Amorphous Elastomeric Ultra-High Molar Mass Polypropylene in High Yield by Half-Titanocene Catalysts

Propylene polymerizations with different ketimide-modified half-titanocene catalysts, Cp’TiCl2(N=CtBu2) [Cp’ = C5H5 (1), C5Me5 (2), Me3SiC5H4 (3)], with MAO as a cocatalyst, were investigated. The obtained polymers were studied in detail by determining their microstructure, molar masses, thermal, and mechanical properties. The Cp*-ketimide, (C5Me5)TiCl2(N=CtBu2) (2), exhibited higher catalytic activities than Cp’TiCl2(N=CtBu2) (1,3), yielding higher molar mass polymers, Mw up to 1400 Kg/mol. All the synthesized polypropylenes (PP) are atactic and highly regioregular, with predominant rrrr pentads, especially PP prepared with catalyst 1. Differential scanning calorimetry (DSC) established that the polymers are fully amorphous aPP, and no melting endotherm events are detected. Glass transition temperatures were detected between −2 and 2 °C. These polypropylenes have been established to be high-performance thermoplastic elastomers endowed with remarkably high ductility, and a tensile strain at break higher than 2000%.


Introduction
Polyolefins are important commercial synthetic polymers, which are predominantly synthesized by transition metal-catalyzed olefin polymerization technology.
The discovery of homogeneous ansa metallocene precatalysts for stereospecific olefin polymerization is thought of as a breakthrough in this leading industrial process [1][2][3][4][5].They allow the production of polyolefins in a much broader variety of compositions and microstructures than any other previous catalytic systems, by virtue of the notable range of ligands which differ in the steric and electronic properties.
The control of polymer molar mass at industrially practical polymerization temperatures is also central to producing poly(α-olefins) with desired properties.Variations in the cyclopentadienyl ligand solved this essential question in the cases of isotactic [11] and syndiotactic [7] polypropylenes, while there are only a restricted number of direct, high-yield processes capable of synthesizing atactic polypropylene (aPP) with high molar masses [9,10].It is probable that the high molar mass has limited the investigation of its properties.However, high molar masses and an absence of crystallinity have a positive effect on the elastic and optical properties of aPP [9,10].Thus, the synthesis of amorphous, high molar mass aPP is still considered a very attractive goal.
Polymers 2024, 16 In particular, Nomura developed a class of nonbridged cyclopentadienyltitanium aryloxide and ketimide complexes [14] as high performance catalysts for the copolymerization of ethylene with α-olefins to achieve high molar mass copolymers, for high α-olefin homopolymerization, and for the copolymerization of cyclic olefin with high α-olefin or ethylene to produce copolymers with very interesting properties in high yields.No studies for the synthesis of high molar mass polypropylene with this type of catalyst have been reported so far.
Recently, ketimide-modified half-titanocenes, Cp'TiCl 2 (N=CtBu 2 ), showed high catalytic activity with efficient norbornene (N) incorporation in the terpolymerization of N with ethylene and α-olefins affording high molar mass polymers with uniform molar mass distribution and compositions [23].Very recently, propylene copolymers with a series of cyclic olefins with interesting thermal properties were obtained with aryloxide complexes, such as Cp*Ti(O-2,6-iPrC 6 H 3 )Cl 2 in the presence of MAO [24].
Thus, we thought to explore propylene homopolymerization with these half-titanocenes.In particular, Cp'TiCl 2 (N=CtBu 2 ) were selected as those that gave the highest activity and molar masses in poly(E-ter-N-ter-O) [25].Polymerization reactions were investigated by using the half-titanocene based precursors shown in Scheme 1 activated with methylaluminoxane (MAO).The influence of polymerization pressure (2 and 4 bar) and temperature (40 and 60 • C) on polymerization behavior were investigated.Polymer analysis, which includes the NMR analysis of microstructure and chain end groups and the determination of molar mass, thermal and mechanical properties, was performed.The production of ultra-high molar mass amorphous atactic PP (aUHMWPP) with excellent mechanical properties was shown.
Polymers 2024, 16,512 Progress in bridged and nonbridged half-metallocenes [12][13][14][15], and so-called metallocene type [16][17][18][19][20] catalysts, offers several new possibilities for the purpose [ In particular, Nomura developed a class of nonbridged cyclopentadienyltitani yloxide and ketimide complexes [14] as high performance catalysts for the copolym tion of ethylene with α-olefins to achieve high molar mass copolymers, for high α homopolymerization, and for the copolymerization of cyclic olefin with high α-ol ethylene to produce copolymers with very interesting properties in high yields.No ies for the synthesis of high molar mass polypropylene with this type of catalyst hav reported so far.
Thus, we thought to explore propylene homopolymerization with these halfcenes.In particular, Cp'TiCl2(N=CtBu2) were selected as those that gave the highest ity and molar masses in poly(E-ter-N-ter-O) [25].Polymerization reactions were i gated by using the half-titanocene based precursors shown in Scheme 1 activate methylaluminoxane (MAO).The influence of polymerization pressure (2 and 4 ba temperature (40 and 60 °C) on polymerization behavior were investigated.Polyme ysis, which includes the NMR analysis of microstructure and chain end groups a determination of molar mass, thermal and mechanical properties, was performe production of ultra-high molar mass amorphous atactic PP (aUHMWPP) with ex mechanical properties was shown.

