Thiol-(Meth)Acrylate Photopolymerizations: Controlled Network Structures for Dental Applications

Objectives: Thiol-ene systems exhibit reduced volume shrinkage per double bond (12-15 ml/mol) as compared to typical (meth)acrylic polymerizations which exhibit volume shrinkage of ~23 ml/mol. The dramatic reduction of volume shrinkage, double bond concentration and shrinkage stresses developed in thiol-ene systems make them attractive for dental applications. In this study we investigate thiol-(meth)acrylate systems, which exhibit a unique combination of step and chain growth mechanisms. Therefore, these polymerizations are representative of both step growth thiol-ene and chain growth (meth)acrylate systems. These novel systems form polymers having low volume shrinkage, reduced shrinkage stresses, and high glass transition temperatures (Tg) with narrow Tg widths.

Methods: Commercially available pentaerythritol tetra-(3-mercaptopropionate) (thiol) and tricyclodecane dimethanol diacrylate (acrylate) monomers are utilized in this study. Mechanical properties of the films are characterized by DMA and a tensometer.

Results: In thiol-acrylate systems control over Tg and modulus is exercised by changing either the thiol functionality or its stoichiometry. The Tg in this thiol-acrylate system can be varied from 40 oC to 80 oC with changes in stoichiometry. Further, while the half maximum Tg in pure acrylate systems is typically around 100-200 oC, in the thiol-acrylate systems it is ~35 oC. The low Tg widths of the thiol-acrylate systems reflect their ability to form homogeneous networks. Further, the unique step-chain polymerization mechanism of thiol-acrylate systems leads to delayed gelation and hence forms films having reduced shrinkage stresses. While the polymer formed from pure acrylate system exhibits a shrinkage stress of 4 MPa, the 1:1 thiol: acrylate system exhibits a shrinkage stress as low as 0.5 MPa.

Conclusion: Thiol-(meth)acrylate polymerizations lead to formation of films having high Tg, low Tg width, and low shrinkage stresses. This unique mix of properties is not facilitated individually by either thiol-ene or (meth)acrylic systems, thereby making thiol-acrylate systems very attractive for dental applications.