Synthesis of Ethylene or Propylene / 1 , 3-Butadiene Copolymers Possessing Pendant Vinyl Groups with Virtually No Internal Olefins

In general, ethylene/1,3-butadiene copolymerizations provides copolymers possessing both pendant vinyls and vinylenes as olefinic moieties. We, at MCI, studied the substituent effects of C2-symmetric zirconocene complexes, rac-[Me2Si(Indenyl’)2]ZrCl2 (Indenyl’ = generic substituted indenyl), after activation on the ratio of the pendant vinyls and vinylenes of the resultant copolymers. Complexes examined in this study were rac-dimethylsilylbis (1-indenyl)zirconium dichloride (1), rac-dimethylsilyl-bis[1-(2-methyl-4,5-benzoindenyl)] zirconium dichloride (2), rac-dimethylsilyl-bis[l-(2-methyl -4-phenylindenyl)]zirconium dichloride (3), rac-dimethy1si1y1bis(2-ethyl-4-phenylindenyl) zirconium dichloride (4), rac-dimethylsilyl-bis[l-(2-n-propyl -4(1-naphthyl)indenyl)]zirconium dichloride (5), rac-dimethylsilyl-[1-(2-ethyl-4-(5-(2,2dimethyl-2,3-dihydro-1H-cyclopenta [a]naphthalenyl)indenyl))][1-(2-n-propyl-4-(5-(2,2dimethyl-2,3-dihydro-1H-cyclopenta[a] naphthalenyl)indenyl))]zirconium dichloride (6), rac-dimethylsilyl-bis[1-(2-ethyl-4-(9-phenanthryl)indenyl)]zirconium dichloride (7), and rac-dimethylsilyl-bis[l-(2-n-propyl-4-(9-phenanthryl)indenyl)]zirconium dichloride (8). We found that the ratio of the pendant vinyls and vinylenes is strongly affected by the bulkiness of the substituent on the complexes examined. The vinyl content increased linearly in the following order, 8 > 7 > 6 > 5 > 4 > 3 > 2 > 1. Notably, complex 8/DMAO formed ethylene/1,3-butadiene copolymers possessing predominant vinyl groups, which can be OPEN ACCESS Catalysts 2015, 5 2002 crucial precursors for functionalized polyolefins. Likewise, complex 8/DMAO afforded propylene/1,3-butadiene copolymers with predominant vinyl groups.


