PP chains with high micro-isotacticity.
In order to evaluate the performance of different De types more comprehensively, propylene polymerization runs were conducted at 0.5 MPa propylene pressure in the presence of H2 at 1% or 4% volume percentage. Each of the produced PP samples was fractionated into C8-sol, C7-sol, and C7-insol fractions, and the fraction distributions of the samples are shown in Figure 7. Table 3 shows the polymerization activity and thermal properties of the PP’s C7-insol fractions. Among the three external donors, Donor-C shows the worst ability in enhancing isotacticity, similarly to the results of propylene polymerization at 0.1 MPa. The variation in fraction distribution among different external donors exhibited a similar trend when the H2 concentration was increased from 1% to 4%. An increase in H2 concentration barely varied the amount of the C8-sol fraction, but it enhanced that of C7-sol while reducing that of C7-insol. However, the extent of the C7-insol fraction’s reduction under high H2 concentrations with Donor-Py was smaller than that with Donor-C and Donor-D. For instance, there was not much difference in the fraction distribution between Donor-D and Donor-Py at 1 mol% H2, but the percentage of the C7-insol fraction produced by Donor-D was reduced from 96.1 wt% to 93.9 wt%, in contrast to that of Donor-Py, which was only reduced from 96.0 wt% to 95.2 wt% when H2 concentrations were further increased to 4%. This indicates that Donor-Py is more suitable for producing iPP with the desired combination of high isotacticity and high MFR.
As seen in Table 3, whether at high or low hydrogen concentrations, the melting temperature of the C7-insol fraction produced by Donor-Py was the highest among all samples. It was more than 1 °C higher for Donor-Py than Donor-D, which is different compared to the result from propylene polymerization at 0.1 MPa. The advantage of Donor-Py in enhancing isotacticity becomes more obvious than that of Donor-D in the presence of H2 at high polymerization pressure when the melting temperature and the contents of the C7-insol fraction are both taken into consideration.
The 13C-NMR spectra of the C7-insol fractions of PP samples produced with different external donors are shown in Figure 8. According to the literature, three peaks located at 46~47 ppm, 28~29 ppm, and 21~22 ppm are, respectively, assigned to methine, methylene, and methyl carbon atoms [53]. The content of [mmmm] determined by the methyl signals can be used to evaluate the micro-isotacticity of PP chains. This reveals that the [mmmm] content declines in the order of Donor-Py = Donor-D > Donor-C, Donor-IE > Donor-NM. However, due to the limited signal-to-noise ratio (SNR) of the NMR spectra, the little difference in micro-isotacticity between the samples produced with Donor-Py and Donor-D cannot be clearly distinguished. Although the 13C-NMR results are consistent with those of the SSA-DSC analysis, the latter method can differentiate the micro-isotacticity of different samples with higher precision.

