Preparation of Well-Controlled Isotactic Polypropylene-Based Block Copolymers with Superior Physical Performance via Efficient Coordinative Chain Transfer Polymerization

The synthesis of isotactic polypropylene (iPP)-based block copolymers by means of controlled isoselective polymerization of propylene with high turnover frequencies is an ongoing challenge in industry and academia. Herein, we report a new strategy for reversible coordinative chain transfer polymerization of propylene and propylene–ethylene with a pyridylamido hafnium/[Ph3C][B(C6F5)4] catalyst system and iBu3Al as a chain transfer agent under conditions that were optimized so that the rate constants for chain transfer and chain propagation were similar (kp ≈ kct). Using this new strategy, we could obtain iPP-b-poly(ethylene-co-propylene) and iPP-b-poly(ethylene-co-propylene)-b-iPP block copolymers with controlled molecular weights, hard/soft block ratios, and compositions via a continuous-monomer-feeding approach like that used in living polymerization systems. The polymerization reactions produced uniform block copolymers with previously inaccessible microstructures. The copolymers have both a semicrystalline iPP block ([mmmm] > 99%, melting temperature > 155 °C) and soft ethylene–propylene copolymer blocks (glass transition temperature < −30 °C), with tunable high molecular weights (∼210 kDa) and Schulz–Flory distributions (PDI < 2.5). These promising block copolymers possess dramatically improved tensile properties (30–50-fold increase in elongation at break) relative to those of the iPP homopolymer, whereas the yield strength and strength at break were similar to those of the iPP homopolymer. By tuning the block composition and hard/soft block ratio, we could obtain block copolymers ranging from ductile elastomers to tough plastics. Our new strategy not only sheds significant light on olefin CCTP but also has great potential utility for the industrial production of high-performance polyolefin materials.