Molecular weight control, tetrahydrofuran polymerization

Silyl ketene acetals are relatively stable species and require activation by a catalyst in order to initiate polymerization of a,3-unsaturated monomers. Numerous catalysts for GTP polymerizations have been examined " and these studies have revealed that bifluorides and bioxyanions such as tris(dimethylamino) sulfonium bifluoride (TASHF2) and tetra-n-butyl ammonium bibenzoate (TBABB), respectively, afford optimum polymerization characteristics. Note. Bifluoride based catalysts are not soluble in tetrahydrofuran (THF) and thus acetonitrile is used as the solvent in these cases in general TBABB is the optimum catalyst as it is readily soluble in THF and affords better control of molecular weight, conversion, and polydispersity.) GTP has been shown to be robust < 1.1) and is compatible with... 

Synthetic applications of the cationic polymerization of tetrahydrofuran (THF) are related to two groups of products, namely high-molecular-weight polymer and lower-molecular-weight products with controlled DPn and well defined end-groups which are being used as telechelics. 

This equation assumes each initiator and alcohol molecule to be a potential polymer chain. Alcohol or other protonic substances can thus be used to control polymer molecular weight. The molecular weight limitation due to exchange reactions, as represented by Eq. (10.11), does not, however, apply to polymerizations initiated by alkoxides and hydroxides in aptotic polar solvents, nor does it apply to polymerizations initiated by other initiators such as metal alkyls and aryls and the various coordination initiators, since the latter initiators are dissolved in aprotic solvents such as benzene or tetrahydrofuran (Odian, 1991). 

Control of the molecular weight of poly(vinyl acetate) has also been accomplished in one recent example by an extension of what appears to be a redox polymerization in nonaqueous solutions. In this work, the reducing component is an organochromiiun(II) complex, the oxidizing agent is dibenzoyl peroxide (BPO), and the solvent is tetrahydrofuran (THF). 

Living polymerization of azo monomers is one of the most effective ways to prepare well-defined azo BCs. Generally, a monodispersed macroinitiator should be prepared first. It is then used as an initiator for the subsequent polymerization of azo monomers. Finkelmann and Bohnert (1994) first reported the synthesis of LC-side chain AB azo BCs by direct anionic polymerization of an azo monomer. As shown in Scheme 12.1, the polymerization of polystyrene (PS)-based diblock copolymers was carried out from a PS-lithium capped with 1,1-diphenylethylene (DPE), whereas the poly(methyl methacrylate) (PMMA)-based diblock copolymers were prepared by addition of methyl methacrylate (MMA) monomers to the living azo polyanion, obtained by reaction of l,l-diphenyl-3-methylpentylithium (DPPL) with the azo monomer in tetrahydrofuran (THF) at lower temperature. By this method, a series of well-defined azo BCs were obtained with controlled molecular weights and narrow polydispersities (Lehmann et al., 2000). 

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