2.4. Discussions on Mechanism of the External Donor and Hydrogen Effects

Considering the mechanism of external donor effects on propylene polymerization with Z-N catalysts, it is accepted that the external donor can coordinate with both the active sites and the cocatalyst [16,54,55]. In a recent work by V. Busico et al. [18], a modified model of external donor effects was proposed, which explained the De effects through its chelating coordination on Mg near the active center and its ability to prevent the desorption of the AlEt2Cl coordinated on Mg in the second coordination sphere of the central Ti (see Scheme 2). The shielding effect of the adsorbed De is enhanced when it has bulkier R substituents. It is the adsorbed AlEt2Cl that plays the role of directly enhancing the stereoselectivity of the active center. The bulky R substituents of De can prevent the adsorbed AlEt2Cl from complexing with the TEA cocatalyst and then detaching from the catalyst’s surface. Bearing two bulky cyclopentyl groups, Donor-D can realize a larger extent of isotacticity improvement than Donor-C. In contrast, Donor-Py also carries two bulky piperidyl groups, but the existence of electron-donating N atoms can enhance its ability to coordinate with the Ti of the active center (see Scheme 2), thus causing a larger extent of [C*]’s decrease. On the other hand, the presence of N with lone-pair electrons in Donor-Py could strengthen its adsorption on Mg in the vicinity of the central Ti of the active center, thus leading to a more stable state of high stereoselectivity (status (c) of Scheme 2). This could be the main reason for the larger kp value and thus the higher stereoselectivity of the active centers in the C7-insol fraction when Donor-Py is used as De. This also explains the higher molecular weight of PP produced by Donor-Py than the other donors, as higher stereoselectivity is directly related to the higher molecular weight of the polymer chain [56].
In contrast to the R1R2Si(OCH3)2 (R = alkyl or amino)-type external donors, the alkoxysilanes carrying three alkoxy groups always produce PP samples with relatively lower isotacticity, because they can be easily removed from the adsorbed Mg atom through its coordination with alkylaluminum by the third alkoxy group.
As proposed in the literature, the effects of hydrogen on propylene polymerization with Z-N catalysts can be largely explained by the activation of dormant sites through the chain transfer of the Ti–CH(CH3)CH2–P-type dormant species with H2 to form Ti–H, which is then resumed relative to the active propagation center after the 1,2-insertion of propylene in Ti–H. If this is the only route for hydrogen reactions with catalytic species, the activation of Ti–CH(CH3)CH2–P-type dormant sites via hydrogen will not change the [C*]/[Ti] ratio, as the Ti–CH(CH3)CH2–P-type dormant sites can still be quench-labeled via an acyl chloride quencher and counted into [C*]. However, the kp value determined using the polymerization rate (Rp) and [C*] data increases for enhanced Rp in the presence of hydrogen.
In 2002, Y. V. Kissin et al. proposed another explanation of the hydrogen activation effect [57]. According to their mechanistic model, Ti–CH(CH3)2-type dormant sites are formed via the 2,1-insertion of propylene in Ti–H or via the chain transfer of the propagation center with propylene when propylene is coordinated relative to Ti in the secondary orientation. This kind of dormant site can also be reactivated via its hydrogenolysis, leading to Ti–H. For the lower probability of 2,1-propylene insertions in Ti–H than that of 1,2-propylene insertions, the added hydrogen can convert most dormant Ti–CH(CH3)2 sites into active Ti–CH2CH2CH3 sites, resuming their catalytic activity. Since the product of quenching Ti–CH(CH3)2 via TPCC, (2-thiopenyl)(isopropyl)ketone is a low-molecular-weight chemical that can be removed from the polymer via the purification process, the activation of Ti–CH(CH3)2 by H2 will cause an increase in [C*]. Considering the hydrogen activation effects on these two kinds of dormant sites, we can expect that adding hydrogen in propylene polymerization may cause different extents of [C*] increase depending on the relative shares of the two kinds of dormant sites. When there are no Ti–CH(CH3)2-type dormant sites, there will be no change in [C*], but the kp value will be enhanced for the resumed chain propagation on the Ti–CH(CH3)CH2–P-type dormant sites in the presence of H2. When the Ti–CH(CH3)2-type dormant sites cannot be neglected, an increase in [C*] via H2 can be expected, but the change in kp value will be hard to predict. If the active centers reactivated from the Ti–CH(CH3)2-type dormant sites have evidently lower reactivity than the propagation centers active in the absence of hydrogen, a decrease in the kp value via H2 can also be expected.
In the catalytic systems studied in this work, when Donor-C or Donor-D was used as external donors, the addition of H2 caused an evident increase in active centers in the C7-sol fraction, in contrast to only a slight increase in [Cm*] in the system with Donor-Py. It seems that there is a high fraction of Ti–CH(CH3)2-type dormant sites in the active centers producing medium-isotactic PP when Donor-C or Donor-D is the external donor. In contrast, the fraction of Ti–CH(CH3)2-type dormant sites should be rather low when Donor-Py is added as the external donor. The kp value of the active centers in the C7-sol fraction was raised moderately (15–25%) via the addition of H2, implying that the activation of Ti–CH(CH3)CH2–P-type dormant sites via H2 should be present in these active centers with medium stereoselectivity.
For the active centers producing highly isotactic PP (C7-insol fraction), though the [Ci*]/[Ti] ratio was almost unchanged by the 2% of H2 when either Donor-Py, Donor-C, or Donor-D was used as external donors, further raising H2 concentrations from 2% to 5% caused a 20% decrease in [Ci*]/[Ti] in the system with Donor-Py in contrast to the nearly invariant [Ci*]/[Ti] in the Donor-C and Donor-D systems. For all three donors, the kp value of the isospecific active centers increased by 40–60% by adding 2% H2 in the system, but further increasing H2 concentrations to 5% caused no changes in the kp value in the Donor-C and Donor-D systems. However, in the system with Donor-Py, the kp value was further enhanced by 40% by increasing the H2 concentration to 5%. These phenomena also showed the unique hydrogen responses in the polymerization system with Donor-Py. Considering that [Ci*]/[Ti] declined by 20% by raising H2 concentrations from 2% to 5% in the Donor-Py system, the larger kp value at 5% H2 could be attributed to the deactivation of a part of the active centers with lower kp values under higher hydrogen concentrations. The slow insertion of propylene in the Ti–H bond formed by the hydrogenolysis of these active centers could be a reason. The capability of Donor-Py to more stably adsorb on the catalyst’s surface than Donor-C and Donor-D could be the reason for its unique hydrogen responses.
Comparing the hydrogen responses of the medium isospecific centers (Cm*) and highly isospecific centers (Ci*), it is clear that the former contains a high fraction of Ti–CH(CH3)2-type dormant sites, but dormant sites in the latter should be mainly composed of the Ti–CH(CH3)CH2–P-type dormant sites. This means that Ti–CH(CH3)2-type dormant sites are mainly formed by active centers with medium stereoselectivity. For the non-stereospecific centers producing the C8-sol fraction, both types of dormant sites seem to be unimportant, possibly owing to easier propylene insertion in such sites with a rather open stereochemical environment.

