Cationic Copolymerization of 1,3,5-Trioxane with 1,3-Dioxepane: A Comprehensive Approach to the Polyacetal Process

The bulk copolymerization of 1,3,5-trioxane (TOX) with 1,3-dioxepane (DXP) initiated by perchloric acid hydrate(s) is studied at 80 degrees C with DXP/TOX initial ratios from 20/80 to 3/97 w/w. As usually observed for the polyacetal synthesis, the polymerization takes place within seconds to tenths of seconds and proceeds in two steps, a homogeneous (induction) period, followed by a very rapid heterogeneous propagation-crystallization step. The influence of the reaction conditions was systematically examined (diluent of initiator, comonomer concentration, transfer to residual water), and the products formed during the short induction period were identified using NMR. The difference between the respective basicities of the comonomers is such that during the induction period the initiation takes place on the DXP, and for DXP initial concentrations above its equilibrium value [DXP](eq,ss) the early formed soluble polymer is either the homopolymer or a DXP highly enriched copolymer. The precipitation step starts only after the concentration of DXP is lowered enough to allow the copolymerization with TOX to proceed significantly and the poly(oxymethylene) -[CH2-O](n)(-) sequences to reach their critical crystallization length. This critical concentration was found constant whatever the initial DXP/TOX ratio. For initial concentrations lower than [DXP](eq,ss) (i.e., DXP/TOX <= 5/95 w/w), the DXP cannot homopolymerize and should be consumed by single unit insertion during a copolymerization effective since the early beginning. Therefore, during the further heterogeneous propagation-crystallization step, the insertion of two successive DXP units in the copolymer chain becomes highly improbable. The copolymer formed by both propagation and transacetalization processes is either inserted into the crystal (or its immediate interface) if only isolated DXP units are present along the poly(oxymethylene) chain or rejected in the amorphous phase if preformed DXP sequences are present. The latter situation occurs for DXP/TOX initial ratios 10/90 w/w. This is in agreement with our previous measurements of the crystal partitioning coefficient, which confirmed that the tetramethyleneoxy -[(CH2)(4)-O]- units from DXP are progressively rejected into the increasing amorphous fraction for initial DXP richer compositions. Coherent chemical pathways are proposed for the overall process, which are in accordance with the experimental behaviors (luring both homogeneous and heterogeneous steps.