Mmmm Pentads Research Articles

Tacticity Changes during Controlled Degradation of Polypropylene

The controlled reactive degradation of polypropylene (CPP) in a twin-screw extruder has been investigated from the perspective of tacticity. CPP is initiated by the decomposition of a peroxide producing radicals that abstract hydrogen from the chain backbone creating tertiary radicals. Since C atoms with tertiary radicals have no longer sp3 configurations, they temporarily lose their stereospecificity. The C atoms resume stereospecificity upon termination, whereby methyl groups may assume a different orientation than before, causing a tacticity change. Degradation experiments in a twin-screw extruder of isotactic PP samples have been performed and tacticity changes are observed by C-13 NMR measurements revealing a decrease of isotactic mmmm pentads and increases of pentads with racemic sequence pairs. Statistical analysis of NMR data shows that the change of tacticity is nonrandom, i.e., units on the chain backbone are more likely to change orientation if they are close together. We attribute the nonrandomness to isomerization reactions, where through cyclic intermediates, a radical from one tertiary C carbon is transferred to another—radical chain walking. To validate this chain-walking hypothesis, a kinetic Monte Carlo simulation algorithm has been developed that predicts the change of the pentad distribution from the reactions taking place: initiation of tertiary radicals, transfer to polymer, chain walking, and termination by disproportionation. PP chains simulated consist of around 109 units with orientations of methyl groups inferred from the pentad distribution measured by NMR. Simulated pentad changes are compared to those measured from NMR, which reveal a relatively high racemic content. kMC simulations show that this can only be explained by assuming chain walking. Also, this isomerization via ring-shaped intermediates influences the patterns of changes in pentad distribution. These findings demonstrate that molecular weight changes are accompanied by tacticity changes. The kMC models confirmed by NMR data show that these tacticity changes are clustered locally along PP chains and that tacticity changes happen within the length scale of individual pentads. Chain-walking isomerization as a main cause of the tacticity changes is supported by predictions from the kMC model.

Connection of Stereoselectivity, Regioselectivity, and Molecular Weight Capability in rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 Type Catalysts

A set of 19 silicon-bridged C2-symmetric zirconocenes rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 of varying steric demand in position 4 were synthesized and screened in propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability (R2 ≈ 0.8–0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure–activity relationship based on one single computational descriptor, Δ%VBur—using the difference in the percent of buried volume between the “blocked” and “open” quadrants of the catalyst precursor—is established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG‡enantio), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (Mw > 1 MDa) and relatively high regioselectivity.

Regulation of Molecular Weight Characteristics and Microtacticity of Polyhexene Produced over Highly Active Supported Titanium–Magnesium Catalysts

AbstractThe data on the effect of polymerization temperature of 1‐hexene within the 30–70 °C range in the presence of a highly active supported titanium–magnesium catalyst on molecular weight characteristics and microtacticity of polyhexene, with cocatalyst composition being additionally varied (AlEt3 or Al(i‐Bu)3), in the absence and presence of an external stereoregulating electron‐donating compound and hydrogen, are reported. Polymerization conditions, making it possible to specifically regulate molecular weight and molecular weight distribution of polyhexene over a broad range ((Mw = 7 × 104–2.2 × 106 g mol−1; Mw/Mn = 3.7–33) and regulate isotacticity of polyhexene (content of mmmm pentads from 56% to 96%), while retaining high catalyst activity, are determined.

Effect of Varying Hydrogen Concentration, External Donor Concentration, and Temperature on Propylene Polymerization Kinetics and Microstructure of Polypropylene Made with a 4th Generation Ziegler-Natta Catalyst

This paper presents the results of a detailed investigation on propylene polymerization kinetics and microstructural characterization of polypropylene samples made with a 4th generation Ziegler–Natta catalyst under varying hydrogen pressures, external donor concentrations, and polymerization temperatures. Hydrogen had a positive effect on the mmmm pentad fraction at low donor concentrations, in agreement with our previous Monte Carlo model. Estimated activation energies for activation, propagation, and deactivation were very close to the values previously reported by our group for a narrower set of experiments with the same catalyst.

