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Summary Photodimers of 9-chloroanthracene, formed by a [4+4] cycloaddition reaction of 9-chloroanthracene, were used as initiators in the atom transfer radical polymerization of styrene and compared to results previously obtained using photodimers of 9-bromoanthracene as the initiator. Heat-induced cleavage of the photodimer coupled with slow initiation from the bridgehead radical have been used to account for the lack of control in the systems, and thus changing the halogen on the initiating photodimer could support or refute this model. Reactions performed using analogous procedures produced polymers with number average molecular weight (Mn) values significantly higher in the case of 9-chloroanthracene photodimer-initiated systems, with similar polydispersity index (PDI) values observed in trials catalyzed with CuCl or CuBr. Polymer products showed absorbance bands indicative of regenerated anthracene in all cases, consistent with heat-induced cleavage of the photodimer during the course of the polymerization. Kinetic plots demonstrated that the polymerizations initiated with photodimers of 9-chloroanthracene showed maximumMnvalues were obtained after approximately 10% monomer conversion, with a decline inMnas a function of monomer conversion after this point. The data support slower initiation in the case of 9-chloroanthracene photodimers, followed by heat-induced cleavage throughout the polymerization system. [ABSTRACT FROM AUTHOR]