Polymerization of α‐amino acid <i>N</i>‐carboxy anhydrides. III. Mechanism of polymerization of <scp>L</scp>‐ and <scp>DL</scp>‐alanine NCA in acetonitrile

Abstract

Abstract The polymerization of L ‐ and DL ‐alanine NCA initiated with n ‐butylamine was carried out in acetonitrile which is a nonsolvent for polypeptide. The initiation reaction was completed within 60 min.; there was about 10% of conversion of monomer. The number‐average degree of polymerization of the polymer obtained increased with the reaction period, and it was found to agree with value of W / I , where W is the weight of the monomer consumed by the polymerization and I is the weight of the initiator used. The initiation reaction of the polymerization was concluded as an attack of n ‐butylamine on the C 5 carbonyl carbon of NCA. The initiation, was followed by a propagation reaction, in which there was attack by an amino endgroup of the polymer on the C 5 carbonyl carbon of NCA. The rate of polymerization was observed by measuring the CO 2 evolved, and the activation energy was estimated as follows: 6.66 kcal./mole above 30°C. and 1.83 kcal./mole below 30°C. for L ‐alanine NCA; 15.43 kcal./mole above 30°C., 2.77 kcal./mole below 30°C. for DL ‐alanine NCA. The activation entropy was about −43 cal./mole‐°K. above 30°C. and −59 cal./mole‐°K. below 30°C. for L ‐alanine NCA; it was about −14 cal./mole‐°K. above 30°C. and −56 cal./mole‐°K. below 30°C. for DL ‐alanine NCA. From the polymerization parameters, x‐ray diffraction diagrams, infrared spectra, and solubility in water of the polymer, the poly‐ DL ‐alanine obtained here at a low temperature was assumed to have a block copolymer structure rather than being a random copolymer of D ‐ and L ‐alanine.