Kinetics of Anionic Ring-Opening Polymerization of Variously Substituted β-Lactams: Homopolymerization and Copolymerization

Nylon-3 copolymers generated via ring-opening polymerization of beta-lactams have recently been shown to function as selective antibactel al agents or as chemoattractants that can induce fibroblasts to attach to surfaces Undei standing the molecular basis of these activities and developing improved materials requires knowledge of the 'dative reactivities of dial ent beta-lactams, which influence subunit distribution patterns within polymer chains The homopolymerization of a cyclooctyl beta-lactam (2) in the presence of a strong base and mude co-mmator was studied using both gas chromatography (GC) and in situ infrared (I R) spectroscopy The rate of this anionic ring-opening polymci ization reaction exhibits a lirst-oi der dependence on the beta-lactam and co-initiator concennations and a zci o-order dependence on the base concentration Analogous studies of foul othei hearing vai ions substituents lcyclohexyl (1), cyclododecyl (3), and Boc-protected amino gioups (4. 5)1, icvcalcd that different monomers exhibit significantly chat en t homaiolymerization rates Binai y copolymerizations of four beta-lactam pairs (I + 4, 2 + 3, 2 -I- 4, and 2 -I- 5), sem al of which lead to biologically active amphiphilic copolymei s, wete investigated by GC In each of the cc polymerizations, except for 2 + 3, the two beta-lactams were consumed at diffeient rates, leading to compositional drift within the resulting polymers (i c., variable subunit distribution along the length of the polymer chains) The copolymerization lutes of 2 + 3 and 2 + 4 exhibited a monotonic dependence on the staruna beta-laciam composition, whereas the copolymerization of 1 -4-4 and 2 + 5 was slower than whet of the respective beta-lactam homopolymerizations Three methods (Fineman Ross, Kelen Tudos, and Mayo Lewis:, were employed to determine the reactivity ratios of these beta-lactam pairs at low conversions This analysis confirms that the copolymers obtained from 1 + 4. 2 + 4. or 2 + 5 are characterized by some extent of compositional drift, while poly(2 + 3) is an ideally random copolymer These results pi vide valuable insights pertm.:nt to the molecular structure of amphiphihe nylon-3 copolymeis that exhibit bioactivity