INFLUENCE OF INTRACHAIN HYDROGEN-BONDING ON THE CYCLIZATION OF A POLYSTYRENE CHAIN

Cyclization of a polystyrene chain (M, = 10,600; M(w)/M(n) = 1.09) both ends labeled with 4-(1-pyrenyl) butanoamide groups was studied in cyclohexane between 25 and 95-degrees-C. The amide groups (peptide bonds) at both ends can form an intrachain hydrogen bond between the amide hydrogen at one chain end and the carbonyl oxygen at the other. The presence of two sets of conformers, random coils, and chains cyclized through hydrogen bonding, complicates the data analysis. The pyrene excimer kinetics of this polymer is well described by a model composed of two monomers (hydrogen bonded and nonbonded chains) and one excimer, in equilibrium. The cyclization rate constant for hydrogen-bonded chains is larger than the one for nonhydrogen-bonded chains. The pyrene excimer binding energy (ca. 1.6 kcal/mol) is lower than the published value for nonhydrogen-bonded chains (approximately 9 kcal/mol), suggesting that intrachain hydrogen bonding hinders the stabilization of the excimer. (C) 1994 John Wiley & Sons, Inc.