Materials
All manipulations were carried out using standard Schlenk-line and glove box techniques.Propylene and nitrogen were purified by passage through column of BTS-catalysts and molecular sieves.Toluene was dried by distillation from sodium under nitrogen atmosphere prior to use.Methylaluminoxane (MAO, 10 wt% solution in toluene, Merck KGaA Polymers 2024, 16, 512 3 of 11 (Darmstadt, Germany)) was prepared by removing solvent and unreacted trimethylaluminum (50 • C, 3 h, 0.1 mm Hg) and then was stored under nitrogen.CpTiCl 2 (N=CtBu 2 ) (1) and Cp*TiCl 2 N=CtBu 2 (2) were synthesized according to reference [26].C 2 D 2 Cl 4 was purchased from Merck KGaA and used as received.
Differential scanning calorimetry (DSC) measurements were performed on a Perkin-Elmer (Waltham, MA, USA) DSC 8000 instrument equipped with a liquid nitrogen device.The scans were carried out from 50 to 200 • C under nitrogen atmosphere using heating and cooling rates of 20 • C/min.Glass transition temperature (T g ) values were recorded during a second heating scan.Molar masses (M n and M w ) and molar mass distribution (M w /M n ) were determined by high temperature size exclusion chromatography (SEC) using a Waters (Milford, MA, USA) GPCV2000 system.Measurements were carried out in o-dichlorobenzene at 145 • C, a polystyrene standard calibration in the 162-5.6 × 10 6 g/mol range was used.
The samples for the mechanical analysis were molded in a heated press at 180 • C and 50 bar for 5 min, afterwards the cooling the press plates were performed at 20 • C/min to room temperature, obtaining films with a thickness of approximately 200 µm.Dog-boneshaped specimens (length overall 75 mm, gauge length 25 mm, and width of narrow section 4 mm) were tested at room temperature using a Zwick Roell (Ulm, Germany) ProLine Z010 dynamometer equipped with a XforceP (50 N) load cell at a crosshead speed of 15 mm/min.In the hysteresis tests, conducted at the fixed strain of 300%, the specimens were loaded and unloaded in uniaxial stretching for 10 cycles.The strain recovery (SR) was determined as SR = 100(ε a − ε r )/ε a , where ε a is the applied strain and ε r is the strain in the cycle at zero load following the applied strain.For each material, at least five specimens were tested for tensile tests and two specimens for recovery experiments.

Polymerization Procedure
In a typical homopolymerization reaction, a 250 mL Büchi (Uster, Switzerland) stainlesssteel autoclave, equipped with a mechanical stirrer and with an external thermostatic bath connected to a temperature control unit for temperature regulation, was evacuated for 120 min at 80 • C and conditioned three times with nitrogen.After cooling down to room temperature, the reactor was filled with toluene and with a solution of 25 mmol of MAO in toluene, which was previously prepared.After the thermal equilibration of the reactor system at the proper temperature, propylene was continuously added until saturation to the desired pressure.The reaction was initiated by injection of the precatalyst dissolved in toluene.The solution was stirred for 15 min, and the propylene pressure was kept constant during the polymerization reaction.The polymerization was terminated by the addition of a small amount of ethanol and hydrochloric acid and the polymer was precipitated upon pouring in ethanol, to which concentrated HCl had been added.The polymer was collected by filtration and dried under a vacuum at 70 • C until a constant weight was reached.

Results and Discussion
Our exploration of half-titanocenes Cp'TiCl 2 (N=CtBu 2 ) selected for the synthesis of propylene homopolymers included polymerization reactions by catalyst precursors, displayed in Scheme 1, in the presence of MAO, the analysis of polymer microstructure and chain end groups, molar masses and polymer properties.