Synthesis and microstructural study of stereoblock elastomeric polypropylenes from metallocene catalyst (2‐PhInd)2ZrCl2 activated with cocatalyst mixtures

AbstractVarious elastomeric polypropylenes (PPs) are synthesized through homogeneous propylene polymerization with metallocene catalyst (2‐PhInd)2ZrCl2 in the presence of different cocatalyst mixtures: triethylaluminum (TEA)/methylaluminoxane (MAO) or triisobutylaluminum (TIBA)/MAO in the range of AlAlR3/AlMAO = 0.0–0.9. The cocatalyst formulation impacts the resultant polymer microstructure and the thermal and dynamic mechanical properties of the produced PPs. 13C NMR analysis of the polymers reveals essentially atactic PP, with mmmm = 7.9%, when AlAlR3/AlMAO = 0.0. The mmmm pentad content is maximized when AlAlR3/AlMAO = 0.8; for TIBA, mmmm = 23.5%; and for TEA, mmmm = 17.6%. Differential scanning calorimetry analysis and dynamic mechanical thermal analysis corroborate these findings. Specifically, Tm, ΔHm, and Tg are essentially maximized under these conditions, and the minimum damping is observed for AlAlR3/AlMAO = 0.6–0.8. 1H NMR analysis of the mixtures of catalyst and cocatalysts (without monomer) shows very minor differences for [Zr]:AlAlR3 in the range of 1:1–1:5. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Synthesis of Stereoblock Elastomeric Poly(propylene)s Using a (2‐PhInd)2ZrCl2 Metallocene Catalyst in the Presence of Co‐Catalyst Mixtures: Study of Activity and Molecular Weight

AbstractThe effect of adding various aluminum alkyls (R = Et, i‐Bu) on the polymerization of propylene is studied using a (2‐PhInd)2ZrCl2 pre‐catalyst. A mild deactivating effect is found upon addition of TIBA, whereas TEA shows a sharp deactivating effect. Increasing amounts of AlR3 results in a significant activity increase for TIBA, but an activity plateau for TEA. AlR3 imposes remarkably different effects on the molecular weight and stereochemical microstructure of polymers. As the TIBA concentration increases, $\overline {M} _{{\rm v}} $ increases at first, growing from 49 000 to 72 000, but subsequently drops to 40 000. For TEA, $\overline {M} _{{\rm v}} $ decreases sharply, plummeting from 49 000 to 17 000. Both TIBA and TEA increase the mmmm pentad content from 7.9 to 23.5% and 17.6%, respectively. magnified image

Synthesis of long-chain branched isotactic-rich polystyrene via cationic polymerization

Cationic polymerization of styrene was conducted with 1-chloro-1-phenylethane (SCl)/AlCl3/phenyl methyl ether (PME) initiating system in hexane/CH2Cl2 (60/40, v/v) at −80 °C. The kinetics for cationic polymerization of styrene was investigated by in-situ ATR-FTIR spectroscopy. The isotactic-rich polystyrene (iPS) with m dyad of 81%, mm triad of 63% and mmmm pentad of 50% could be synthesized. Small amounts of crystalline regions in iPS formed after flow-induced crystallization and the crystallinity increased with increasing the molecular weight of iPS. Furthermore, the long-chain branched isotactic-rich polystyrene (biPS) with around 12 times higher molecular weight than that of corresponding iPS could be synthesized via cationic polymerization of styrene by introducing a small amount of isoprene (Ip) as a comonomer and branching sites as well. The possible mechanism for long-chain branching formation via intermolecular alkylation reaction by using Ip structural units along polymer chain as branching sites was proposed. The nucleation rate of biPS could be greatly enhanced with increasing the content of branching sites, leading to an obvious increase in crystallinity. The multi-melting temperatures from 140 °C to 237 °C were observed in DSC curves of these PS products. The tensile strength of commercial atactic polystyrene could be improved remarkably from 41.4 MPa to 55.7 MPa by adding 16.7% of biPS.

