No abstract

Biomedical polymers that are designed to degrade faster at slightly acidic pH values may potentially be developed for use in drug delivery, pharmaceutical formulation and regenerative medicine. Water-soluble polyacetals derived from two well known pendent functionalised tyrosine-based diphenol monomers were prepared. Acid catalysed copolymerisation of triethylene glycol divinyl ether with the diphenol was first used to confirm the 1 H-NMR signals for the acetal moieties. A ter-polymerisation process employing the divinyl ether and the diphenol with poly(ethylene glycol) (PEG, 3 400 g mol 21 ) gave the desired water-soluble polyacetals with weight average molecular weights ranging from 24 000-71 000 g mol 21 (PDI 1.6-2.9). Diphenol incorporation into the polymer mainchain was less efficient than aliphatic hydroxyl incorporation from PEG. Thermal properties positively correlated with the relative amount of PEG that was incorporated into the polymer mainchain. The ter-polyacetals displayed enhanced rates of hydrolysis at pH 5.5 with little hydrolysis occurring at pH 7.4 over a 4 day period. Degradation was faster overall than that observed for polyacetals derived from aliphatic monomers only. Air-water contact measurements were lower for the ter-polyacetals than for PEG alone. Overall the properties of the ter-polyacetals were influenced more by the characteristics of PEG in the polymer mainchain than the structure of the two pendent chains that were examined.

The kinetics of the pyridine-initiated polymerisation of simple 1,3-oxathiolan-2,5-diones (3a-c) in nitrobenzene was studied at various temperatures. Results show that, although the substituents in 4-position have a marked effect on rate of polymerisation, the polymerisation of the three monomers studied is linked by a common mechanism. Thus, the pyridine, nucleophile attacks the carbonyl in 5-position and forms a charge transfer complex which decomposes to form a highly reactive a-thiolactone intermediate 8 together with carbon dioxide and pyridine. Chain addition continues by the direct attack on the thiolactone by the terminal-SH group of traces of parent acid. The reaction of each of the three monomers with pyridine was found to show first order kinetic dependence on both monomer and pyridine, although the initiator (i. e. pyridine) is not consumed. The resultant polymers (molecular weight typically around 20000) are highly crystalline and relatively insoluble in common organic solvents.