Polyorthoesters
These polymers appear to be particularly useful for controlled drug release. Initially poly(orthoesters) were prepared by condensation of 2,2-diethoxytetrahydrofuran and a dialcohol47 B ecause the tetrahydrofuran component of the polymers does not provide the necessary rigidity (Tg varies from body temperature or lower), this material had limited utility as a solid implant designed for drug delivery. An alternative route uses 3,9-bis(methylene)-2,4,8,10-tetraoxaspirol [5,5] undecane and a dialcohol.48 Mechanical properties of the polymer can be controlled by the choice of diols used in the condensation reactions. Use of rigid diols (trans-cyclohexane dimethanol) produces a rigid polymer whereas the use of 1,6-hexanediol produces a flexible material. Polymers with molecular weights in excess of 100,000 can be synthesized. Poly(orthoesters) are hydrophobic materials and are stable during storage without careful exclusion of moisture.
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