A Living Polymerization of Ylides

Controlled synthesis of linear hydrocarbon polymers has been achieved by the Lewis-acid-catalyzed/initiated polymerization of ylides and ylide-like monomers. The hydrocarbon backbone is extended one carbon at a time. Dimethylsulfoxonium methylide 1 has been the most common monomer used for these polymerizations (Shea et al, 1997). 

Oxidation of star polymethylene 6 affords 3 equiv. of a-hydroxy-co-(4-methoxyphenyl) polymethylene 5 with DP = n. The strict correspondence between stoichiometry and DP in these examples is consistent with a rapid initiation and the absence of termination or chain transfer reactions in the polymerization of ylide 1. The absence of a termination step is reasonable, since the living end of this polymer is a trialkylborane, which is stable under the reaction conditions. Dimethyl sulfoxide (DMSO), a byproduct that accumulates during the reaction does not inhibit the initiator/catalyst. 

The polyhomologation reaction has been used to prepare polymethylene with aMn as high as 354 KDa corresponding to an intermediate tris(polymethylene)borane with Mn 1.0 x 10 (Busch et aL, 2002). A of 1.06 million for the star tris(polymethylene)borane represents approximately 7.6 x 10 turnovers per boron atom. The reaction is complete in 10 min, from which one estimates a lower limit turnover frequency of 6.4 x lO g of polymethylene (mol boron) h at 120 °C. The turnover frequency for the boron catalytic center is comparable to some of the most efficient homogeneous ethylene polymerization catalysts, such as neutral anilinotropone-Ni(II) (8.8 x 10 g of polyethylene (mole catalyst) h ) (80 °C, 200 psi ethylene) (Hicks and Brookhart, 2001). 

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