STEREOSPECIFIC ANIONIC POLYMERIZATION OF STYRENE IN THE PRESENCE OF BARIUM DODECYL BENZENE SULFONATE

The anionic polymerization of styrene has been studied using dodecyl benzene sulfonic acid barium salt(SDBB) /tetrahydrofuran as ligands,n-BuLi as initiator in cyclohexane.The results showed the remarkable effect of SDBB on polymerization.With increasing [SDBB]/[Li],the polymerization rate and monomer conversion decreased,and the gel formed gradually.The polymerization was completely retarded at [SDBB]/[Li]≥ 0.8 at 60℃.The stereospecific polystyrene was obtained at 60℃,and its weight content of methyl ethyl ketone(MEK) insoluble fraction reached about 80%.The 13C-NMR spectra indicated that its microstructure sequence was isotactic-rich polystyrene.The contents of mm triad and mmmm pentad were equal to 57% and 45%,respectively.The Tg of 100℃ and melting temperature at 211℃ were observed in DSC curves.The isotactic content of MEK soluble fraction was also higher than that of PS obtained by conventional anionic polymerization.The GPC results displayed an obvious difference between the theoretical molecular weight and experimental molecular weight.It also indicated a much broader polydispersity of the experimental sample.

Structure and isothermal crystallization behavior of polypropylene prepared at high polymerization temperature

AbstractThe structure, morphology, and isothermal crystallization behaviors of polypropylene (PP) prepared with heterogeneous Ziegler‐Natta catalyst at high temperature (100°C) were investigated with differential scanning calorimetry, wide‐angle X‐ray diffraction, temperature‐rising elution fractionation, gel permeation chromatography, and 13C NMR. The results reveal that the crystalline structure changes with variation of the composition of the PP. The isotactic PP (iPP)1 prepared with Et3Al and “TMA‐depleted” methylaluminoxane crystallizes from the melt in the mixtures of the α and β forms, whereas each fraction obtained from pure PP1 does not show β‐PP crystal at the same crystallization condition. In addition, the γ‐PP crystal is appeared for the fractions of low mmmm%‐[mmmm] (mmmm pentad content) values and molecular weight. Moreover, it was found that the iPP2 or iPP3 prepared with Hex3Al crystallizes from the melt in mixtures of the α and γ forms, even at atmospheric pressure and for high molecular weight. The microstructure showed in the PP samples obtained at high temperature could be well explained with the shift in the alkylaluminium‐donor equilibrium reactions at high polymerization temperature. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Synthesis of Bulky Zirconocene Dichloride Compounds and Their Applications in Olefin Polymerization

The bulky substituted cyclopentadienyllithium derivatives, LiC5H4(CHMeR) (R = C6H5 (1), 1-naphthyl (2), 9-anthryl (3)), were synthesized from the reaction of 6-phenylfulvene, 6-(1-naphthyl)fulvene or 6-(9-anthryl)fulvene with LiMe. The ansa-bis(cyclopentadiene) ligands Me2Si(C5HMe4){C5H4(CHMeR)} (R = C6H5 (4), 1-naphthyl (5), 9-anthryl (6)), and their lithium derivatives Li2(Me2Si(C5Me4){C5H3(CHMeR)}) (R = C6H5 (7), 1-naphthyl (8), 9-anthryl (9)) have been prepared. The zirconocene complexes, [Zr(η5-C5H5){η5-C5H4- (CHMeR)}Cl2] (R = C6H5 (10), 1-naphthyl (11), 9-anthryl (12)) and [Zr{η5-C5H4(CHMeR)}2Cl2] (R = C6H5 (13), 1-naphthyl (14), 9-anthryl (15)), were synthesized by the reaction of lithium derivatives 1-3 and [Zr(η5-C5H5)Cl3] or ZrCl4, respectively. The reaction of the lithium ansa-derivatives 7-9 and zirconium tetrachloride yielded the ansa-zirconocene complexes, [Zr(Me2Si(η5-C5Me4){η5-C5H3(CHMeR)})Cl2] (R = C6H5 (16), 1-naphthyl (17), 9-anthryl (18)). The zirconocene complexes have been tested in the polymerization of ethene and propene. The polymerization of propene with the ansa-zirconocene catalysts 16-18 gave polypropylene with 70% mmmm pentads and the symmetric zirconocene catalysts 13-15 30-60% mmmm pentads.

