The polymerization of propylene oxide by trimethyl aluminum

The polymerization of propylene oxide at 80°C. catalyzed by aluminium trimethyl has been studied kinetically, using a dilatometric method combined with a high-vacuum technique to exclude adventitious water and oxygen. Both in bulk and in methylene chloride solution, the reaction is not stereospecific in the absence of oxygen or moisture. When catalytic quantities of water are present, approximately 5% of the product is crystallizable. The reaction shows first-order dependence on monomer concentration and a complex variation with water and catalyst concentrations, the critical factor being their molar ratio. In bulk, the reaction goes to completion smoothyl, giving maximum molecular weights of approximately 300,000 for the crystalline polymer and 20,000 for the amorphous product. In solution, the reaction involves two distinct stages: in the first, a rapid reaction up to a conversion determined by the water/catalyst ratio is followed by a much slower rate; only the second slow stage appears to produce crystallizable polymer. At critical ratios of the water/catalyst concentrations, the system becomes heterogeneous and the initial rate falls sharply. In the bulk reaction, these two effects occur simultaneously at a ratio [H2O]/[Al2Me6] = 3:1. In solution, the rate falls at a ratio 0.4:1, and the reaction mixture becomes heterogeneous at a 1:1 ratio. Mechanisms are proposed for the various stages of the reaction, and a bridge-bonded hydrolytic complex is suggested as the active catalyst for the stereospecific stage.