“Living” radical polymerizations of vinyl monomers initiated by aged “Cr2++ BPO” in homogeneous solution

Abstract

Monomer is added to chromium(II) acetate (Cr²⁺) and benzoyl peroxide (BPO) previously treated at 10°C in dimethylformamide (DMF)(the ageing process), and polymerizations are performed. It is found that the degree of polymerization of poly(methyl methacrylate)(PMMA) obtained at 30°C or below increases with an increase in monomer conversion and the single peak of a molecular weight distribution curve shifts with monomer conversion. These results suggest that a “living” polymerization occurs in the homogeneous polymerization of MMA initiated by the aged “Cr²⁺+ BPO” system.

The polymerizations of various vinyl monomers with this initiator system are carried out at 25°C. The polymerization activity is relatively high for MMA, acrylonitrile, methyl acrylate and acrylamide but low for styrene, vinyl acetate and vinyl chloride. The molecular weights of polyacrylonitrile and poly(methyl acrylate) also increase with the monomer conversion.

By copolymerizing MMA with acrylonitrile, it is found that polymerizations with aged “Cr²⁺+ BPO” proceed via a free radical intermediate. The polymerization rates (R_p) of MMA at 25 and 30°C are expressed by the following relation, R_p= const.[aged “Cr²⁺+BPO”]^0.7–1.0[MMA].

The mechanism of a “living” radical polymerization is discussed in terms of a transition metal complex with free radicals.