Introduction
Polyolefins, represented by polyethylene, polypropylene, and amorphous ethylene/α-olefin copolymers, can display useful mechanical properties (high impact strength, hardness/softness, durability), chemical stability (low reactivity, oil resistance, weathering resistance), thermal stability (heat/cold resistance), processability, lightweight, cost-effectiveness, and recyclability.However, due to their non-polar nature, they show poor affinity for polar materials such as polar organic polymers, glasses, and metals.To overcome this drawback, several attempts have been made in an effort to introduce polar groups into polyolefins, which include copolymerization of ethylene with polar monomers via a high pressure radical polymerization and the modification of polyolefins by a radical reaction [1].However, these methods have disadvantages to control over polymer architecture (e.g., contents and distributions of polar groups, product molecular weights, molecular weight distributions, and branching extent) due to inherently poor reaction selectivity and unfavorable side reactions.Conversely, the direct synthesis of functionalized polyolefins by the anionic insertion copolymerization of olefins and polar CH2=CHX monomers (X = polar groups) using organometallic complexes has extensively been investigated [1].Although these investigations have made an impact on polymer synthesis, these are far from being of practical use because of low productivity and a low degree of polar monomer incorporation.
In this study, we focused on the copolymerization of butadiene with olefins for the purpose of creating new functionalized polyolefinic materials using cost-competitive feedstocks.Because butadiene is a conventional monomer, yet it is able to generate a vinyl group onto the polymer backbone.The pendant vinyl group can be converted to functional groups, or utilized by itself.Thus it is that we explored catalysts that can copolymerize butadiene with olefin(s), where the insertion reaction of the butadiene proceeds preferentially via a 1,2-insertion without the accompanying subsequent cyclization, since the formation of the internal olefin, the cause of the degradation of the polymer generated by the 1,4-insertion, is unfavorable.
A rare example of such polymerization is propylene/butadiene copolymerization using rac-[Me2Si (2-methyl-4-phenylindenyl)2]ZrCl2 in the presence of hydrogen [10,11].However, while this method provides a copolymer having pendant vinyl groups in the main chain with a minimal number of internal olefins, there is still room for investigation regarding the substituent effects on the number of both internal olefins and vinyl groups.
Thus, we preliminary investigated a wide variety of catalysts including metallocene and post-metallocene catalysts to find catalysts that are capable of generating copolymers with predominant pendant vinyl groups in ethylene/butadiene copolymerization [12].As a result, we found a qualitative correlation between the catalyst structure and the insertion mode of butadiene.As a general rule, catalysts incorporating α-olefins with 1,2-insertion favor the 1,2-insertion of butadiene.On the other hand, catalysts incorporating α-olefins wit 2,1-insertion prefer the 1,4-insertion of butadiene.Namely, the catalysts inserting α-olefins by the 1,2-insertion tend to produce olefin/butadiene copolymers having pendant vinyl groups.This tendency is, in general, pronounced when steric regulation becomes significant.From our preliminary results obtained through the screening study, C2-symmetric zirconocene, rac-[Me2Si(2-methyl-4-phenylindenyl)2]ZrCl2 [10,11] was an especially efficient catalyst to incorporate butadiene by 1,2-insertion to obtain the desired polymer structure of an ethylene/butadiene copolymer.
Interestingly, this phenomenon is supported by molecular modeling calculations for an analogous primary coordination of butadiene is energetically favorable compared to η2 secondary coordination and the η4-cis coordination of butadiene [8].In addition, the relative amount of constitutional monomer units derived from butadiene are affected by the bulkiness of the substituent.To that end, we have anticipated that tuning the steric property of the rac-[R2Si(Indenyl')2]ZrCl2 (Indenyl' = generic substituted indenyl) would lead to a further increase in the pendant vinyl group while keeping the internal olefins at a negligible level.Herein we describe studies on a series of rac-[SiMe2(Indenyl')2]ZrCl2 catalysts with diverse steric properties to clarify factors that control the insertion mode of butadiene, and investigated methods to prepare copolymers that contain predominant pendant vinyl groups and are substantially free of internal olefins (derived from 1,4-or 1,3-addition units).

Ethylene/Butadiene Copolymerization by rac-[SiMe2(Indenyl')2]ZrCl2
Influence of Substituent on Insertion Mode of Butadiene in Ethylene/Butadiene Copolymerization We evaluated eight C2-symmetric zirconocene complexes, rac-[SiMe2(Indenyl')2]ZrCl2 (Figure 1); ), and (8) for ethylene or propylene/butadiene copolymerization in the presence of hydrogen under identical conditions.The resultant copolymers were analyzed by 13 C-NMR, and the butadiene content and relative abundance of various enchained monomer units were determined.The copolymers were also analyzed by 1 H-NMR to determine the quantity of the double bonds (vinyl groups and internal olefins) more accurately, because by 13 C-NMR analysis, the polymer is exposed to harsh conditions such as 140 °C for 12 h, so that the consumption of double bonds, especially the consumption of internal olefins during an analysis is anticipated.Ethylene/butadiene copolymerization results are summarized in Table 1.In the case of complex 1 / i Bu3Al/Ph3CB(C6F5)4, although activity was significantly high, butadiene was hardly incorporated into the resultant polymer, and the majority of butadiene units formed a five-membered ring (Table 1, run 1; Figure 2).However, a strong correlation between the microstructure of the copolymer and the bulkiness of the substituent on rac-[SiMe2(Indenyl')2]ZrCl2 was observed; as bulkiness of the substituent increases, a relative abundance of rings relative to all the structural units derived from butadiene decreased and conversely, that of the vinyl groups increased (Table 1, run 1-8; Figure 2).Surprisingly, complex 8, which has the most bulky substituent generated 18 times as much vinyl groups compared to uncrowded complex 1.These results suggest that, steric hindrance on rac-[SiMe2(Indenyl')2]ZrCl2 is effective to generate pendant vinyl groups by hampering the formation of sterically demanding cyclopropane and the cyclopentane rings.The amount of internal olefins was negligible for all cases regardless of the type of substituent, even according to sensitive 1 H-NMR analysis (Figures 3 and 4).   1.
Interestingly, DMAO is more efficient than i Bu3Al/Ph3CB(C6F5)4 for generating vinyl groups (Table 1, entries 9 and 10).As part of an effort to increase the number of vinyl groups, the butadiene/ethylene ratio in the feed was increased at DMAO activation conditions.As a result, we succeeded in synthesizing ethylene/butadiene copolymers which are dominated by vinyl groups (fv = 79 mol %; Table 1, entry 11).Further increase in butadiene/ethylene feed ratio [13] led to formation of ethylene/butadiene copolymers whose structural units derived from butadiene are mostly vinyl groups (fv = 95 mol %;  4).Although the formation of internal olefins accompanied this condition (1.3 per 1000 Carbon; Table 1, entry 12), it is a quantity far too low compared to that of the vinyl groups.The remaining internal olefins will be eliminated by introducing a higher amount of hydrogen [10,11].