C2- and Cs-symmetric diamidodichlorotitanium complexes: Syntheses, structures and 1-hexene polymerization catalysis

The complexes l-[CMe 2{CHMeN(2-Pr i -C 6H 4)} 2TiCl 2] ( 2) and u - [ CMe 2 { CHMeN ( 2 , 6 - Pr 2 i C 6 H 3 ) } 2 TiCl 2 ] ( 3), C 2- and C s -symmetric analogues, respectively, of McConville’s C 2 v hexene polymerization precatalyst [ ( CH 2 ) 3 { N ( 2 , 6 - Pr 2 i – C 6 H 3 ) } 2 TiCl 2 ] ( 1), were prepared by high-dilution salt-elimination from the lithium amides and characterized spectroscopically and crystallographically. Complex 2, though less active than 1, was a highly active catalyst of polymerization of 1-hexene when activated by MAO. Complex 3 was inactive under similar conditions. NMR analysis confirmed that there were more mmmm pentads in polymer produced by 2 than in the statistically atactic material produced by 1, though the isotacticity index was not high. The results are interpreted in terms of an isotactic/atactic block structure, caused by syn/ anti fluxion of the two 2-isopropylphenyl rings in 2. Kinetic profiles and polydispersities were consistent with a slow initiation step involving monomer, followed by rapid propagation, with some chain transfer to aluminium and only a small extent of β-hydride elimination. The rubber-like polymers were indistinguishable by thermal analysis from those prepared by Ziegler–Natta catalyst systems.

C1‐Symmetric Heterocyclic Zirconocenes as Catalysts for Propylene Polymerization, 1

AbstractSummary: The synthesis of C1‐symmetric zirconocene complexes bearing the 2,5‐dimethyl‐7H‐cyclopenta[1,2‐b:4,3‐b′]dithiophene ligand (S2‐3) linked to substituted cyclopentadienes is described. Different syntheses of S2‐3, the common intermediate for the preparation of these complexes, are discussed. Many of these complexes have been found to be highly active in propylene polymerization, to require very low amounts of methylalumoxane to be activated, and to produce poly(propylene)s of low isotacticity and melting points. 13C NMR analysis shows that the poly(propylene)s are fully regioregular and that the stereoerrors are randomly distributed, as shown by the enantiomorphic‐site triad test E ≈ 1. The experimental pentad distribution was fitted using a two‐site model with different probability parameters for each site. The probability of chain back‐skip was also taken into account. The molecular weight and crystallinity of the poly(propylene)s are dependent upon the type of substituents on the cyclopentadienyl ring, and the correlation between mmmm content and melting point of the PP confirms the random distribution of stereoerrors.Correlation between % mmmm pentad and melting point.magnified imageCorrelation between % mmmm pentad and melting point.

Synthesis of vinyl-terminated isotactic poly(propylene)

Isotactic poly(propylene)s with 60–80% vinyl chain-end selectivity were synthesized with metallocene catalysts. Some of these vinyl-terminated poly(propylene)s are highly stereoregular (mmmm pentads up to 95%) and have high crystalline melting points in the range of 140–150°C. Chain-end analysis using 13C NMR indicates the vinyl chain-ends in the polymer are most likely formed through β-methyl elimination in the chain termination step.

Propylene polymerization with dimethylsilylbis(3-methylcyclo-pentadienyl)MCl2 [M = Ti, Zr, Hf] in combination with methylaluminoxane

Propylene polymerizations with rac-dimethylsilylbis(3-methylcyclopentadienyl)MCl (M = Ti, Zr, Hf) in combination with methylaluminoxane (MAO) were carried out to study the effect of the metal species on catalytic activity, stereoregularity, regioregularity, and molecular weight of poly(propylene). The rac-Me2Si(3-MeCp) 2 TiCl 2 /MA catalyst showed lowest activity among the three catalysts. The stereospecificity of the titanium based catalyst is very high and the portion of the mmmm pentads is roughly as high as for the zirconium and hafnium analogue. On the other hand, the regiospecificity of the titanium based catalyst is low and only the 1,3-regioirregular structure was observed, whereas the 2,1-regioirregular structure was detected in poly(propylene)s obtained with the zirconium and hafnium analogue. As a result, the melting point of poly(propylene) obtained with the titanium based catalyst is lower than that of poly(propylene) obtained with the zirconium or hafnium analogue. Addition of hydrogen with the titanium based catalyst gave rise to an increase in activity; but this accelerating effect was not observed in ethylene polymerization.

