Preparation of High-Molecular-Weight Polyacetals

As already mentioned, the polymerization of 1,3,5-trioxane and bicyclic acetals (including anhydro sugars) is discussed separately. In this section we shall only give the conditions for preparation of high-molecular-weight polymers from the parent monomer of this class 1,3-dioxolane (DXL). The majority of conclusions applies also to the 7-membered acetal 1,3-dioxepane (DXP). 

The preparation of high polymers under living polymerization conditions requires that the polymerization is carried out in the presence of counterions which do not lead to irreversible termination 27 29). Noncomplex counterions, derivatives of strong protic acids (superacids), e.g. the CF3SOf anion, may react with cationic growing species. This reaction is reversible and affects only the polymerization kinetics but not the molecular weights 30). These counterions are classified as stable. 

Stable counterions thus include the most stable complex anions SbF and AsFf 27,31-32) as well as the noncomplex anion CF3SOf. Counterions such as SbCl, BF and even PF should be classified as unstable, because in their presence irreversible termination is frequently observed 27 29). 

Carbenium salts [e.g. (C6H5)3C A ] initiate the polymerization of DXL by hydride-transfer which introduces an additional complication 28,29-33,34). 

Taking the above discussed factors into consideration we may select the group of initiators which initiate quantitatively by addition, the terminationless polymerization of DXL and which are either commercially available or can easily synthesized  

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Acetals are one of a few groups of monomers for which the best general polymerization conditions can be given. Thus, if the aim is the preparation of high-molecular-weight polyacetals we would advise to use <0.1 mol % trifluoromethanesulfonic anhydride well below 0 °C in bulk. If the polymerization proceeds violently under these conditions and the time for mixing is insufficient to obtain uniform solution, both temperature and initiator concentration should be decreased. 

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