New Mechanism for Concurrent Photo-polymerization and Stress Relaxation

Objective: Here we examine the effect of an allyl sulfide functionality on polymerization-induced shrinkage stress development. Previous studies have focused on novel stress relieving properties of fully cured networks containing allyl sulfides within their backbone. In this work, we study the ability of the allyl sulfide functionality to undergo reversible addition-fragmentation chain transfer during photo-polymerization, allowing for stress relaxation via bond rearrangement in the latter stages of photo-polymerization (i.e., post gel-point). Methods: Two divinyl-ether monomers were synthesized: one containing an allyl sulfide functionality and the other a propyl sulfide analogue. The propyl sulfide functionality, nearly identical in structure, is unable to undergo addition-fragmentation and is thus inactive as a stress relieving agent. Dynamic mechanical analysis (DMA) was utilized to compare mechanical differences in the glassy and rubbery moduli as well as the glass transition temperatures (T_gs). Tensile stress and functional group conversion were measured simultaneously during the polymerization. Results: 2-Methylene-propane-1,3-di(thioethyl vinyl ether) (MDTVE) and 2-methyl-propane-1,3-di(thioethyl vinyl ether) (MeDTVE) were synthesized and each photo-polymerized with a stoichiometric amount of pentaerythritol tetrakis(3-mercaptopropionate) (PETMP). The resultant networks exhibited nearly identical glassy and rubbery moduli and slightly offset T_gs, indicative of similar network structures. The rate of polymerization for networks containing the allyl sulfide functionality (i.e., MDTVE/PETMP) was decreased due to chain transfer; however, this phenomenon was accompanied by as much as a three-fold reduction in stress buildup. Moreover, stress development curves decrease over time, consistent with relaxation of the stress in the latter stages of polymerization. Conclusion: Inclusion of the allyl sulfide functionality within the monomer backbone provides a unique mechanism for the relief of polymerization shrinkage stress; this provides an avenue for alleviating a primary cause of premature failure in dental restorations. Supported by NIH DE Grant #10959.