Random and Block Copolymers of Isotactic Polypropylene with Higher α-Olefins: The Role of the Chain Topology on the Side-Chains Confined Crystallization

Samples of isotactic propylene-1-octadecene (iPP-co-C18) random copolymers and of diblock copolymers (BCPs) of isotactic polypropylene (iPP) linked to a propylene-1-octadecene block (P-co-C18), iPP-b-P(P-co-C18) with tuned block length and different comonomer content (up to 24 mol %) have been prepared using two different organometallic Hf-based catalysts. Both catalysts ensure good activity, high comonomer incorporation preserving high molecular weight, and a living behavior with the Cs-symmetric pyridylamido-Hf dimethyl complex. The influence of the copolymer topology with random and block architecture on the crystallization behavior and, specifically, the crystallization of side-chains has been analyzed. In random copolymers iPP-co-C18, the main-chains crystallize in the α form and in a mesophase of iPP, whereas at low temperature, the side-chains of the C18 co-units crystallize forming PE-like crystals. The crystallization of the lateral chains is surprisingly observed even in the sample with low C18 concentration and low branches’ frequency, where the randomly placed branches are highly spaced. This indicates that these random copolymers show unexpected nanophase separation between iPP chains and alkyl side-chains, with aggregation of side-chains that facilitates their crystallization even at low branches’ frequency. In the iPP-b-P(P-co-C18) BCPs, the iPP blocks crystallize in the common α form and the side-chains of the C18 co-units in the P(P-co-C18) block crystallize at low temperature, forming polyethylene-like crystals. The confined crystallization at low temperature of side-chains of the C18 co-units is driven by both phase separation of the dissimilar blocks (further driven by iPP crystallization) as well as by phase separation induced by the local incompatibility inside the P(P-co-C18) block between main-chain and C18 side-chains. This cooperative mechanism further facilitates the crystallization of the C18 side-chains compared to random copolymers and provides indirect evidence of microphase separation in these systems. We have demonstrated that branches are intriguingly segregated in nanodomains, favoring crystallization after self-aggregation, and can crystallize in random copolymers with a perfectly uniform and random distribution of C18 co-units even at very low comonomer content, therefore, even for low frequency of branching, when randomly placed branches are highly spaced.