3. Experimental Section

3.1. Materials

Tetrahydrofuran (99.5%, dried with molecular sieves) and n-butyllithium in the n-hexane solution (2.2 M) were used as received. Piperidine (≥99%) and tetramethoxysilane (≥98%) were both purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as received. A commercial MgCl2-supported Ziegler–Natta catalyst MgCl2/Di/TiCl4 (Di = non-phthalate-type internal donor, provided by ExxonMobil Asia Pacific Research and Development Company, Ltd., Shanghai, China) with Ti contents of 2.87 wt% was used for polymerization. Propylene samples (polymerization grade, product of SINOPEC Shanghai Petrochemical Co., Ltd., Shanghai, China) were further purified by passing them through a column packed with deoxygen reagents and molecular sieves. Hydrogen (>99.999%) was further purified by passing it through a column packed with molecular sieves. Al(C2H5)3 (TEA) was purchased from Albemarle Co. (Charlotte, NC, USA) and diluted in n-heptane to 2 mol/L before use. 2-Thiophenecarbonyl chloride (TPCC, purchased from J&K Scientific, San Jose, CA, USA) was diluted with n-heptane to 2 mol/L before use. Cyclohexylmethyldimethoxysilane (Donor-C) was supplied by Shandong Lujing Chemical Technology Co., Ltd. (Dezhou, China) and distilled before use. Dicyclopentyldimethoxysilane (Donor-D), trimethoxy(propyl)silane (Donor-NM), triethoxy(propyl)silane (Donor-NE), and triethoxy(isobutyl)silane (Donor-IE) were all purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) and used as received. All external donors were diluted in n-heptane to 0.25 mol/L before use.

