Organozinc monomers, polymerization

PolymerizatiOTi of synunetrical dibromo monomer 40, consisting of thiophene, naphthalenediimide, and thiophene, also proceeds in chain-growth polymerizatiOTi manner (Scheme 48) [136]. The first attempt to form a Grignard mmiomer from 40 for Kumada-Tamao coupling polymerization failed. Activated Zn was next reacted with 40 for generation of an organozinc monomer. Remarkably, however, acidic... 

The propylene oxide complex not only dissociated into its components but also transformed to either an oligomer or a polymer of propylene oxide when it was allowed to stand in solution. This transformation could be followed by H-NMR techniques with the use of a-deuterated propylene oxide instead of the non-deuterated one. Its rate depended on the nature of solvent and on the temperature. This experimental result implies that the monomer liberated by dessociation of the complex is polymerized by the catalyst, that only a minute fraction of the organozinc component of the complex actually acts as a catalyst for polymerization, and that the rate of propagation is far faster than that of initiation. These implications together with the evidence that coordination of the monomer to the catalyst is a prerequisite for the stereospecific polymerization led us to the detailed studies of the bulk polymerization, that is, the polymerization of propylene oxide in propylene oxide solution. 

Organozinc compounds have been widely used as components of initiators for polymerizations of various monomers. Although a tremendous number of the research works on the subject are available in the literature, this chapter describes only the main features of the use of organozinc compounds as initiators for polymerizations. All patents were omitted in this review. 

Cationic organozinc compounds are expected to be good catalysts for ring opening polymerization reactions of epoxides and lactones because the enhanced Lewis acidity (see Lewis Acids Bases) of the zinc center favors its coordination to the monomer. For example, Walker and coworkers have found that the cationic zinc substituted cyclopentadienyl complex [3,5-Me2C6H3CH2CMe2C5H4Zn(TMEDA)]+ [EtB(C6F5)3] is an active initiator species for the polymerization of cyclohexene oxide and e-caprolactone. ... 

Explain the mechanism postulated by Tsumta of steric control in polymerizations of oxiranes with the aid of organozinc compounds, giving the structure of the catalyst and the mode of monomer insertion and the mode or ring opening. 

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