Silicon-Modified Ziegler−Natta Polymerization. Catalytic Approaches to Silyl-Capped and Silyl-Linked Polyolefins Using “Single-Site” Cationic Ziegler−Natta Catalysts

This contribution reports efficient and selective organotitanium-mediated silanolytic (PhSiH 3, PhMeSiH 2, Me 2 SiH 2, Et 2 SiH 2 ) chain transfer in the homogeneous polymerization and copolymerization of a variety of α-olefins. The result is several broad classes of silyl-capped and silyl-linked polyolefins which have been characterized by 1 H/ 13 C/ 29 Si NMR and IR spectroscopy as well as by GPC. “Single-site” Ziegler−Natta catalysts effect this chain transfer with significant efficiency and scope. For [Me 2 Si(Me 4 C 5 ) t BuN]TiMe + B(C 6 F 5 ) 4 - -mediated propylene polymerization, the mechanism of chain transfer is supported by the observation that M n of the capped atactic polypropylenes produced at constant [catalyst], [PhSiH 3 ], and [propylene] is inversely proportional to [PhSiH 3 ]. Using the same catalyst, this process also efficiently produces silyl endcapped poly(1-hexene), ethylene + 1-hexene copolymers, ethylene + styrene copolymers (both with high degrees of comonomer incorporation), and (at low temperatures) polyethylene. In the case of rac -C 2 H 4 (Ind) 2 TiMe + B(C 6 F 5 ) 4 - + propylene and (Me 5 C 5 )TiMe 2 + B(C 6 F 5 ) 4 - + styrene, PhH 2 Si-functionalized isotatic polypropylene and syndiotactic polystyrene, respectively, are produced. Using 1,4-disilabenzene and 1,3,5-trisilabenzene as chain-transfer agents in the presence of [Me 2 Si(Me 4 C 5 ) t BuN]TiMe + B(C 6 F 5 ) 4 - + propylene, linear diblock and starlike atactic polypropylene structures, respectively, are produced. In addition to primary silanes, secondary silanes (PhMeSiH 2, Me 2 SiH 2, Et 2 SiH 2 ) are efficient and selective chain-transfer agents in this organotitanium-mediated polymerization. In the presence of [Me 2 Si(Me 4 C 5 ) t BuN]TiMe + B(C 6 F 5 ) 4 - + propylene, (Me 5 C 5 )TiMe 2 + B(C 6 F 5 ) 4 - + propylene, and (Me 5 C 5 )TiMe 2 + B(C 6 F 5 ) 4 - + styrene, PhMeHSi- or Me 2 HSi-capped atactic polypropylene and PhMeHSi-, Me 2 HSi-, or Et 2 HSi-capped syndiotactic polystyrene are produced, respectively. Using PhH 2 Si-capped polypropylene as a chain-transfer agent, an atactic polypropylene−syndiotactic polystyrene AB block copolymer is produced in the presence of (Me 5 C 5 )TiMe 2 + B(C 6 F 5 ) 4 - + styrene.