First Emulsion Polymerization of Styrene with Sodium Borohydride:Evidence of the Generation of Radical Intermediates by Sodium Borohydride in H_2O

摘要

Emulsion polymerization of styrene with sodium borohydride (NaBH4) in an aqueous sodium dodecyl sulfate (SDS) solution was successfully accomplished for the first time.Polystyrene with a high molecular weight (M_w>2 000 000) and a broad molecular weight distribution (MWD approx=3.5) was obtained in a conversion of less than 30%.Several pieces of evidence that the polymerization proceeded through radical intermediates were observed.Variations in the concentration of NaBH_4 showed a critical range in said concentration,i.e.,a borderline that determined whether the main reaction was directed to either a polymerization or a competed reaction with variations in the NaBH_4 level.Kinetic studies on the emulsion polymerization of styrene with NaBH4 performed at 50,55,and 60 deg C showed that the initiator had an approximately 50-min induction period.A plot of -ln(l-X),where X is the fractional conversion,as a function of time resulted in a linear relationship,showing that the present initiator system followed first-order kinetics with respect to monomer concentration.The Arrhenius plot between In k vs 1/T gave a good linear relation,and the overall activation energy was observed to be about 37.5 kcal/mol.The employment of CH_3I with NaBH_4 significantly increased conversion (>95%) and provided polystyrene with a well-controlled M_w and MWD (<2.3).