Propylene/Butadiene Copolymerization by Complexes 3 and 8
Following the ethylene/butadiene copolymerization, propylene/butadiene copolymerization was conducted using complexes 3 and 8 combined in activation with MMAO so as to compare with the previous study [10,11].As with the results obtained by the ethylene/butadiene copolymerization, a similar substituent effect on the vinyl content was observed.Therefore, in this case again, we succeeded in synthesizing a propylene/butadiene copolymer possessing predominant vinyl groups (Table 2; Figure 5; Figure 6).Notably, the vinyl content obtained by complex 8 overwhelmed that by the previously reported complex 3 [10,11].Likewise, increasing the butadiene concentration in the feed led to the formation of propylene/butadiene copolymers abundant with vinyl groups (quantity of vinyl groups = 86 per 1000 carbon; Table 2, entry 3; Figure 5).Although an extremely high butadiene/propylene feed ratio entails the formation of a small amount of internal olefins (quantity of internal olefins = 3.7 per 1000 carbon; Table 2, entry 3; Figure 5), the quantity is still far too low compared to that of the vinyl group.The remaining internal olefins will be eliminated by introducing a higher amount of hydrogen, as mentioned above [10,11].2. Figure 6. 13 C-NMR spectra of the propylene/butadiene copolymers obtained from the runs 1, 2, and 3 of Table 2.

General
All manipulations were performed using dry box techniques under a purified N2 atmosphere or Schlenk techniques under N2 atmosphere, or on a high-vacuum line, unless otherwise indicated.

Ethylene/Butadiene Copolymerization (Table 1)
A prescribed amount of hexane was introduced into a SUS autoclave (1000 mL; entry 1-8, 4000 mL; entry 9-12) equipped with two propeller-like stirrers under nitrogen at an atmospheric pressure of 25 °C.A toluene solution of DMAO or i Bu3Al was loaded.Then a prescribed amount of butadiene was charged.Subsequently, the mixture was heated to a prescribed polymerization temperature, and a prescribed amount of hydrogen (the volume under atmospheric pressure at 20 °C) was added, then ethylene was introduced into the reactor up to the polymerization pressure (0.78MPa).For runs in which i Bu3Al/Ph3CB(C6F5)4 was employed as a co-catalyst, Zr complex/toluene, i Ph3CB(C6F5)4/toluene, and toluene wash (2 mL) were sequentially injected into the reactor to start the reaction.For runs in which the DMAO was employed as a co-catalyst, Zr complex/toluene and toluene wash (2 mL) were injected sequentially into the reactor to start the reaction.The pressure was kept constant at 0.78 MPa by feeding ethylene on demand.After the reaction was performed at a prescribed polymerization temperature for 20 min, the polymerization was terminated by adding a small amount of methanol.The resulting mixture was poured into a large excess of methanol containing hydrochloric acid to precipitate the polymer.The polymer was separated by filtration, and admixed with a stabilizing agent, 0.1 wt % of antioxidant (SUMILIZER ® GS(F): 2-[1-(2-Hydroxy-3,5-di-tert-pentylphenyl)ethyl]-4,6-di-tert-pentylphenyl acrylate, sumitomo chemical co.ltd, Tokyo, Japan), then dried under reduced pressure at 20 °C for 24 h.

