Unexpected Formation of Atactic Blocks in Propylene/1‐Pentene Copolymers from rac‐Me₂Si(2‐MeBenz[e]Ind)₂ZrCl₂

Abstract

In a previous paper, [1] that has inspired the Note by van Reenen, [2] we reported on a study of propylene/1-pentene copolymerisation in the presence of two isospecific methylaluminoxane (MAO) activated zirconocenes: rac-Et(Ind) 2 -ZrCl 2 (EI) and rac-Me 2 Si(2-MeBenz[e]Ind) 2 ZrCl 2 (MBI).Two series of propylene/1-pentene copolymers with a wide range of compositions were prepared with both catalysts. We observed a surprising result concerning the influence of the ligand structure on the copolymer stereoregularity. In fact, with the moderately isospecific catalyst EI, the isotacticity value of the poly(propylene) homopolymer was maintained for the entire series of copolymers, while with the highly stereospecific catalyst MBI the isotacticity of the copolymers differed from that of the reference homopolymer and gradually decreased at increasing comonomer content and also at increasing the steric hindrance of the comonomer. [1,3] For the sake of clarity, here we present again the methyl region of 13 C NMR spectra of the highly isotactic homopolymer from MBI (Figure 1a, run 11 [1] ) and of two copolymers containing 6.90 and 10.31 mol-% of 1-pentene, respectively (Figure 1b and 1c, run 15-16 [1] ).In Table 1, the distribution of the methyl centered steric pentads of the samples of Figure 1 and of the corresponding Summary: Supplementary experimental findings are reported in addition to those presented in a previous paper dealing with the microstructure of propylene/1-pentene copolymers prepared with rac-Me 2 Si(2-MeBenz[e]Ind) 2 ZrCl 2 . The fractionation results which demonstrate that the atactic part of the copolymer can not be separated from the isotactic one confute the hypothesis suggested by other authors according to which the catalyst used could contain a percent of impurities of the meso (aspecific) form. This evidence is in agreement with the observed narrow polydispersities and the almost random distribution of the comonomers (r P Á r Cm % 1) which are typical of single site catalysts. 13 C NMR methyl region of pentane-soluble (a) and pentaneinsoluble (b) fractions of copolymer prepared with MBI.

Thermal Behavior, Structure and Morphology of Propene/Higher 1-Olefin Copolymers

“…comonomers [21][22][23][24] and it has been reported and widely discussed in previous papers [21,22] for propene/1-pentene copolymers.…”

“…[21][22][23] Their molecular and microstructural features were related to the catalytic system [21,22] and their crystallization and melting behavior was correlated with the chain microstructure. [23] Afterwards, using the same EI and MBI catalysts and at a fixed feed ratio, two series of propene/1-olefin copolymers were synthesized with 1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene and 4-methyl-1-pentene (MP).…”

“…[24] The two isospecific catalysts in propene homopolymerization yielded two families of copolymers with different microstructural features in the presence of higher 1-olefins. [21][22][23][24] With EI, all the copolymers have similar microstructure; with a fixed feed of linear 1-olefins they have nearly the same comonomer content and both isotacticity and stereosequence distribution do not significantly differ from those of the reference polypropene. [24] Furthermore, an increase of 1-olefin content in the copolymers barely affects isotacticity and distribution of stereoerrors.…”

“…The length of the atactic blocks depends on both amount and type of comonomer. [21][22][23][24] The availability of structurally well defined isotactic polypropenes has renewed the interest on the formation of the g-polymorph, which displays unique crystallographic structure [25,26] and is typical of polypropenes with short isotactic sequences. [11,27,28] The recent extensive investigations performed on propene homopolymers with various contents of regio and stereo defects and on random copolymers with 1-olefins by Alamo et al [29][30][31][32][33][34] and De Rosa et al [35][36][37][38][39][40][41][42] have shown that random constitutional defects essentially behave as any other type of irregular unit in the development of the g-polymorph.…”

“…All the details on polymerization conditions as well as on molecular and microstructural characteristics were reported in a previous paper. [24] Some propene/1-pentene copolymer samples previously synthesized and characterized [21][22][23] were also used for DSC and WAXD analyses. The main molecular and microstructural features of all the investigated samples are reported in Table 1.…”