Propylene polymerization with dimethylsilylbis(3-methylcyclo-pentadienyl)MCl2 [M = Ti, Zr, Hf] in combination with methylaluminoxane

Propylene polymerizations with rac-dimethylsilylbis(3-methylcyclopentadienyl)MCl2 (M = Ti, Zr, Hf) in combination with methylaluminoxane (MAO) were carried out to study the effect of the metal species on catalytic activity, stereoregularity, regioregularity, and molecular weight of poly(propylene). The rac-Me2Si(3-MeCp)2TiCl2/MAO catalyst showed lowest activity among the three catalysts. The stereospecificity of the titanium based catalyst is very high and the portion of the mmmm pentads is roughly as high as for the zirconium and hafnium analogue. On the other hand, the regiospecificity of the titanium based catalyst is low and only the 1,3-regioirregular structure was observed, whereas the 2,1-regioirregular structure was detected in poly(propylene)s obtained with the zirconium and hafnium analogue. As a result, the melting point of poly(propylene) obtained with the titanium based catalyst is lower than that of poly(propylene) obtained with the zirconium or hafnium analogue. Addition of hydrogen with the titanium based catalyst gave rise to an increase in activity; but this accelerating effect was not observed in ethylene polymerization.

Phosphorus-bridged metallocenes: New homogeneous catalysts for the polymerization of propene

The synthesis of a new class of metallocenes for the syndiospecific, aspecific and isospecific polymerization of propene is reported. This has been achieved by the incorporation of a phosphorus linking the cyclopentadienyl-type rings. The catalyst precursors, syndiospecific PhP(fluorenyl–Cp)ZrCl 2 ( 2), aspecific PhP(fluorenyl) 2ZrCl 2 ( 4), and isospecific PhP(indenyl) 2ZrCl 2 ( 5), RP(2-Me,4-Ph-indenyl) 2ZrCl 2 (R=Ph ( 6); R= i Pr ( 7)) were prepared. Compound 2, after activation by methylaluminoxane (MAO), in LIPP at 67°C affords syndiotactic polypropene (s-PP) with an activity of 155 kg s-PP/g Zr·h. The physical properties of the s-PP (stereoregularity and molecular weight) are similar to that of conventional carbon-bridged systems. Ab initio calculations on model compounds assisted in rationalizing the high syndiospecificity of 2 in contrast to the much poorer stereoregularity of closely related Me 2Si(fluorenyl–Cp)ZrCl 2. Aspecific metallocene 4, after activation with MAO, affords high molecular weight atactic-PP, albeit with a low activity. Metallocenes 6 and 7, activated by MAO, afford isotactic polypropene (i-PP) with extremely high stereoregularity (>98% mmmm pentads), melting points 156–160°C and molecular weights tunable in the range 250,000–1,100,000. Activities of up to 580 kg i-PP/g Zr·h for 6/MAO (LIPP, 67°C, 37 000 equiv. MAO) and 1265 kg i-PP/g Zr·h for 7/MAO (LIPP, 50°C, 37 000 equiv. MAO) have been obtained.

Kinetic investigation of the active sites precursors on MgCl2-supported Ziegler catalyst through interaction with a Lewis base

The initial polymerization stage of propene with a MgC1 2 -supported Ziegler catalyst after the pre-treatment with a Lewis base was examined as a study of the stereospecificity of the active sites precursors. A drastic depressing effect on the catalyst activity occurs, accompanied by an increase in stereospecificity, by the direct interaction of cyclohexylmethyldimethoxysilane (CMDMS) with the catalyst before the polymerization. The kinetic study of the polymerization indicated that the active sites concentration ([C * ]) decreases drastically by the catalyst pre-treatment with CMDMS. On the other hand, the increase of propagation rate constant (k p ) is relatively small and corresponds to the moderate increase of the mmmm pentad fraction. This is an indication that the interaction of the Lewis base with the active sites precursors prevents formation of the active sites and that inhibition occurs preferentially at the aspecific precursor.

