Preparation of Asymmetric Spiro Orthocarbonates via Cyclic Thionocarbonates.

Abstract

The development of monomers that show no shrinkage in volume upon polymerization is highly desirable in the field of materials such as dental fillings, high-strength composites, precision castings, and adhesives. 1 Volume shrinkage during polymerization results from the conversion of van der Waals distances between monomers to covalent bond lengths. Since the van der Waals distance separates monomer molecules approximately three times greater than a covalent bond does, the formation of new covalent bonds upon polymerization is destined to produce the substantial amount of volume contraction. A hopeful approach to prevent this shrinkage could be attained by using monomers that polymerize via the double ring-opening process, which results in two bonds breaking for each new bond formation. Since the double ring-opening polymerization was reported in 1972, 2 various monomers based on spiro orthoester (SOE), 3 bicyclo orthoester (BOE), 4 polycyclic ketal lactone (PKL), 5 cyclic ether, 6 spiroketal, 7 and spiro orthocarbonate (SOC) skeletons 8 have been developed to show the expansion in volume upon polymerization. Among them, spiro orthocarbonates (SOCs) are the most widely investigated monomers because they show the biggest volume expansion. Moreover SOCs containing exo-methylene groups undergo the ring-opening polymerization not only with cationic initiators 9 but also with radical initiators. 10 Asymmetric SOCs possessing one exo-methylene group are more favorable than symmetric SOCs, because the extra exomethylene group remained in the polymer from symmetric monomers affords the crosslinking reaction that causes the volume shrinkage. 11 Recently, N,S- 12 and S,S-analogs 13 of