The dynamic mechanical thermal properties of a family of poly(alpha-alkyl beta,L-aspartate)s bearing various cyclic, linear, and branched alkoxycarbonyl groups in the side chain were studied. The measurements carried out by dynamic mechanical thermal analysis (DMTA) revealed the significant influence of the constitution of the side chain on mechanical relaxation phenomena. Three relaxations were observed, which are referred to as gamma, beta, and alpha, in increasing order of temperature. The first two, gamma and beta, are related to the local and global motions of the side chain, respectively. Relaxation alpha is related to the motion of the main chain. Relaxation beta, which is associated with the rotation of the side chain, is the most intense. The magnitude and temperature at which this relaxation occurs depends on the volume, the length, and the degree of branching of the ester group of the side chain. A comparison between the dynamic mechanical properties Of poly(beta,L-aspartate)s and poly(alpha,L-glutamate)s revealed that the two methylene groups spacing the ester group from the main chain provides the poly(alpha-L-glutamate)s with greater mobility, and thus, relaxations a and beta occur at lower temperatures. (c) 2005 Wiley Periodicals, Inc.