Zr−Alkyl Isomerization in ansa-Zirconocene-Catalyzed Olefin Polymerizations. Contributions to Stereoerror Formation and Chain Termination1

In polymers made from (E)- or (Z)-[1-D]propene with methyl alumoxane (MAO)-activated ansa-zirconocene catalysts, signals of deuterium-labeled mrrm pentads document that stereoerrors arise mainly from an isomerization of the Zr-bound chain end. Reduced D-atom redistribution in poly([2-D]propene) indicates a kinetic isotope effect of kH/kD ≈ 3 for the β-H/D transfer associated with the isomerization reaction. In all poly([1- and [2-D]propenes) studied, D-labeled mmmm pentads occur with a probability similar to that of D-labeled mrrm pentads; this observation requires further mechanistic clarification. Olefinic chain ends of polymers obtained with C2H4(thind)2ZrCl2-MAO from (E)- or (Z)-[1-D]propene deviate in their D-label distributions from the expected stereochemistry. Isomerization of the Zr-bound chain end, probably via a Zr-bound tertiary alkyl intermediate, thus contributes also to chain-growth termination. With the sterically hindered catalyst Me2Si(2-Me-4-tBu-C5H2)2ZrCl2-MAO, almost all chain terminations appear to occur via chain-end isomerization, which does not lead to stereoerrors here. With the high-performance catalyst Me2Si(2-Me-benz[e]indenyl)2ZrCl2-MAO, finally, no D-label is found in the mrrm and mmmm pentad signals; here, Zr-alkyl isomerization can apparently not compete with the high rate of olefin insertion.

Effect of phenyltriethoxysilane as external donor on the microstructure of ethylene-propylene copolymers

The effect of an external electron donor, namely, phenyltriethoxysilane (PES) on the compositional heterogeneity and the microstructure of ethylene-propylene copolymers prepared using a titanium-magnesium catalyst has been investigated. Addition of PES causes an increase in the average sequence length of ethylene and propylene. The products of reactivity ratios of ethylene and propylene indicate that the copolymer is blocky in nature. The external donor produces a propylene sequence in the copolymer which is richer in mmmm pentads.

New telechelic polymers and sequential copolymers by polyfunctional initiator‐transfer agents (inifers) LVII. Chlorine telechelic poly(p‐chlorostyrene)

AbstractThe polymerization of p‐chlorostyrene (pClSt) under carbocationic conditions has been investigated. The use of binifers 1,4‐di(2‐chloro‐2‐propyl) benzene (pDCC) and 1,3‐di(2‐chloro‐2‐propyl)‐5‐tert‐butylbenzene (m‐tBuDCC) in conjunction with BCl3 coinitiator in CH3Cl diluent at −60 and −35°C give α,ω‐di‐benzylic chlorine‐capped polymer sClPpClStCls. According to kinetic investigations the inifer mechanism is operative and both binifers are efficient in controlling the end structures, molecular weights, and to a certain extent molecular weight distributions (MWD). Strong circumstantial evidences have been generated to demonstrate the satisfactory synthesis of sClPpClStCls and the absence of indanyl end‐groups, i.e., structural considerations, linearity of inifer plots, quantitative Cl end‐group microanalysis, FTIR spectroscopy, and blocking experiments. The latter involved the blocking of THF from the sec‐benzylic chlorine end‐groups by AgPF6 and thus led to the synthesis of a new triblock copolymer PTHF‐b‐PpClSt‐b‐PTHF. Interestingly, the isotactic content (mm triads and mmmm pentads) of PpClSt is significantly higher than that of polymer prepared by free‐radical induced polymerization. The Tg of a PpClSt of M̄n = 5700 g/mol was 123°C.