The contribution of entropic factors to the penultimate unit effect in free-radical copolymerizations is discussed and exemplified. In addition, significant penultimate unit effects on radical selectivity in transfer reactions are demonstrated and are shown to have a significant polar component. Further, ring-opening copolymerization studies are presented and describe surprising results that seem to originate from strong solvent effects in copolymerization. These results could not have been predicted with current knowledge, prior to the experiment. The present contribution demonstrates in detail that radical reactions are highly complex and there are significant dangers and drawbacks in employing simplified kinetic models when in search of fundamental understanding
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