Polymerizaton
Propylene homopolymerizations were conducted in a 250 mL steel reactor using 10 µmol of catalyst and 25 mmol of MAO in 100 mL of toluene for 15 min with MAO ([MAO]/[Ti] = 2500) acting as a co-catalyst and scavenger, at two polymerization temperatures (40 and 60 • C) and propylene pressures (2 and 4 bar).The polymerization time was set at 15 min.Polymers were characterized by 1 H-and 13 C-NMR, DSC and SEC measurements.The results regarding the synthesis and characterization of selected polymers are summarized in Table 1.A comparison of the entries obtained at 2 and 4 bar at 40 • C evidences that polymerization activities with all catalytic systems increase with propylene pressure for propylene polymerization.A comparison of the entries at 40 and 60 • C shows that catalytic activities increase with temperature only with the permethylated catalyst 2, while, with catalysts 1 and 3, a decrease in activity is observed.This decrease is quite consistent with catalyst 3.In general, polymer molar masses increase with monomer concentration (from 2 to 4 bar)-see entries 1 vs. 2 and 4 vs. 5-and molar mass distribution is narrow at 4 bar and 40 • C. The molar mass distribution of polymers obtained from catalysts 1 and 3 becomes bimodal at 60 • C (Figures S1-S3).This is probably related to the decrease in activity with catalysts 1 and 3. Indeed, although these catalysts are generally stable at 60 • C [14], in the conditions of this study, a partial formation of different active species may occur, probably due to some remaining AlMe 3 in dried MAO.The permethylation of the Cp ligand of catalyst 2 protects the catalyst from deactivation or active site changes; this causes greater activities All samples are soft solids, elastomeric materials, soluble in hydrocarbons and chlorinated solvents.DSC analysis showed no melting events, in agreement with data from the literature that report that moderately syndiotactic polypropylenes, with rrrr contents of about 20-25%, are completely amorphous [28].The observed glass transitions are in a typical range for polypropylene materials with T g values in the range of −2 to 2 • C, where the lower values are those of samples prepared at higher temperatures.

Microstructure
The microstructures of all the polypropylenes obtained with the three above mentioned catalyst systems were investigated by 13 C-NMR spectroscopy (see Figure S4).In Figure 1, the methyl region of the 13 C-NMR spectra of PP, obtained at 40 • C and 4 bar, from the three catalysts is presented.This region of the spectra shows the signals revealing the presence of all the pentads, indicating that all the three catalysts produce nearly atactic polypropylenes.The percentage of all the pentads of all the polymers obtained with the three catalysts, along with triads, the Bernoullian index [9] and regioirregularities, are displayed in Table S1.
bimodal at 60 °C (Figures S1-S3).This is probably related to the decrease in activity with catalysts 1 and 3. Indeed, although these catalysts are generally stable at 60 °C [14], in the conditions of this study, a partial formation of different active species may occur, probably due to some remaining AlMe3 in dried MAO.The permethylation of the Cp ligand of catalyst 2 protects the catalyst from deactivation or active site changes; this causes greater activities by raising the temperature from 40 to 60 °C.Thus, for all the catalysts, the best results are obtained at 40 °C and 4 bar.
All samples are soft solids, elastomeric materials, soluble in hydrocarbons and chlorinated solvents.DSC analysis showed no melting events, in agreement with data from the literature that report that moderately syndiotactic polypropylenes, with rrrr contents of about 20-25%, are completely amorphous [28].The observed glass transitions are in a typical range for polypropylene materials with Tg values in the range of −2 to 2 °C, where the lower values are those of samples prepared at higher temperatures.