3.2. Synthesis of Dipiperidyldimethoxysilane (Donor-Py)

Dipiperidinyldimethoxysilane (Donor-Py) with a purity of above 96% was synthesized according to the literature [21,31,32,33], and the reaction steps are shown in Scheme 3.
In total, 100 mL of THF and 0.2 mol of piperidine were fed into a 500 mL flask in advance. Then, 0.22 mol of n-butyllithium in an n-hexane solution (2.2 M) was slowly dripped into the flask using a dropping funnel with an ice-water bath for the protection of nitrogen. The mixture was stirred at 25 °C for 3 h after the completion of the dropwise addition. Then, 0.1 mol of tetramethoxysilane was dropwise added slowly with an ice-water bath. The mixture was continually stirred at 25 °C for 12 h for the protection of nitrogen when the addition was completed. After the confirmation of sufficient product levels via gas chromatography, the mixture was filtered using a G4 glass filter at room temperature to remove the precipitate under the protection of a nitrogen atmosphere. The solid precipitate was sufficiently washed using dry n-heptane. The filtrate was distilled under reduced pressure to evaporate the solvent, and the remaining liquid was subjected to secondary distillation to recover the intended product. The product had a boiling point of 104.5 °C per 3.5 mbar. The yield of Donor-Py was 81.9% based on the fed piperidine. The 1H NMR spectra of Donor-Py in CDCl3 are shown in Figure S3 of the Supporting Information.

3.3. Propylene Polymerization

Propylene polymerizations were conducted in a 100 mL flask equipped with a magnetic stirring bar. The flask was dried by heating it under a vacuum and refilling it with nitrogen three times. In a propylene atmosphere of 1 atm, the designated amount of n-heptane (60 mL), AlEt3 (Al/Ti = 100), external donors (Si/Ti = 5), and hydrogen was successively introduced into the flask. The solid catalyst powder (~50 mg) was then added to start the polymerization at 60 °C. Gaseous propylene at 1 atm was continuously supplied to the flask during polymerization. After 10 min, a thiophene-2-carbonyl chloride (TPCC) heptane solution (TPCC/Al = 2) was injected into the flask to quench polymerization, and the liquid phase was further stirred at 60 °C for 5 min. Subsequently, an ethanol/HCl mixture (95/5) was added to decompose the catalyst and quencher, and the polymer was precipitated with excess ethanol. After being repeatedly washed with ethanol and filtered, the produced PP powder was then dried at 60 °C under vacuum for 6 h.
Before fractionation and characterization, the quenched polymer was purified to remove the remaining sulfur-containing compounds in the polymer. The quenched polymer was first treated by refluxing in excess of the ethanol/HCl mixture for 60 min. The collected polymer was then washed and dried and thoroughly purified via three dissolution–precipitation operations. Finally, it was extracted with absolute ethanol in a Soxhlet extractor for 12 h and dried in a vacuum at 60 °C.

3.4. Polymer Fractionation

Each purified PP sample was fractionated into three fractions in two steps: (1) About 1~2 g of the purified PP sample was heated to carry out refluxing in 200 mL of n-octane for 2 h, and then, it was cooled to room temperature (25 °C) and kept at 25 °C for 8–12 h. The suspension was separated into solution and solid parts via filtration. The solid part was dried in a vacuum and called the n-octan-insoluble part. The solution part was concentrated via rotary evaporation and then dried in a vacuum. This fraction was named the n-octane-soluble part (RTC8-soluble or C8-sol). The n-octane-insoluble part was further extracted by boiling n-heptane for 12 h in a Kumagawa-type extractor. Then, the n-heptane-soluble part (BC7-soluble or C7-sol) was recovered via the rotary evaporation of the solution. The n-heptane-insoluble part is named BC7-insoluble (C7-insol). As disclosed via the 13C NMR and DSC analyses in our previous work, the C8-sol, C7-sol, and C7-ins fractions are composed of atactic PP (aPP), medium-isotactic PP (miPP), and isotactic PP (iPP) chains, respectively [16,35,42].

