Polyacetals and Polyketals
Polyacetals and polyketals are polyethers that form (1) through condensations of glycols with carbonyl compounds, (2) by exchange reactions of acetals or ketals, and (3) by additions of diols to dialkenes [109]:
An acid-catalyzed exchange reaction of glycols and acetals yields polyacetals as follows:
These reactions often lead to cyclic acetals that interfere with the formation of high molecular weight products .Useful polyacetals, however, can be formed from pentaerythritol and acetals of dialdehydes:
Acetals generally exhibit poor resistance to hydrolytic attack. Some, however, are much more resistant than others, depending upon the glycol. The following formal was reported to exhibit good hydrolytic stability under both acidic and basic conditions [109]:
Pentaerythritol yields spiro polymers by this reaction. The products offer superior thermal resistance [114]:
The addition of diols to dialkenes can be illustrated on addition of a glycol to a divinyl ether:
Commercially, large volume acetals are only produced by polymerizations of formaldehyde and by ring opening polymerizations of trioxane. These reactions are discussed in chapters three and four. Two such materials are manufactured in this country. One is a homopolymer of formaldehyde, polyoxymethylene. It is sold under a trade name of Delrin. The material is end-capped to prevent depolymerization by acetylating the terminal hydroxyl groups. The other one, a copolymer of formaldehyde with small quantities of a comonomer, is sold under the trade name of Celcon. Copolymerization accomplishes the same objective as end-capping. It also makes it more resistant to attacks by bases. Polyoxymethylene is highly crystalline. This is due to easy packing of the simple, polar chains. The crystallinity is estimated to be 60-77%. Polyoxymethylene is a strong material with good resistance to creep, fatigue, and abrasion.