Propylene/Butadiene Copolymerization (Table 2)
A prescribed amount of hexane was introduced into a SUS autoclave (1000 mL) equipped with two propeller-like stirrers under nitrogen at an atmospheric pressure of 25 °C.A toluene solution of MMAO was loaded, then a prescribed amount of butadiene was charged.Subsequently, the mixture was heated to a prescribed polymerization temperature, and a prescribed amount of hydrogen (the volume under atmospheric pressure at 20 °C) was added.Then, propylene was introduced into the reactor up to the polymerization pressure (0.78 MPa).Zr complex/toluene and toluene wash (2 mL) were injected sequentially into the reactor to start the reaction.The pressure was kept constant at 0.78 MPa by feeding propylene on demand.After the reaction was performed at 60 °C for 20 min, the polymerization was terminated by adding a small amount of methanol.The work-up procedure for the propylene/butadiene copolymer was identical to that for the ethylene/butadiene copolymerization.

Polymer Characterization
3.4.1.Microstructure Analysis of Ethylene/Butadiene Copolymer by 13 C-NMR 13 C-NMR spectra of the ethylene/butadiene copolymers were recorded on an ECA500 spectrometer (125 MHz) from Japan Electron Optics Laboratory Co. Ltd. (JEOL, Tokyo, Japan), using 1,1,2,2-tetrachloroethane-d2 as a solvent at 140 °C.Chemical shifts were referenced to the residual solvent peak (δ13C = 72.4ppm).The spectrum was analyzed with reference to the chemical shifts of the signals assigned to the vinyl groups, the cyclopropane skeleton, the cyclopentane skeleton, the 1,4-addition units and the 1,3-addition units in the ethylene/butadiene copolymers which were described in previous papers [8,18].The determination of the butadiene content (1) and the vinyl groups content (2); the cyclopropane skeleton content (3); the cyclopentane skeleton content (4); the 1,4-addition units content (5) and the 1,3-addition units content (6) relative to all the butadiene-derived structural units was performed in accordance with the following Equations (1) to (6).
Proportion of structural units derived from butadiene relative to all the monomer units in the copolymer is given by Equation (1).

BD content (mol %) = 100 × (A + B + C + D + E)/{(A + B + C + D + E) + [(1000
Fraction of vinyl groups (fv), cyclopropane rings (fcycPro), cyclopentane rings (fcycPen), 1,4-addition units (f1,4), and 1,3-addition units (f1,3) relative to all the structural units derived from butadiene is given by Equations ( 2)-( 6 The letters A to E indicate the respective integrated values per one carbon of the structural units derived from the structures illustrated below, based on the total integrated value, 1000, of all the peaks in the 13 C-NMR spectrum of the ethylene/butadiene copolymers. 1 H-NMR spectra of the ethylene/butadiene copolymers were recorded on a JEOL270 spectrometer (270 MHz) from Japan Electron Optics Laboratory Co. Ltd. (JEOL, Tokyo, Japan), using o-dichlorobenzene-d4 as a solvent at 120 °C.Chemical shifts were referenced to the residual solvent peak (δ1H = 7.15 ppm).Quantity of the vinyl groups Equation ( 7) and the internal olefins Equation (8) in ethylene/butadiene copolymers were quantified as per 1000 Carbon in accordance with the following Equations ( 7) and (8), respectively.Quantity of vinyl groups per 1000 Carbon = F + G (7) Quantity of internal olefins per 1000 Carbon = H (8) The letters F to H indicate the respective integrated values per one proton of the structural units derived from the structures illustrated below, based on the total integrated value, 2000, of all the peaks in the 1 H-NMR spectrum of the ethylene/butadiene copolymer.  1C-NMR 13 C-NMR spectra of the propylene/butadiene copolymers were recorded on an ECA500 spectrometer (125 MHz) from Japan Electron Optics Laboratory Co. Ltd. (JEOL, Tokyo, Japan), using 1,1,2,2-tetrachloroethane-d2 as a solvent at 140 °C.Chemical shifts were referenced to the residual solvent peak (δ13C = 72.4ppm).The spectrum was analyzed with reference to the chemical shifts of the signals assigned to the vinyl groups, the cyclopropane skeleton, the methylcyclopentane skeleton [19], the 1,4-addition units and the 1,3-addition units in the propylene/butadiene copolymers which were described in previous papers [8,10,11].The determination of the butadiene content (Equation ( 9)) and the vinyl groups content (Equation ( 10)), the cyclopropane skeleton content (Equation ( 11)), the methylcyclopentane skeleton content (Equation ( 12)), the 1,4-addition units content (Equation ( 13)), the 1,3-addition units content (Equation ( 14)), and the hydrogenated 1,4-addition units content (Equation ( 15)) relative to all the butadiene-derived structural units was performed in accordance with the following Equations ( 9) to (15).

