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Synthesis of Monomers That Expand on Polymerization

For a number of industrial applications, such as strain-free composites, potting resins, and binders for solid propellants, it was highly desirable to have monomers that have essentially zero shrinkage on polymerization. However, for many other applications, such as precision castings, high strength adhesives, prestressed castings and dental fillings, it would be highly desirable to have monomers that would undergo expansion on polymerization. Ring-opening polymerizations usually involves somewhat less shrinkage than either addition or condensation polymerization because for every new bond that is formed involving a shift from a Van der Waals' distance to a covalent distance, another bond is broken involving a shift from a covalent distance to a near Van der Waals' distance. There is reason, therefore, to believe that if monomers were utilized in which two or more bonds were broken for every new bond that is formed in the polymerization process, near zero shrinkage or even expansion would be possible. Among the monomers that do indeed give essentially zero contraction or some expansion on polymerization are the spiro ortho esters, such as 2,4,6-trioxaspiro[4,4] nonane, spiro orthocarbonates, and bicyclo ketal lactones.