Polymerization shrinkage and the accompanying stress development have been significant limiting factors in the performance of Dental polymers. The volumetric shrinkage, which results in some degree from all conventional polymerization processes, is responsible for marginal gap formation, microcracking and tooth flexure. Undercut cavity preparations, complicated multistep bonding pretreatments and application of restoratives in incremental layers are all concessions the Dentist must make to deal with the polymerization shrinkage issue. Shortcomings related to polymerization shrinkage are echoed in a wide variety of polymer applications that extend beyond Dentistry; therefore, practical solutions achieved here have the potential to be very far-reaching. This investigation focuses on the double ring-opening polymerization of Spiro-orthocarbonates (SOC), which occurs with volumetric expansions typically of 2.5 to 3.5 percent. While substantial accomplishments have been made in this field since its initial introduction in the 1970s, there remain some glaring gaps in the basic understanding of this unique polymerization process. These deficiencies in the knowledge base are largely responsible for the difficulties that have been encountered in efforts to integrate SOC monomers with other monomer systems. Thus, the goals of this project are to: 1) determine SOC photopolymerization kinetics with efficient cationic initiator systems; 2) identify the polymeric structures generated by cationic photopolymerization of model SOC monomers; 3) develop a practical multifunctional SOC oligomer that undergoes cationic ring-opening polymerization via cross-link formation; and 4) demonstrate the potential of cationic/free radical, visible light cured resins and composites that produce hybrid network polymers with minimal shrinkage and stress development. In the end, a fundamental understanding of the complex SOC ring-opening photopolymerization process, which has not been available to this point, will emerge along with a practical cationic/free radical dual cure resin system that will produce only cross-linked polymers. The current application offers an alternative approach to SOC development that acknowledges the deficiencies that have held back this technology and provides remedies that will address each of the potential negative areas. The information gained on the hybrid materials will be directly applicable to the development of improved Dental restoratives with minimal shrinkage and stress.
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