Microstructure of Metallocene‐Catalyzed Propene/1‐Pentene Copolymers

Abstract

The microstructure of propene/1‐pentene copolymers prepared with the metallocene catalysts rac‐Et(Ind)2ZrCl2 and rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 was studied by 13C NMR spectroscopy. Both catalysts lead to the formation of random copolymers although rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 favors a somewhat higher incorporation of 1‐pentene than rac‐Et(Ind)2ZrCl2. Surprisingly, the presence of 1‐pentene has a significant influence on the stereoregularity of the copolymers formed with rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2. Propene/1‐pentene copolymers produced with rac‐Et(Ind)2ZrCl2 retained the isotacticity of polypropene and featured only the stereoerror pentads mmmr, mmrr and mrrm typical for enantiomorphic site control. However, propene/1‐pentene copolymers obtained with rac‐Me2Si(2‐MeBenz‐[e]Ind)2ZrCl2 are characterized by a gradual loss of tacticity which is concurrent with an increase in the amount of all irregular pentads. It is proposed that the presence of 1‐pentene can lead to a reversible loss of stereocontrol over a whole sequence of inserted monomer units. Subsequently, more or less atactic blocks are formed besides isotactic blocks.Percentage of 1‐pentene in the copolymer as function of the feed composition for 1‐pentene/propene copolymers prepared with EI and MBI.magnified imagePercentage of 1‐pentene in the copolymer as function of the feed composition for 1‐pentene/propene copolymers prepared with EI and MBI.

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

“…[24] Furthermore, an increase of 1-olefin content in the copolymers barely affects isotacticity and distribution of stereoerrors. [21] On the contrary, the copolymers obtained with MBI exhibit significant differences among themselves and with respect to the reference propene homopolymer. They show a rather low isotacticity and, in contrast with the highly stereoregular corresponding polypropene, highly isotactic sequences and atactic blocks, which occasionally follow the insertion of a comonomer unit, are present along the chain.…”

“…[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.…”