acetal copolymers of trioxane and 1 , 3 - dioxep - 5 - ene having a high degree of crystallinity and specific mole percent incorporations of 1 , 3 - dioxep - 5 - ene are synthesized . the invention also provides a process for producing highly crystalline acetal copolymers by copolymerizing triozane and 1 , 3 - dioxep - 5 - ene and precipitating the copolymer product from solution . the copolymers show much improved stability against halogen degradation .