Microstructure
The microstructures of all the polypropylenes obtained with the three above mentioned catalyst systems were investigated by 13 C-NMR spectroscopy (see Figure S4).In Figure 1, the methyl region of the 13 C-NMR spectra of PP, obtained at 40 °C and 4 bar, from the three catalysts is presented.This region of the spectra shows the signals revealing the presence of all the pentads, indicating that all the three catalysts produce nearly atactic polypropylenes.The percentage of all the pentads of all the polymers obtained with the three catalysts, along with triads, the Bernoullian index [9] and regioirregularities, are displayed in Table S1.Peak labels refer to the pentad assignments reported in [29].In the expanded region, regioirregular sequences are shown [30].
All the polymers are slightly syndiotactoid; the rrrr and the ratio between rrrr/mmmm follow the order 1 > 3 > 2. The Bernoullian factor B is below 1 for all polymers and tends to increase with temperature.
PPs produced by catalyst 2 are highly regioregular, those by catalyst 1 and 3 contain small amounts of both erythro and threo 2,1 misinsertions, as visible in the inserts of Figure 1a,c.They are around 5 mol% in the runs at 60 • C and 4 bar.
Scheme 2 displays the typical termination processes in propylene polymerization, which lead to the creation of the unsaturated end groups.All the polymers are slightly syndiotactoid; the rrrr and the ratio between rrrr/mmmm follow the order 1 > 3 > 2. The Bernoullian factor B is below 1 for all polymers and tends to increase with temperature.
PPs produced by catalyst 2 are highly regioregular, those by catalyst 1 and 3 contain small amounts of both erythro and threo 2,1 misinsertions, as visible in the inserts of Figure 1a,c.They are around 5 mol% in the runs at 60 °C and 4 bar.
Scheme 2 displays the typical termination processes in propylene polymerization, which lead to the creation of the unsaturated end groups.Scheme 2. Unsaturated chain end groups in polypropylene.
1 H-NMR end-group analysis can give information about the chain transfer mechanisms operating with a certain catalyst.The expansions of the 1 H-NMR spectra of polypropylenes obtained at 40 °C and 4 bar by catalyst 1, 2 and 3 are displayed in Figure S5.The signal-to-noise ratio is quite high due to the high molar mass of the polymers.However, only signals of chain end groups formed after 1,2 insertions are visible.
The percentage of vinylidene, isobutenyl, and allyl end groups of PP samples obtained with the three catalysts are listed in Figure 2. Allyl chain end groups arising from β-methyl transfer increase with monomer concentration and decrease with the temperature.Vinylidene chain end groups, indicating the β-hydrogen elimination and/or chain transfer to monomer, show the opposite trend.The sum of vinylidene plus isobutenyl end groups is, in general, similar to the amount of allyl chain end groups.The relative decrease in allyl groups with temperature indicates an increase in β-hydrogen elimination in agreement with the decrease in molar masses with temperature.However, differences are not significant enough to give an indication of the differences in the polymerization mechanism.

Scheme 2. Unsaturated chain end groups in polypropylene.
1 H-NMR end-group analysis can give information about the chain transfer mechanisms operating with a certain catalyst.The expansions of the 1 H-NMR spectra of polypropylenes obtained at 40 • C and 4 bar by catalyst 1, 2 and 3 are displayed in Figure S5.The signal-to-noise ratio is quite high due to the high molar mass of the polymers.However, only signals of chain end groups formed after 1,2 insertions are visible.
The percentage of vinylidene, isobutenyl, and allyl end groups of PP samples obtained with the three catalysts are listed in Figure 2. Allyl chain end groups arising from βmethyl transfer increase with monomer concentration and decrease with the temperature.Vinylidene chain end groups, indicating the β-hydrogen elimination and/or chain transfer to monomer, show the opposite trend.The sum of vinylidene plus isobutenyl end groups is, in general, similar to the amount of allyl chain end groups.The relative decrease in allyl groups with temperature indicates an increase in β-hydrogen elimination in agreement with the decrease in molar masses with temperature.However, differences are not significant enough to give an indication of the differences in the polymerization mechanism.

Mechanical Properties and Recyclibility
To evaluate the influence of molar masses and microstructure on the mechanical properties, uniaxial tensile and cyclic stress-strain tests were performed as well.The stress−strain curves of the investigated PPs generally first exhibit a linear elastic behavior and, subsequently, a progressive stress decrease with increasing strain, whereas no yield-

Mechanical Properties and Recyclibility
To evaluate the influence of molar masses and microstructure on the mechanical properties, uniaxial tensile and cyclic stress-strain tests were performed as well.The stress−strain curves of the investigated PPs generally first exhibit a linear elastic behavior and, subsequently, a progressive stress decrease with increasing strain, whereas no yielding and strain-hardening were observed.The stress-strain curves in the uniaxial tensile test of polypropylenes from catalyst 1 are reported in Figure 3 as an example.

