Random and Block Copolymers of Isotactic Polypropylene\nwith Higher α‑Olefins: The Role of the Chain Topology\non the Side-Chains Confined Crystallization

Samples\nof isotactic propylene-1-octadecene (iPP-<i>co</i>-C18)\nrandom copolymers and of diblock copolymers (BCPs) of isotactic\npolypropylene (iPP) linked to a propylene-1-octadecene block (P-<i>co</i>-C18), iPP-<i>b</i>-P(P-<i>co</i>-C18)\nwith tuned block length and different comonomer content (up to 24\nmol %) have been prepared using two different organometallic Hf-based\ncatalysts. Both catalysts ensure good activity, high comonomer incorporation\npreserving high molecular weight, and a living behavior with the <i>C</i>s-symmetric pyridylamido-Hf dimethyl complex. The influence\nof the copolymer topology with random and block architecture on the\ncrystallization behavior and, specifically, the crystallization of\nside-chains has been analyzed. In random copolymers iPP-<i>co</i>-C18, the main-chains crystallize in the α form and in a mesophase\nof iPP, whereas at low temperature, the side-chains of the C18 co-units\ncrystallize forming PE-like crystals. The crystallization of the lateral\nchains is surprisingly observed even in the sample with low C18 concentration\nand low branches’ frequency, where the randomly placed branches\nare highly spaced. This indicates that these random copolymers show\nunexpected nanophase separation between iPP chains and alkyl side-chains,\nwith aggregation of side-chains that facilitates their crystallization\neven at low branches’ frequency. In the iPP-<i>b</i>-P(P-<i>co</i>-C18) BCPs, the iPP blocks crystallize in\nthe common α form and the side-chains of the C18 co-units in\nthe P(P-<i>co</i>-C18) block crystallize at low temperature,\nforming polyethylene-like crystals. The confined crystallization at\nlow temperature of side-chains of the C18 co-units is driven by both\nphase separation of the dissimilar blocks (further driven by iPP crystallization)\nas well as by phase separation induced by the local incompatibility\ninside the P(P-<i>co</i>-C18) block between main-chain and\nC18 side-chains. This cooperative mechanism further facilitates the\ncrystallization of the C18 side-chains compared to random copolymers\nand provides indirect evidence of microphase separation in these systems.\nWe have demonstrated that branches are intriguingly segregated in\nnanodomains, favoring crystallization after self-aggregation, and\ncan crystallize in random copolymers with a perfectly uniform and\nrandom distribution of C18 co-units even at very low comonomer content,\ntherefore, even for low frequency of branching, when randomly placed\nbranches are highly spaced.