Abstract
The polymorphic behaviors of long-chain n-alkyl acrylates and methacrylates (C12-C20) as well as vinyl esters of long-chain fatty acids (C12-C18) were investigated by thermal analysis combined with the analyses by X-ray diffraction and infrared spectroscopy. Effects of the chemical structure of functional groups and the length of hydrocarbon chains on the thermal behavior of these long-chain vinyl compounds were elucidated. n-Alkyl acrylates and methacrylates with sufficiently long chains, such as octadecyl acrylate, exhibit five crystalline modifications, α (hexagonal), sub α (hexagonal, partly diaxial orientation), β0 (uncertain), β1 (monoclinic with M|| subcell), and β2 (triclinic with T|| subcell) forms, while vinyl stearate exhibits three crystalline modifications, α, β1, and β'2 (monoclinic with O⊥ subcell) forms. With shortening of the alkyl chain the polymorphic behavior simplifies. The packing modes of long-chain vinyl compounds as well as the rotational freedom around the molecular axis and the conformational freedom of functional groups estimated from the entropy changes for fusion and transformation were also discussed in relation to the polymerizability.