3.5. Measurement of the Molecular Weight

The molecular weights and molecular weight distributions of the fractions were measured via gel permeation chromatography (GPC) in a PL 220 GPC instrument (Polymer Laboratories, Ltd., Church Stretton, UK) at 150 °C in 1,2,4-trichlorobenzene with 0.0125% bibutyl hydroxy toluene (BHT). Three PL mixed B columns (500~107) were used. Universal calibration against narrow polystyrene standards was adopted.

3.6. Sulfur Content Determination

The sulfur contents of each purified PP sample along with its three fractions were measured in a YHTS-2000 ultraviolet fluorescence sulfur analyzer with a lower detection limit of 0.05 ppm (Jiangyan Yinhe Instrument Co., Taizhou, China). The polymer sample for analysis was solid powder (2–4 mg, weighed to ±0.01 mg), and the average value of three parallel measurements was taken as the sulfur content. The number of active centers ([C*]/[Ti]) was calculated according to the sulfur content. The propagation rate constant (kp) of polymerization was calculated according to the following equation:
Rp = kp[C*][M]
where Rp is the rate of polymerization calculated by dividing the polymer yield with polymerization time, [C*] is the concentration of active center, and [M] (=0.25 mol/L) is the equilibrium propylene concentration in n-heptane at 1 bar and 60 °C.

3.7. Thermal Analysis

3.7.1. Differential Scanning Calorimetry (DSC)

DSC analysis was carried out using a TA DSC25 thermal analyzer under a high-purity nitrogen atmosphere. About 3 mg of the sample was sealed in an aluminum pan and first melted at 190 °C for 5 min in order to erase the previous thermal history; then, it was cooled down to 40 °C at a rate of 10 °C/min. Finally, it was heated up to 190 °C at the rate of 10 °C/min again, and the second melting endotherm was recorded.

3.7.2. Successive Self-Nucleation and Annealing (SSA)

In the SSA experiment, the most important parameter is the first self-nucleation temperature (Ts1). According to the self-nucleation (SN) technique, the Ts1 temperature of each sample was determined in this research. The complete thermal treatment comprised the following steps: (a) erasure of the crystalline thermal history by heating the sample to 200 °C and holding for 3 min; (b) cooling the sample at 20°C/min to 50 °C and holding for 2 min; (c) the sample was heated at 20 °C/min from 50 °C to the Ts1 temperature; (d) the sample was kept at Ts1 for 5 min; (e) DSC cooling scan at 20 °C/min from Ts1 to 50 °C, where the effects of the thermal treatment will be reflected on the crystallization of the sample; (f) repeat step “c” to “e” at a new lower Ts temperature, which was varied from Ts1 to 121 °C at 5 °C intervals for a total ten self-nucleation/annealing steps. (g) Finally, the sample was heated at 20 °C/min from 50 °C to 200 °C, and multiple melting endotherms were obtained. Scheme 4 shows the temperature–time profile of the SSA experiment.

3.8. Nuclear Magnetic Resonance (NMR)

13C NMR spectra were recorded on a Varian Mercury 300-plus spectrometer (Varian, Palo Alto, CA, USA) at 75 MHz. o-Dichlorobenzene-d4 was used as the solvent, and the sample’s concentration was 10 w/v%. The spectra were recorded at 120 °C with hexamethyldisiloxane as an internal chemical shift reference. Cr(acac)3 was used to reduce the relaxation time of carbon atoms, and the delay time was set as 3 s. The pulse angle was 90°, and more than 3000 transients were collected.

