A variety of initiating systems for the preparation of macromolecules by nitroxide-mediated "living" free radical procedures have been prepared and evaluated. The systems can be divided into two classes, unimolecular initiators in which alkylated TEMPO (2,2,6,6-tetramethylpiperidinyloxy) derivatives dissociate to provide both the initiating radical and the stable radical, and bimolecular systems in which a traditional free radical initiator, such as BPO or AIBN, is used in conjunction with TEMPO. For the unimolecular initiators it was found that an R-methyl group is essential for "living" character, while a variety of substituents could be placed on the phenyl ring or the β-carbon atom without affecting the efficiency of the unimolecular initiator. It was also found that the rate of polymerization is approximately the same for both the unimolecular and corresponding bimolecular systems; however, the unimolecular initiators afforded better control over molecular weight and polydispersity.

No abstract

The efficiency of exchange between the mediating nitroxide moieties at the termini of growing polymer chains during “living” free radical polymerizations has been probed by a series of crossover experiments using functionalized unimolecular initiators. The design of appropriately substituted initiators permitted the synthesis of specifically functionalized model polymers which could be readily distinguished using high-performance liquid chromatography (HPLC). Using these models, the mixture of macromolecules obtained from a 1:1 combination of disparate initiators was separated and identified. The results reveal that exchange of the mediating nitroxide free radicals is a facile process and at essentially all stages of the polymerization a nearly statistical mixture of crossover products is obtained. The HPLC techniques developed are also useful in evaluating the extent of chain termination in nitroxide mediated “living” free radical polymerizations.

The autopolymerization of styrene, styrenic derivatives, and styrene/(meth)acrylate comonomer mixtures in the presence of stable nitroxide free radicals has been shown to be a “living” process. Molecular weight can be controlled by varying the ratio of vinyl monomer to TEMPO and low-polydispersity materials are obtained. Significantly, a definite incubation period is observed during these polymerizations, and the length of this incubation period increases with increasing amounts of TEMPO. The structures of the in situ generated unimolecular initiators which are formed during this incubation period correspond to those expected from a Mayo mechanism for the autopolymerization of styrene. The isolated and purified adducts, 4 and 5, were shown to be effective unimolecular initiators leading to low-polydispersity, controlled molecular weight polymers.

The synthesis of a range of narrow polydispersity, well-defined poly(4-hydroxystyrene)s via a “living” radical polymerization technique is reported. The synthetic route chosen involves the 2,2,6,6-tetramethyl-1-piperidinyloxy mediated free radical polymerization of acetoxystyrene, followed by deacetylation with base. The efficiency and molecular weight control of the “living” free radical polymerization of 4-acetoxystyrene is compared to styrene. Further, the effect of initiating system, “unimolecular” or “bimolecular”, on the “living” radical polymerization of 4-acetoxystyrene is also discussed. The physical and structural properties of these well-defined polymers were used to compare poly(4-hydroxystyrene) prepared via “conventional” free radical polymerization. A fundamental study of the dissolution of poly(4-hydroxystyrene) in aqueous base was undertaken to elucidate the effect of polydispersity, molecular weight, and structure on the dissolution behavior of these polymers.