Mechanical Properties and Recyclibility
To evaluate the influence of molar masses and microstructure on the mechanical properties, uniaxial tensile and cyclic stress-strain tests were performed as well.The stress−strain curves of the investigated PPs generally first exhibit a linear elastic behavior and, subsequently, a progressive stress decrease with increasing strain, whereas no yielding and strain-hardening were observed.The stress-strain curves in the uniaxial tensile test of polypropylenes from catalyst 1 are reported in Figure 3 as an example.In Table 2, the Young's modulus (E) was reported for all the samples and was found to be around 2.0-3.0MPa for samples from catalysts 1 and 2. Slightly higher values have been observed for entries 7 and 8 prepared by catalyst 3. The maximum tensile strength (σ max ) values were in the range of 0.3 to 1.2 MPa (Table 2).As already mentioned for Young's modulus, σ max reached higher values for entries 7 and 8.The break features could not be determined for all samples; indeed, the elongation at break (ε) was higher than could be measured (>2000%) for all the samples prepared at 40 • C and 4 bar, independently from the catalyst, and for entries 1 and 7 prepared at 2 bar.This suggests tough, highly ductile materials.The tensile behavior is similar to that described by O'Hare et al. for fully amorphous ultra-high molar mass atactic PP (M w up to 2.0 MDa and a low amount of 2,1-misinsertions) prepared in the presence of ansa-permethylindenyl-phenoxy titanium complexes immobilized on solid MAO [22].The PP samples prepared at 60 • C and characterized by lower molar masses exhibited ε values spanning from 500 to 1000%.The elastic recovery is the aptitude of an elastomer to restore its original shape and size after the removal of the force and is a crucial factor in assessing its properties.Thus, the samples obtained by catalyst 1 were continuously cycled 10 times at 300% strain, and the elastic recovery was evaluated after the removal of the strain for all cycles (Figure 4 and Table S3).The elastic recovery is the aptitude of an elastomer to restore its original shape and size after the removal of the force and is a crucial factor in assessing its properties.Thus, the samples obtained by catalyst 1 were continuously cycled 10 times at 300% strain, and the elastic recovery was evaluated after the removal of the strain for all cycles (Figure 4 and Table S3).The highest extent of irreversible deformation occurs in the first cycle; after that, a slight enhancement in the unrecovered strain takes places in following cycles (Figure 4a).Entries 1 and 2 exhibit an excellent elastic recovery over the entire cyclic test, that is, 93 and 88% after the first cycle, and 86 and 75% after being cyclically stretched at 300% strain The highest extent of irreversible deformation occurs in the first cycle; after that, a slight enhancement in the unrecovered strain takes places in following cycles (Figure 4a).Entries 1 and 2 exhibit an excellent elastic recovery over the entire cyclic test, that is, 93 and 88% after the first cycle, and 86 and 75% after being cyclically stretched at 300% strain 10 times, respectively (Figure 4b and Table S3).Conversely, entry 3 shows a high extent of irreversible deformation after the first cycle with SR equal to 70% and low elastic recovery (47%) at the end of the last cycle (Figure 4b and Table S3).
These outstanding mechanical properties are justified assuming that, since the samples possess an amorphous nature, the elastic network derives from entanglements caused by the ultra-high molar masses, which restrict permanent deformation by decreasing the mobility of the chains [22,31].In detail, entanglements, defined as physical constraints to macromolecular dynamics in the amorphous regions, influence the mechanical behavior of a polymer system.Thus, ultra-high molecular mass polymers, having a high density of entanglements due to their long chain lengths, present a higher elasticity with respect to lower molecular mass polymers.Indeed, entries 1 and 2, obtained by catalyst 1, and entries 7 and 8, obtained by catalyst 3, possess M w higher than 1000 Kg/mol and exhibit better mechanical performances in terms of elongation at break and strain recovery.
The reprocessability of entry 2, that is, the polypropylene with the best tensile properties, was assessed by repeatedly melting and molding the fractured specimens at the processing conditions of the initial preparation, namely in a press at 180 • C and 50 bar for 5 min, thus obtaining recycled films with a 200 µm thickness.To evaluate the recyclability of entry 2, the reprocessing test was replicated three times, and the reshaped specimens underwent uniaxial tensile and 300% strain cycle tests.The stress-strain diagrams and mechanical properties of the pristine and reprocessed polypropylenes from entry 2 are shown in Figure 5a and Table S4, respectively.The stress-strain curves of the reprocessed specimens nearly overlap with the original one; therefore, even after the third reprocessing cycle, the mechanical properties of entry 2 change marginally.
Polymers 2024, 16, 512 9 of 11 of entry 2, the reprocessing test was replicated three times, and the reshaped specimens underwent uniaxial tensile and 300% strain cycle tests.The stress-strain diagrams and mechanical properties of the pristine and reprocessed polypropylenes from entry 2 are shown in Figure 5a and Table S4, respectively.The stress-strain curves of the reprocessed specimens nearly overlap with the original one; therefore, even after the third reprocessing cycle, the mechanical properties of entry 2 change marginally.Notably, after being recycled three times, entry 2 still showed great elasticity with strain recoveries similar to those of pristine samples (Figure 5b and Table S5).

