Ring-Opening Polymerizations of Cyclic Acetals

The solubility of polyoxymethylene is very poor so that the ring-opening polymerization of 1,3,5-trioxane proceeds heterogeneously both in bulk (melt) and in solution. 1,3,5-Trioxane can also be readily polymerized in the solid state this polymerization can be initiated both by high-energy radiation and by cationic initiators (see Example 3-24). 

Generally, the molecular weight and the molecular-weight distribution are determined by two side reactions. Moreover, the end groups and in case of copolymers, their sequence length distribution are determined by the following two side reactions  

Hydride transfer or hydride migration is initiated by the electrophilic attack of the poly(oxymethylene) cation from the methylene bridge of its own or of a neighboring macromolecule. A hydride ion is thus split off, and a methoxy end group is formed. 

The newly created cation is stabilized through conjugation with the free electron pairs of the neighboring 0 atoms and is broken into a chain with the formate end group and another poly(oxymethylene) cation. 

Both end groups can be determined quantitatively. A second side reaction is the transacetalization. Here a poly(oxymethylene) cation attacks an oxygen of a poly(oxymethylene) chain with formation of an oxonium ion that decomposes. Through continued cleavage and recombination of poly(oxymethylene) chains one obtains polymers which are chemically and molecularly largely homogeneous. For the case of a trioxane/ethylene oxide copolymer the following reaction scheme can be formulated  

Like THF, cyclic acetals (e.g., 1,3-dioxolane and 1,3,5-trioxane) are polymerizable only with cationic initiators. The ring-opening polymerization of 1,3,5-trioxane (cyclic trimer of formaldehyde) leads to polyoxymethylenes (see Example 3.24), which have the same chain structure as polyformaldehyde (see Example 3.22). They are thermally unstable unless the semiacetal hydroxy end groups have been protected in a suitable way (see Example 5.7). Like the cyclic ethers, the polymerization of 1,3,5-trioxane proceeds via the addition of an initiator cation to a ring oxygen atom, with the formation of an oxonium ion which is transformed to 

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In connection with studies on the ring-opening polymerization of cyclic acetals, we have undertaken investigations on the polymerization of bicyclic acetals, bicyclic oxalactone, and bicyclic oxalactam, which yield polysaccharide analogs, macrocyclic oligoesters, and a hydrophilic polyamide, respectively, some of which can be expected to be useful as novel speciality polymers. The monomers employed in the studies were prepared via synthetic routes presented in Scheme 1, starting from 3,4-dihydro-2H-pyran-2-carbaldehyde (acrolein dimer) I. 

The polymer has a relatively low ceiling temperature (119 °C), but this is the highest of all the formaldehyde polymers. The lack of success at polymerizing other monomers was due to the low ceiling temperatures (e.g. —39 C for acetaldehyde) (Odian, 1991). Aldol condensation can be a side reaction competing with polymerization for these monomers. There are other routes, such as cationic ring-opening polymerization of cyclic acetals, to achieve the same polymer (Penczek and Kubisa, 1989). 

Recent review articles on the following topics were published the controversy concerning the cationic ring-opening polymerization of cyclic acetals (213), photoinitiators for cationic polymerization (21A), living polymerization and selective dimerization (215). raacroraonomers (216), and functional polymers and sequential copolymers by carbocationic polymerization (217). 

Ring-Opening Polymerizations of Cyclic Acetals Termination... 

It should be noted that ring-opening polymerization of cyclic acetals is not the only route to polyacetals. Polyacetals are also formed by ionic polymerization of aldehydes, by polycondensation of aldehydes and diols, or by polyaddition of divinyl ethers to diols. " In this chapter, however, only cationic ring-opening polymerization of cyclic acetals will be discussed. 

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