Side-group Influence on Degradation Kinetics

The nature of the organic side groups has a decisive influence on the degradation rate of the resulting poly(organo)phosphazene. 

The most hydrolysis-susceptible side-substituent is chlorine and hence if preparing polymers from a poly(dichloro)phosphazene precursor, knowledge of the residual chlorines is required [27, 30]. Not only is the P-Cl moiety itself extremely labile, its hydrolysis leads directly to the degradation intermediate hydroxyphosphazene [31] and also produces HCl as a by-product, which is known to further catalyse hydrolysis (see Section 1.1). Although through the partial substitution of poly(dichloro)phosphazene it is thus possible to prepare degradable materials, in practice however, full substitution is usually strived for, not least for the sake of reproducibility. 

Howevei although general tendencies can be observed, it would appear that hydrophilicity alone cannot explain hydrolytic sensitisation for all polyphosphazenes. For example, oligoethyleneoxy side chains, including poly[ /s(2-(2-methoxyethoxy)ethoxy)phosphazene], and its many structural variations [33], used as solid polymer electrolytes, are reported to be biostable, whereas structurally similar polymers with P-NH-R attachments, instead of the P-O-R are known to undergo hydrolytic degradation [34]. Also, ethyl esters of serine and 

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