4. Conclusions

Three categories of external donors, including dialkyldimethoxysilane (Donor-C and Donor-D), diaminodimethoxysilane (Donor-Py), and alkoxysilane carrying three alkoxy groups (Donor-NM, Donor-NE, and Donor-IE), were applied to propylene polymerization in the presence or absence of hydrogen. By analyzing the effects of external donors on the catalytic activity, molecular weight distribution, fraction distribution, number, and reactivity of active centers and the thermal property of the PP product, the efficiency of enhancing PP isotacticity for each De was comparatively studied, and the mechanism of De performance was also explained. The introduction of hydrogen caused enhancements in polymerization activity no matter what kind of De was used. However, the extent of activation is dependent on the type of De. Alkoxysilanes carrying three alkoxy groups generally have lower polymerization activities than dialkyldimethoxysilane-type and diaminodialkoxysilane-type De due to their stronger electron-donating ability. Donor-Py produced the PP sample with the highest MW among the De used in this research, and it was more sensitive to hydrogen than Donor-D. According to the fractionation results, Donor-D can reduce the percentage of atactic PP (C8-sol fraction) more efficiently than other De. However, Donor-Py produced PP with the lowest contents of medium-isotactic PP (C7-sol) and the highest content of highly isotactic PP (C7-insol), especially at high hydrogen concentrations. By raising hydrogen concentrations in propylene polymerization, the number of active centers was also enhanced in the systems with Donor-C and Donor-D, while it was reduced when Donor-Py was the De. Bulkier alkoxy and alkyl groups in Donor-IE also caused a reduction in the number of active centers and an increase in the kp value. When Donor-Py was used as De, the effects of H2 concentrations on active center distributions and the reactivity (kp value) of different active centers were evidently different in comparison with those of Donor-C and Donor-D. With an increase in H2 concentration, the polymerization system with Donor-Py showed unchanged [Cm*] and lower [Ci*], and the kp value of its isospecific active centers was sharply increased. In contrast, an increase in H2 concentration enhanced [Cm*] and only moderately increased the kp value of isospecific active centers in systems with Donor-C and Donor-D. According to the SSA results, the C7-insol fraction produced by Donor-Py had the highest melting temperature. When the melting temperature and percentage of the C7-insol fraction are both taken into consideration, the advantage of Donor-Py in enhancing isotacticity becomes more obvious than Donor-D in the presence of H2 and increased propylene pressure. It can be concluded that the siloxane external donor carrying two piperidyl groups (Donor-Py) is the best candidate among the De used in this work for producing PP with both high isotacticity and satisfactory processability.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15040330/s1. Table S1. Activity of PP produced with different external donors; Table S2. Weight average molecular weight and polydispersity of the PP products; Table S3. Influence of De on the number of active centers and chain propagation rate constants of different active centers; Table S4. SSA results of C7-insol fractions produced with different external donors; Figure S1. The second DSC melting traces of C7-insol fractions of the PP samples produced at different H2 concentrations; Figure S2. The second DSC melting traces of C7-insol fractions of the PP samples produced with external donors bearing three alkoxy groups. Figure S3. Melting endotherms of C7-insol fractions annealed at different Ts temperatures for 5 min (rate of the melting scan is 10oC/min). Figure S4. The 1H NMR spectra of Donor-Py in CDCl3.

Author Contributions

Conceptualization: Z.F. (Zhiqiang Fan), B.Z. and Z.F. (Zhisheng Fu); methodology, B.Z., X.L., B.J., M.D., Z.F. (Zhisheng Fu) and Z.F. (Zhiqiang Fan); validation, Z.F. (Zhisheng Fu) and Z.F. (Zhiqiang Fan); formal analysis, B.Z., X.L. and W.G.; investigation, B.Z., X.L., B.J. and W.G.; resources and supervision, Z.F. (Zhisheng Fu), M.D. and Z.F. (Zhiqiang Fan); data curation and writing—original draft preparation, B.Z. and X.L.; writing—review and editing, Z.F. (Zhiqiang Fan). All authors have read and agreed to the published version of the manuscript.

Funding

National Key Research and Development Program of China (Grant Number: 2021YFB2401501) and Exxon Mobil Corp.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors gratefully thank Qingyun Qian and Pengjia Yang of ExxonMobil Asia Pacific Research & Development Co., Ltd. for their collaboration and assistance in the execution of this research work.

