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 (PhSiH₃, PhMeSiH₂, Me₂SiH₂, Et₂SiH₂) 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 ¹H/¹³C/²⁹Si NMR and IR spectroscopy as well as by GPC. 'Single-site' Ziegler-Natta catalysts effect this chain transfer with significant efficiency and scope. For [Me₂Si(Me₄C₅)(t)BuN]TiMe⁺B(C₆F₅)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], [PhSiH₃], and [propylene] is inversely proportional to [PhSiH₃]. 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-C₂H₄(ind)₂TiMe⁺B-(C₆F₅)₄^- + propylene and (Me₅C₅)TiMe₂+B(C₆F₅)₄^- + styrene, PhH₂Si-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 [Me₂Si(Me₄C₅)¹BuN]TiMe+B(C₆F₅)₄^- + propylene, linear diblock and starlike atactic polypropylene structures, respectively, are produced. In addition to primary silanes, secondary silanes (PhMeSiH₂, Me₂SiH₂, Et₂SiH₂) are efficient and selective chain-transfer agents in this organotitanium-mediated polymerization. In the presence of [Me₂Si(Me₄C₅)(t)BuN]TiMe+B(C₆F₅)₄^- + propylene, (Me₅C₅)TiMe₂⁺B(C₆F₅)₄^- + propylene, and (Me₅C₅)TiMe₂⁺B(C₆F₅)₄^- + styrene, PhMeHSi- or Me₂HSi-capped atactic polypropylene and PhMeHSi-, Me₂HSi-, or Et₂HSi-capped syndiotactic polystyrene are produced, respectively. Using PhH₂Si-capped polypropylene as a chain-transfer agent, an atactic polypropylene-syndiotactic polystyrene AB block copolymer is produced in the presence of (Me₅C₅)TiMe₂⁺B(C₆F₅)₄^- + styrene.