| Abstract: | γ-Ray-initiated postpolymerization of octadecyl methacrylate in polymorphic crystals and melt has been investigated to clarify the effect of molecular arrangement of the monomer on polymerizability. From thermal, x-ray, and infrared (IR) analyses this long-chain monomer exhibited three crystalline modifications that we refer to as α-, sub-α, and β-forms. The β-form (mp 28.7–29.7°C), which is obtainable from solution, is a stable state with triclinic chain packing. The α-form (mp 19.5–20.0°C), which is obtained first from the melt but transforms into β-form on storing, is a metastable state with hexagonal chain packing. The sub-α-form appears transiently in α→β transition. The polymerizability of octadecyl methacrylate in the β-form is extremely low, whereas the α-form can polymerize easily and the initial polymerization rate, saturated conversion, and polymer molecular weights increase with temperature. Polymerizability in the molten state at fairly high temperature is rather low, however. Thus maximum polymerizability is obtained just above the melting point of α-form. It has been found that particular orientation and suitable packing mode with some freedom of rotational motion of the monomer molecules in layered structure accelerate the polymerization reaction. |
