Living and syndioselective polymerization of methacrylates by constrained geometry titanium alkyl and enolate complexes

This paper reports the homopolymerization and block copolymerization of methacrylates by the cationic titanium methyl complex, CGCTiMe(+)MeB(C6F5)(3)(-) (1, CGC = Me2Si(Me4C5)(t-BuN)), as well as the synthesis and methyl methacrylate (MMA) polymerization of three new neutral and cationic CGC Ti ester enolate complexes. Unlike the isostructural, cationic CGC Zr methyl complex, which is inactive for polymerization of MMA, CGC Ti methyl complex 1 effects living polymerizations of MMA and BMA (BMA = n-butyl methacrylate) at ambient temperature, producing syndiotactic PMMA ([rr] = 82%, P-r = 0.90) and PBMA ([rr] = 89%, P-r = 0.95). Sequential copolymerization using 1 and starting from either MMA or BMA produces the well-defined, syndiotactic diblock copolymer PMMA-b-PBMA with narrow molecular weight distribution (M-w/M-n, = 1.08). On the other hand, copolymerizing MMA and BMA simultaneously affords the homogeneous random copolymer PMMA-co-PBMA. Two neutral CGC Ti ester enolate complexes, CGCTiCI[OC((OPr)-Pr-i)=CMe2] (2) and CGCTiMe[OC((OPr)-Pr-i)=CMe2] (4), have been synthesized, and the molecular structure of 4 has been determined by X-ray crystallography. The reaction of 4 and B(C6F5)(3).THF (THF = tetrahydrofuran) in methylene chloride readily generates the cationic CGC Ti ester enolate complex, CGCTi(+)(THF)[OC((OPr)-Pr-i)=CMe2] [MeB(C6F5)(3)]- (5). All three CGC Ti ester enolate complexes have been investigated for MMA polymerization; of particular interest are the polymer characteristics (molecular weight, molecular weight distribution, and syndiotacticity) of the PMMA formed by 5, which are remarkably similar to those of the PMMA by 1, suggesting that the cationic ester enolate 5 is an appropriate model for the propagating species involved in the MMA polymerization by the alkyl cation 1. The microstructures of PMMA have been analyzed at the pentad level, and the polymerization mechanism has also been discussed.