Studies in polymerization - VII. The polymerization of N -carboxy-α-amino acid anhydrides

Abstract

The kinetics of the polymerization of the N -carbonic anhydrides of DL-leucine, DL-phenylalanine and sarcosine in nitrobenzene solution have been studied. When preformed polymer is used as initiator, and in the complete absence of water, the first two anhydrides polymerize at rates which show first-order dependence on the monomer (anhydride) and initiator con­centrations. The reaction of the sarcosine derivative, however, while first order in monomer, is very nearly second order in initiator, and is strongly catalyzed by carbon dioxide. This latter effect is attributed to the substituted carbamic acids formed by interaction of carbon dioxide and bases in the system. Other acids have been shown to be catalysts for the reaction. The observations are consistent with a reaction scheme (5) in which a complex is formed reversibly from the monomer and a base; this complex may then decompose with evolution of carbon dioxide by routes involving base or acid catalysis. The polymerizations of the DL-leucine and DL-phenylalanine carbonic anhydrides are simple cases of this reaction mechanism in which the formation of the complex is the rate-determining step. This difference explains the lower order of these reactions in the initiator concentration, the absence of catalysis, and also the observation that unlike the formation of polysarcosine these polymerizations have a positive temperature coefficient. When the dimethylamide of an amino-acid is used as initiator the first stage of the reaction is faster than the subsequent stages involving polymers, since the initiator is a stronger base than the polymers. The kinetics for these reactions have been worked out, and shown to agree with experiment. The tertiary bases pyridine and quinoline when quite pure have been shown to be ineffective as initiators of the polymerization of sarcosine carbonic anhydride. Initiation by water and amino-acids is discussed. The molecular weights and molecular weight distribution are not affected by the presence of acid catalysts. The distribution for a reaction in which the first step has a rate different from that of the subsequent ones has been calculated.