Polypropylene Thermosets by Functional Nitroxyl-Mediated Radical Crosslinking
New chemistry for preparing polypropylene (PP) thermosets is described, wherein radical degradation suffered during conventional peroxide treatment is overcome using a nitroxyl additive bearing a polymerizable functional group. A small amount of 4-vinylbenzoic-2,2,6,6-tetramethylpiperidin-N-oxyl (VBTEMPO) is used to trap peroxide-derived, alkyl radical intermediates during the early stages of the modification process, yielding a macromonomer derivative in situ that crosslinks in the later stages through side-chain oligomerization. This process has proven capable of producing thermosets with gel contents in excess of 90%, while providing the induction period needed to shape the formulation before it rendered thermoset. Control over PP crosslinking dynamics and yields have been gained by appropriate selection of peroxide initiator, polymerizable functional structure, and additional cure formulation components. The thesis concludes with preliminary assessments of the stability of PP-derived alkoxyamines, and their potential to confer oxidative stability to a thermoset.