화학공학소재연구정보센터
Macromolecules, Vol.41, No.22, 8452-8457, 2008
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)](2)ZrCl2/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 degrees 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.