Synthesis and Solution Properties of Norbornene Based Polybetaines

Ring-opening metathesis polymerization (ROMP) was used to synthesize novel norbornene based polycarboxy- (d-Poly 3a and d-Poly 3b) and polysulfobetaines (Poly 3c) using the third generation Grubbs' catalyst (G3) as the initiator. Hydrophobicity of the polycarboxybetaines was varied by changing the bridging group of the norbornene backbone. A protective group approach was utilized to prevent any possible retardation in the polymerization due to interactions of the carboxylate functionality with the catalyst and to provide ease of characterization. The tert-butyl ester protected precursor polymers (Poly 3a and Poly 3b) were deprotected under acidic conditions to yield the corresponding polycarboxybetaines with very narrow polydispersity indices, ranging from 1.03 to 1.15. This method allowed excellent control over the molecular weight distributions compared to the direct polymerization approach. When molecular weight was plotted against the theoretical degree of polymerization (DP), linear relationships were obtained for both H-1 NMR and GPC-MALLS data. Oxanorbornene based polycarboxybetaine (d-Poly 3a) was studied in aqueous 0.1 M NaBr solution by dynamic light scattering (DLS) and no significant aggregation was observed. An attempt was made to determine the acid ionization constant (K-a) of the carboxylate group, which led to the discovery that the cyclic imide of these particular monomers is easily ring opened under basic conditions.