Conclusions
Propylene polymerization by ketimide-modified half-titanocene catalysts, Cp'TiCl2(N=CtBu2), in the presence of MAO has been studied.These catalysts were demonstrated to be highly active for propylene polymerization, especially at 4 bar and 40 °C.The Cp*-ketimide, C5Me5TiCl2(N=CtBu2) (2), exhibited higher catalytic activities, while Cp'TiCl2(N=CtBu2) (1,3) yielded very high molecular mass polymers, Mw up to 1400 Kg/mol. 13C-NMR analysis of the polymer microstructure showed that all the synthesized polypropylenes are atactic and highly regioregular, with predominant rrrr pentads, especially with catalyst 1. Fully amorphous aPP, with no melting endotherm events and glass transition temperatures between −2 and 2 °C, were observed by DSC.These aPP, thanks to their high molar mass, behave like high-performance thermoplastic elastomers Notably, after being recycled three times, entry 2 still showed great elasticity with strain recoveries similar to those of pristine samples (Figure 5b and Table S5).

Conclusions
Propylene polymerization by ketimide-modified half-titanocene catalysts, Cp'TiCl 2 (N=CtBu 2 ), in the presence of MAO has been studied.These catalysts were demonstrated to be highly active for propylene polymerization, especially at 4 bar and 40 • C. The Cp*-ketimide, C 5 Me 5 TiCl 2 (N=CtBu 2 ) (2), exhibited higher catalytic activities, while Cp'TiCl 2 (N=CtBu 2 ) (1,3) yielded very high molecular mass polymers, M w up to 1400 Kg/mol. 13C-NMR analysis of the polymer microstructure showed that all the synthesized polypropylenes are atactic and highly regioregular, with predominant rrrr pentads, especially with catalyst 1. Fully amorphous aPP, with no melting endotherm events and glass transition temperatures between −2 and 2 • C, were observed by DSC.These aPP, thanks to their high molar mass, behave like high-performance thermoplastic elastomers exhibiting exceptionally high ductility, and a tensile strain at break higher than 2000%.It is worth emphasizing that these materials maintain outstanding mechanical properties, namely high elongation and strain recovery, even after being molded and recycled several times.Thus, for the first time, it was demonstrated that ketimide-modified half-titanocene catalysts selected give a high yield of aUHMWPP, with exceptional tensile and elastic properties, similar to commercially engineered thermoplastic elastomers.
temperature from 40 to 60 • C. Thus, for all the catalysts, the best results are obtained at 40 • C and 4 bar.

Figure 2 .
Figure 2. Chain end group trend as a function of temperature and pressure for catalyst 1 (a), catalyst 2 (b) and catalyst 3 (c).

Figure 2 .
Figure 2. Chain end group trend as a function of temperature and pressure for catalyst 1 (a), catalyst 2 (b) and catalyst 3 (c).

Figure 3 .
Figure 3. Stress-strain curve in the uniaxial tensile test of polypropylenes from catalyst 1.Figure 3. Stress-strain curve in the uniaxial tensile test of polypropylenes from catalyst 1.

Figure 3 .
Figure 3. Stress-strain curve in the uniaxial tensile test of polypropylenes from catalyst 1.Figure 3. Stress-strain curve in the uniaxial tensile test of polypropylenes from catalyst 1.

Figure 4 .
Figure 4. (a) Stress-strain curve in the hysteresis test for ten cycles at 300% strain; (b) strain recovery as a function of cycle number at 300% strain of polypropylenes from catalyst 1.

Figure 4 .
Figure 4. (a) Stress-strain curve in the hysteresis test for ten cycles at 300% strain; (b) strain recovery as a function of cycle number at 300% strain of polypropylenes from catalyst 1.

Figure 5 .
Figure 5. (a) Stress-strain curve in tensile test and (b) strain recovery as a function of cycle number at 300% strain in the hysteresis experiment for the pristine and recycled entry 2 samples.

Figure 5 .
Figure 5. (a) Stress-strain curve in tensile test and (b) strain recovery as a function of cycle number at 300% strain in the hysteresis experiment for the pristine and recycled entry 2 samples.

Table 2 .
Tensile mechanical properties of polymers described in Table1.