Poly(2,2-dimethyltrimethylenecarbonate) (PDTC) and its cyclic monomer 5,5-dimethyl-1,3-dioxan-2-one (DTC) are characterised by X-ray diffraction, calorimetry, dilatometry and conformational energy calculations. Three crystalline modifications are identified for PDTC: form I from solution, form III in melt spun fibers, while form II, obtained from the melt at surprisingly small undercoolings shows unusually large coherent domain dimensions, chain extended morphology and a modest enthalpy of fusion. These features indicate that form II should be a high entropy, conformationally disordered phase. Conformational calculations identify four regular, extended conformations with similar low energy, which are consistent with the observed polymorphic behaviour of PDTC. The cyclic monomer DTC also shows three crystalline modifications: a low temperature, ordered form and two high temperature phases, which like the polymer, are characterised by high entropy presumably due to conformational and orientational disorder. (C) 1997 Elsevier Science Ltd.