One of the greatest limitations to the application of free radical photopolymerizations is the inhibitory effect that oxygen has on these types of reactions. The inability of free radical photopolymerizations to overcome oxygen inhibition has limited their applicability or forced the application of expensive inerting equipment. Oxygen interacts with photopolymerizing systems both at the initiation stage and in the propagation stage. Its presence generally leads to the formation of an inhibition layer at the top surface of the sample in which the diffusion rate of oxygen into the sample from the surrounding environment is more rapid than its consumption by the polymerization. Also, unique kinetic behavior is observed in the presence of oxygen as two rate maxima are often observed, and the presence of even a small amount of a monovinyl compound can have pronounced effects. In this project we are developing an improved understanding of the effects of oxygen on both the initiation and propagation steps of the polymerization. Mitigation of oxygen's effects will be attempted through the application of three component initiating species and the presence of highly abstractable hydrogens.

     In this work, we are pursuing an approach designed to understand and mitigate the effects of oxygen on the photopolymerization reaction.

     Project Focus Summary:

• investigating the effect of oxygen on the initiation step for commercial single component initiators as well as two and three component initiator systems;

• determining the effects of oxygen on the propagation step by examining the photopolymerization kinetics and surface polymer properties in environments with systematically varying oxygen levels, by examining the cure in thick films, by following the formation and subsequent degradation of peroxy functionalities, and by systematically adding compounds that have specific interactions with oxygen and peroxy radicals; and

• developing methodologies for mitigating the impact of oxygen on the photopolymerization kinetics and photopolymer properties.