Synthesis of chain end functional isotactic polypropylene by the combination of metallocene/MAO catalyst and organoborane chain transfer agent

This paper discusses a simple and effective route to prepare chain end functionalized PP and PP diblock copolymers, with high molecular weight and high purity. The reaction scheme involves a versatile "intermediate" of borane-terminated isotactic polypropylene (PP-t-B), which was prepared under a specific reaction condition, involving an iso-specific metallocene catalyst, purified MAO activator (without TMA), 9-borabicyclononane (9-BBN) dimmer chain transfer agent, and ambient reaction temperature. During the propylene polymerization mediated by the rac-Me₂Si[2-Me-4-Ph(Ind)] ₂ZrCl₂/MAO catalyst system, the PP propagating chain end (C-Zr active site) engages in a selective ligand exchange reaction with a B-H group in the 9-BBN dimmer. Evidently, the side reaction between the 9-BBN dimmer and the purified MAO is very slow at <35 °C, so that the normal polymerization-chain transfer reaction mechanism can effectively take place to form the PP-t-B polymer. The polymer molecular weight is inversely proportional to the molar ratio of [9-BBN]/[propylene] with a chain transfer constant (k _tr/k_p) of 1/65. Despite the high molecular weight and heterogeneous reaction conditions, the terminal borane group in PP-t-B is quantitatively interconverted to the OH group under a mild ionic oxidation reaction condition. It is also selectively auto-oxidized by oxygen to form a control radical macroinitiator that affords the chain extension with methymethacrylate monomers to form PP-b-PMMA diblock copolymers.