Dynamic mechanical spectra (—180 > 140° ˜1 cps) of amorphous stereoregular poly(methyl methacrylate)s and poly(t-butyl methacrylate)s with assigned microtacticities are presented and discussed. An intermolecular argument (“interlocking”) is invoked to account for the higher Tg of syndiotactic vis a vis iso tactic PMMA, in spite of the higher density of the latter at 30° C. An argument is presented to show that the T/Tg ratio is not only a measure of the degree of coupling of the T and Tgprocesses, but also of the degree to which intermolecular factors influence these processes. The greater extent of the low-temperature irreversibilities observed in the thermomechanical spectra of poly(t-butyl methacrylate)s is attributed to the brittle character induced by the bulky side groups which presumably weaken cohesive forces. Furthermore, internal stress generation and brittleness of the polymers are shown to be connected to freezing out of side group motions. Copyright © 1974 by Marcel Dekker. Inc. All Rights Reserved.