7.1. Hydrolytic: non-enzyme and enzyme-mediated degradation

The O-protected L-threitol, L-arabinitol, and xylitol diols (24–26, 29, 31, 32) were used [14] to prepare a series of linear [AABB]-type PU (Figure 16) by polyaddition reaction to HDI (3) and MDI (23). The O-methyl-protected polyurethanes derived from 38 and 39 were amorphous and their T<sup>g</sup> was highly dependent on the aliphatic or aromatic nature of the diisocyanate used. The hydrodegradability of these PU was conditioned by the number of methoxy side groups present in the repeating unit.

The hydrophilic polyurethanes PU(X-MDC) and PU(X-MDC) (Figure 13) obtained by the reaction of xylitol with dimethyl hexamethylene dicarbamate (HDC) or di-tert-butyl-4,4'-diphenyl methyl dicarbamate (MDC) [6] were hydrolytically degradable under physiological conditions, in contrast with less-hydrophilic linear polyurethanes previously described [14].

In order to enhance degradability, a series of homo- and copolyurethanes containing the hydrophilic monomer [triethylene glycol (TEG)] and 1,4-di-S-benzyl-D,L-dithiothreitol (DTTSBn) (34) with HDI (3) [19] was synthesized (Figure 17). Enzymatic degradation studies were carried out with a variety of proteolytic and esterase enzymes. They were thermally stable up to 250C, and PU(TEG-HDI) homopolymer was degraded under physiological conditions. In addition, a study of the properties of this PU as matrix-forming excipient for controlled drug delivery was carried out [20].
