Controlling Polymer Architecture in the Thermal Hyperbranched Polymerization of l-Lysine

Because of the unequal reactivity of the two amine groups of l-lysine hydrochloride, thermal polymerization of this asymmetrical AB2 monomer results in hyperbranched polymers, which contain ∼2.5 times more Nε-linked linear compared to Nα-linked linear structural units. This report discusses the feasibility of three approaches to control polymer architecture during the thermal hyperbranched polymerization of l-lysine hydrochloride. The reactivity of the more reactive ε-NH2 group was modulated by introducing temporary protective groups that preferentially block the ε-NH2 position. This was achieved by (i) addition of o-vanillin to the polymerization, (ii) copolymerization of Nε-benzylidene-l-lysine, and (iii) copolymerization of α-amino-ε-caprolactam. Analysis of the degree of branching (DB) and the average number of branches (ANB) of the obtained polymers did not provide evidence for any significant structural changes. Analysis of the distribution of structural units, in contrast, revealed major structural changes and indicated a rearrangement from Nε-linked linear structural units to Nα-linked linear structural units upon addition of o-vanillin and Nε-benzylidene-l-lysine. Unlike o-vanillin and Nε-benzylidene-l-lysine, α-amino-ε-caprolactam was found to be rather ineffective in modulating polymer architecture.