DSC Analysis
The glass transition temperature (Tg) and the melting point (Tm) of the polymers were determined by differential scanning calorimetry (DSC) with a Shimadzu DSC-60 instrument (Shimadzu Corporation, Kyoto, Japan).In a N2 (nitrogen) atmosphere, the polymer was heated from ordinary temperature to 200 °C at a temperature increasing rate of 50 °C/min, held at the temperature for 5 min, cooled to −100 °C at a temperature decreasing rate of 10 °C/min, and held at the temperature for 5 min.The temperature was again increased to 200 °C at a temperature increasing rate of 10 °C/min.The Tg and the Tm were obtained from an endothermic curve recorded during the second scanning.

Conclusions
In this study, we investigated the substituent effects of C2-symmetric zirconocene complexes 1-8, rac-[Me2Si(Indenyl')2]ZrCl2, after activation, on the structures of the resultant copolymers in the copolymerization of ethylene or propylene with butadiene.As a result, we can reveal a larger steric hindrance on the complex tends to suppress the formation of cyclopropyl and cyclopentyl moieties in the copolymer.Additionally, we have found that the steric hindrance exercises a great influence on the ratio of the pendant vinyls and vinylenes (internal olefins), and complex 8 having the largest steric hindrance provides copolymers having predominant pendant vinyl groups with virtually no internal olefins.These results indicate that the steric hindrance on the zirconocene complex employed can control the insertion mode of butadiene and, moreover, mitigate the cyclization process leading to the generation of cyclopropyl and cyclopentyl moieties.The copolymers can be transformed into functionalized copolymers using procedures from the literature.The functionalized copolymers we obtained will have a wide variety of applications covering various fields.
a Conditions: hexane 250 mL, butadiene 25 g, ethylene 0.51 MPa (partial pressure at 60 °C), H 2 372 mL (volume under atmospheric pressure at 20 °C), Zr 0.002 mmol, i Bu 3 Al 0.5 mmol, Ph 3 CB(C 6 F 5 ) 4 0.012 mmol, polymerization time 20 min, 60 °C, total pressure 0.78 MPa; b Activity based on polymer yield; unit: kg-polymer/(mmol-Zr hr); c Weight average molecular weight determined by GPC using polystyrene calibration; d Butadiene content in the copolymer determined by13C-NMR; e Relative abundance of various enchained monomer units relative to all the structural units derived from butadiene determined by13C-NMR; f determined by 1 H-NMR; g The total amount of f 1,4 and f 1,3 ; h The total amount of pendant vinyl groups and terminal vinyl groups; i hexane 1000 mL, butadiene 100 g, ethylene 0.55 MPa (partial pressure at 60 °C), H 2 620 mL (volume under atmospheric pressure at 20 °C), Zr 0.004 mmol, i
1H-NMR spectra of the ethylene/butadiene copolymers obtained from entries 1, 8, and 12 of Table

Table 1
Notably, the quantity of vinyl groups per 1000 carbon reached 76 for this condition (Table 1, entry 12; Figure

Table 2 .
Propylene/butadiene copolymerization results using complexes 1-8 a .Relative abundance of various enchained monomer units relative to all the structural units derived from butadiene determined by13C-NMR; f determined by 1 H-NMR; g The total amount of f 1,4 and f 1,3 ; h The total amount of pendant vinyl groups and terminal vinyl groups; i butadiene 200 g, propylene 0.26 MPa (partial pressure at 60 °C).