Conflicts of Interest

Authors Xiaoyu Liu and Biao Zhang were employed by the company Zhejiang Sinopont Technology Co. Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 15 00330 sch001
Scheme 1. Structures of external donors bearing different substituents.
Scheme 1. Structures of external donors bearing different substituents.
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Figure 1. Propylene homopolymerization activity with different external donors.
Figure 1. Propylene homopolymerization activity with different external donors.
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Figure 2. Molecular weight and its distribution of PP produced with different external donors (the number in the columns represents the hydrogen volume percentage).
Figure 2. Molecular weight and its distribution of PP produced with different external donors (the number in the columns represents the hydrogen volume percentage).
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Figure 3. Fractionation results of PP samples produced with different external donors (the number in the columns represents the hydrogen volume percentage).
Figure 3. Fractionation results of PP samples produced with different external donors (the number in the columns represents the hydrogen volume percentage).
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Figure 4. Influence of De and H2 on the number of active centers (a) and the apparent propagation rate constants (b) of three groups of active centers (the number in the columns denotes the volume percentage of hydrogen).
Figure 4. Influence of De and H2 on the number of active centers (a) and the apparent propagation rate constants (b) of three groups of active centers (the number in the columns denotes the volume percentage of hydrogen).
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Figure 5. SSA melting endotherms of the C7-insol fractions produced with different external donors.
Figure 5. SSA melting endotherms of the C7-insol fractions produced with different external donors.
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Figure 6. Deconvolution of the SSA melting endotherms of the C7-insol fractions produced with different external donors (circles: experimental data; red line: Peak 1 assigned to PP chains with the highest isotacticity; blue line: Peak 2 assigned to PP chains with relatively lower isotacticity.).
Figure 6. Deconvolution of the SSA melting endotherms of the C7-insol fractions produced with different external donors (circles: experimental data; red line: Peak 1 assigned to PP chains with the highest isotacticity; blue line: Peak 2 assigned to PP chains with relatively lower isotacticity.).
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Figure 7. Fractionation results of PP samples produced with different external donors at a propylene pressure of 0.5 MPa.
Figure 7. Fractionation results of PP samples produced with different external donors at a propylene pressure of 0.5 MPa.
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Figure 8. 13C-NMR spectra of C7-insol fractions produced with different external donors.
Figure 8. 13C-NMR spectra of C7-insol fractions produced with different external donors.
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Scheme 2. Reversible coordination of De on Ti of an active site located on the (110) lateral cut of the MgCl2 crystallite and nearby Mg of the same (110) cut. (a) The active site without adsorptions of DEAC and the external donor; (b) The active site with the external donor adsorbed on the central Ti and the Al of DEAC; (c) The active site with DEAC and the external donor adsorbed on the adjacent Mg.
Scheme 2. Reversible coordination of De on Ti of an active site located on the (110) lateral cut of the MgCl2 crystallite and nearby Mg of the same (110) cut. (a) The active site without adsorptions of DEAC and the external donor; (b) The active site with the external donor adsorbed on the central Ti and the Al of DEAC; (c) The active site with DEAC and the external donor adsorbed on the adjacent Mg.
