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

Abstract

The bulk copolymerization of 1,3,5-trioxane (TOX) with 1,3-dioxepane (DXP) initiated by perchloric acid hydrate(s) is studied at 80 °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 during both homogeneous and heterogeneous steps.

A Novel Branched Polyoxymethylene Synthesized by Cationic Copolymerization of 1,3,5‐Trioxane with 3‐(Alkoxymethyl)‐3‐ethyloxetane

“…A common method that has been industrialized is the end‐capping reaction of the terminal hemiacetals with acetic anhydride; however, the end‐capping process is very energy‐intensive, requiring high reaction temperature (≈170 °C) due to the poor solubility of POM . The other way to improve the thermal stability of POM is by copolymerizing of 1,3,5‐trioxane (TOX) with cyclic ethers such as ethylene oxide, 1,3‐dioxolane, 1,3‐dioxepane, and 1,3‐dioxep‐5‐ene . The random introduction of ‐[(CH 2 ) n ‐O] ( n ≥ 2)‐ units limits the unzipping of the chain and improves the thermal stability of the ‐(CH 2 ‐O) n ‐ units in the copolymer .…”

“…The other way to improve the thermal stability of POM is by copolymerizing of 1,3,5‐trioxane (TOX) with cyclic ethers such as ethylene oxide, 1,3‐dioxolane, 1,3‐dioxepane, and 1,3‐dioxep‐5‐ene . The random introduction of ‐[(CH 2 ) n ‐O] ( n ≥ 2)‐ units limits the unzipping of the chain and improves the thermal stability of the ‐(CH 2 ‐O) n ‐ units in the copolymer . Compared with the homopolymer, the resulting compolymers exhibit excellent properties, including lower melting temperature, better alkali and hot water resistance, and better molding processability .…”

“…Compared with the homopolymer, the resulting compolymers exhibit excellent properties, including lower melting temperature, better alkali and hot water resistance, and better molding processability . But the copolymerization behavior has remained unclear and is highly related to the properties of the cyclic ethers . Besides the cyclic ethers mentioned above, cyclosiloxanes and cyclic carbonate have also been used to copolymerize with TOX.…”

Semi-Crystalline Polyoxymethylene-co-Polyoxyalkylene Multi-Block Telechels as Building Blocks for Polyurethane Applications

“…Two options for chain propagation seem likely: (i) The oxiranium cation reacts with one of the oxygen centers of trioxane. Opening of the trioxane ring leads to the elongation of the chain by three oxymethylene moieties and the formation of a terminal oxonium cation [ 71 ]. (ii) The opening of the dioxane ring leads to the elongation of the chain by one oxyethylene moiety and, analogously, the formation of a terminal oxonium cation.…”

ABA Triblock Copolymers Based on Linear Poly(oxymethylene) and Hyperbranched Poly(glycerol): Combining Polyacetals and Polyethers

“…However, due to the polyacetal repeating units, the POM homopolymer is temperature- and acid labile and degrades slowly under formaldehyde release. To prevent the underlying “unzipping” mechanism, trioxane is copolymerized with cyclic ethers, e.g., ethylene oxide, 1,3-dioxolane, 1,3-dioxepane, or comparable structures. , In this case, unzipping can still occur, but degradation stops as soon as the polyacetal structure is interrupted by a C–C-bond of the respective comonomer. The copolymerization affects the degree of crystallization of POM, resulting in a lower melting temperature and decreased mechanical properties.…”