Synthesis and Topological Trapping of Cyclic Poly(alkylene phosphates)

The zwitterionic ring-opening polymerization of 2-isopropoxy-2-oxo-1,3,2-dioxaphospholane (iPP) with N-heterocyclic carbenes (NHC) generates poly­(alkylene phosphate)­s with molecular weights of Mn = 55000–202000 Da. MALDI-TOF mass spectrometry provided clear evidence for cyclic poly­(alkylene phosphate)­s (poly­(iPP)) for lower molecular weight fractions (m/z ≤ 3000). The cyclic topology of the higher molecular weight fractions was inferred by trapping of poly­(iPP) in cross-linked 2-hydroxyethyl methacrylate (HEMA) hydrogels. Cross-linked HEMA hydrogels were generated in the presence of a high molecular weight (Mn = 202000 Da) poly­(iPP) generated from the zwitterionic ring-opening polymerization of iPP with the NHC 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene 2. Extraction of the resulting gel with methanol for 11 days revealed that 36% of the poly­(iPP) was retained in the gel, whereas a linear poly­(iPP) was completely extracted under similar conditions. The retention of the poly­(iPP)­s in the gels is attributed to topological trapping of the cyclic poly­(iPP) in the cross-linked network.