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Catalysts 15 00330 sch003
Scheme 3. Synthesis of the external donor Donor-Py.
Scheme 3. Synthesis of the external donor Donor-Py.
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Scheme 4. SSA thermal protocol schematic representation. Cooling and heating scans are performed at a constant scanning rate.
Scheme 4. SSA thermal protocol schematic representation. Cooling and heating scans are performed at a constant scanning rate.
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Table 1. Fractionation results of PP samples produced with different external donors.
Table 1. Fractionation results of PP samples produced with different external donors.
SamplesDeH2 (mL)C8-solC7-solC7-insolC7-sol + C7-insol
PP-C-0Donor-C02.11 13.61 84.27 97.89
PP-C-2Donor-C21.06 14.78 84.16 98.94
PP-C-5Donor-C51.29 16.92 81.79 98.71
PP-D-0Donor-D01.20 8.51 90.30 98.80
PP-D-2Donor-D20.38 10.60 89.02 99.62
PP-D-5Donor-D50.60 10.61 88.79 99.40
PP-Py-0Donor-Py02.51 8.36 89.13 97.49
PP-Py-2Donor-Py20.75 8.22 91.03 99.25
PP-Py-5Donor-Py51.11 7.63 91.26 98.89
PP-NM-2Donor-NM23.10 19.39 77.50 96.90
PP-NE-2Donor-NE22.24 12.27 85.48 97.76
PP-IE-2Donor-IE22.01 10.10 87.88 97.99
Table 2. Proportions of Peak 1 and Peak 2 in the melting endotherms of the C7-insol fraction produced with different external donors.
Table 2. Proportions of Peak 1 and Peak 2 in the melting endotherms of the C7-insol fraction produced with different external donors.
SamplesTm (°C)ΔH (J/g)Peak 1Peak 2
Tm1 (°C) PP1 a (%)ΔHP1 b (J/g)Tm2 (°C) PP2 a (%)ΔHP2 c (J/g)
PP-NM-2170.02125.7170.1862.861.1164.9137.236.3
PP-C-5170.39124.2170.3667.968.9165.2432.232.7
PP-D-5173.39125.9173.5266.274.0167.4333.837.8
PP-Py-5173.36126.2173.5066.276.2167.4333.838.9
a Proportion of Peak 1 or Peak 2 in the melting endotherm of the C7-insol fraction. b Melting enthalpy of the most isotactic PP chains in the C7-insol per gram of the PP sample: ΔHP1 = (ΔH)C7-insol × PP1 × (weight fraction of C7-insol in PP sample). c Melting enthalpy of relatively lower isotactic PP chains in the C7-insol per gram of PP sample: ΔHP2 = (ΔH)C7-insol × PP2 × (weight fraction of C7-insol in PP sample).
Table 3. Thermal properties of the C7-insol fractions of PP synthesized with different external donors at a propylene pressure of 0.5 MPa.
Table 3. Thermal properties of the C7-insol fractions of PP synthesized with different external donors at a propylene pressure of 0.5 MPa.
External DonorSampleActivity (g/g Cat·h)Tm (°C)ΔH (J/g)
Donor-CHP-PP-C1 a1314 ± 2.37%162.5897.74
HP-PP-C4 b1343 ± 0.22%161.96103.4
Donor-DHP-PP-D1 a1597 ± 0.08%163.7198.92
HP-PP-D4 b1553 ± 1.74%163.70101.2
Donor-PyHP-PP-Py1 a1387 ± 0.67%165.05100.8
HP-PP-Py4 b1376 ± 0.45%164.73100.5
a PP was produced in the presence of 1% of H2; b PP was produced in the presence of 4% of H2.
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Liu, X.; Zhang, B.; Guo, W.; Jiang, B.; Du, M.; Fu, Z.; Fan, Z. Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors. Catalysts 2025, 15, 330. https://doi.org/10.3390/catal15040330

AMA Style

Liu X, Zhang B, Guo W, Jiang B, Du M, Fu Z, Fan Z. Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors. Catalysts. 2025; 15(4):330. https://doi.org/10.3390/catal15040330

Chicago/Turabian Style

Liu, Xiaoyu, Biao Zhang, Wenqi Guo, Baiyu Jiang, Miao Du, Zhisheng Fu, and Zhiqiang Fan. 2025. "Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors" Catalysts 15, no. 4: 330. https://doi.org/10.3390/catal15040330

APA Style

Liu, X., Zhang, B., Guo, W., Jiang, B., Du, M., Fu, Z., & Fan, Z. (2025). Hydrogen Responses of Propylene Polymerization with MgCl2-Supported Ziegler–Natta Catalysts in the Presence of Different Silane External Donors. Catalysts, 15(4), 330. https://doi.org/10.3390/catal15040330

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