Enhanced rate of sarcosine N-carboxyanhydride polymerisation from a lysine dendrimer macroinitiator driven by neighbouring H-bonding effects

Abstract

We report on the synthesis of 32-arm polysarcosine (PSar) star polymers initiated from a lysine dendrimer macroinitiator and describe faster rates of polymerisation compared to linear PSar equivalents. These polymerisations were monitored using in situ Fourier Transform Infrared spectroscopy (FTIR) to measure the disappearance of absorption bands related to the N-carboxyanhydride (NCA) monomer carbonyl groups. The star polymerisation rates were found to be faster than the linear equivalents irrespective of chain length and the effect was also observed in solvents with different dielectric constants. This effect was investigated further by synthesising star polymers as tetrablock homopolymers in one-pot reactions that revealed the polymerisation rate decreased with each additional block, confirming that chemical features in the dendrimer core were acting to catalyse the reaction. The increased rate was hypothesised to originate from hydrogen bonding between the lysine tert-butyloxycarbonyl (boc) protecting groups in the final generation of the lysine dendrimer and the NCA. Replacing the lysine –NHBoc group for a –NMeBoc resulted in a drop in reaction rate, confirming the hypothesis. However, the data also indicated to a lesser extent that some of the enhanced rate effect also likely originated from the dendrimer core lysine amides as well. This effect was corroborated by performing a density functional theory study on the mechanism of NCA ring opening, which revealed a high energy barrier for proton transfer in the zwitterion intermediate of the reacting NCA that is facilitated by a proton donor in close proximity. This is the first observation of this behaviour for PSar, a polymer that has found increasing popularity as a hydrophilic polymer in drug delivery systems and demonstrates the value of in situ reaction monitoring for NCA polymerisations.