Polyanhydrides melt condensation

The highest molecular weight polyanhydrides have been obtained by melt condensation. The reaction temperature and time, the presence of catalysts, and the purity are important factors in this respect [253]. 

Polyanhydrides Synthetic Melt condensation, ring-opening polymerization Most polyanhydrides soluble in organic solvents, insoluble in water... 

The most widely used method to synthesize polyanhydrides is melt condensation polymerization, which results in high molecular weight polymers. Other methods include Schotten-Baumann condensation, dehydrative coupling, and ring opening polymerization. 

The polyanhydrides synthesized by melt condensation have fiber-forming properties in the molten state. They hydrolyze when exposed to air and this degradation is mainly controlled by the composition of the polymer. Homopolymers of aromatic monomers, such as CPH, degrade at a rate that is several orders of magnitude lower than that of homopolymers of aliphatic monomers. 

Polyanhydrides have been synthesized by the following methods a) bulk melt condensation of activated diadds, b) ring opening polymerization, c) reaction between dibasic acid and diacid chlorides, and d) interfacial polymerization. A detailed study of these polymerization methods and various polymerization conditions for a range of diadds were previously described [25-27],... 

Polyanhydrides can be synthesized by melt condensation of trimethylsilyl dicarboxylates and diacid chlorides to yield polymers with intrinsic viscosities up to 0.43 dl/g [31, 32] ... 

SCHEME 10.1 Synthesis of polyanhydrides through melt condensation. 

Polyanhydrides can be synthesised by melt condensation of trimethylsilyl dicarbox-ylates and diacid chlorides (Kricheldorf and Lubbers, 1990) as shown in Scheme 5.2. Polymerisation is carried out at 100 °C under vacuum in the presence of benzyltriethy-lammonium chloride. This method possesses several disadvantages over the acetic anhydride melt condensation method it requires pure chloride and trimethylsilyl... 

Scheme 5.2 Synthesis of polyanhydrides by melt condensation of trimethylsilyl dicarboxylates and diacid chlorides.

The condensation reaction of diacetyl mixed anhydrides of aromatic or aliphatic diacids is carried out in the temperature range 150—200°C. The synthesis of polyanhydride from RA half-esters with maleic and succinic anhydride has been reported (Teomim and Domb, 1999). Ricinoleic acid maleate (RAM) or ricinoleic acid succinate (RAS) diacid half esters were polymerised by melt condensation as shown in Scheme 5.3. 

Polyanhydrides have been synthesized by melt condensation of activated diacids (Domb et al, 1987), ring opening pol) merization, dehydrochlorination, and dehy-drative coupling agents (Leong et al, 1987 Domb et al, 1988). Solution polymerization yielded in general low molecular weight pol miers. The most widely used method is the melt condensation of dicarboxylic acids treated with acetic anhydride ... 

Polyanhydrides can be synthesized by melt condensation of trimethylsilyl dicar-boxylates and diacid chlorides to yield polymers with an intrinsic viscosity 0.43 dl/g (Gupta 1988). Direct polycondensation of sebacic acid and adipic acid at a high temperature under vacuum resulted in low molecular weight oligomers (Knobloch et al, 1975). The preparation of adipic acid polyanhydride from cyclic adipic anhydride (oxepane-2,7-dione) was investigated (Albertsson et al., 1990). 

General methods for the synthesis of poly(amide-anhydrides) and poly(amide-esters) based on naturally occurring amino acids were described (Domb et at, 1990). The polymers were synthesized from dicarboxylic acids prepared by amidation of the amino group of an amino acid with a cyclic anhydride, or by the amide coupling of two amino acids with a diacid chloride. Low molecular weight polymers from methylene bis(p-carboxybenzamide) were symthesized by melt condensation (Hartmann et al, 1989). A series of amido containing polyanhydrides based on p-aminobenzoic acid were sy nthesized by melt condensation. The polymers melted at 58 to 177°C and had a molecular weight of 2500 to 12400. 

Poly(ester-Anhydrides) and Poly(ether-anhydrides). 4,4 -alkane-and oxa-alkanedioxydibenzoic acids were used for the s5mthesis of polyanhydrides. These polyanhydrides melted at a temperature range of 98-176°C and had Mw up to 12,900. Di- and triblock copolymers of poly(caprolactone) (PCL), poly(lactic acid) (PLA), and poly(hydroxybutyrate) (PHB) have been prepared from carboxylic acid terminated low M prepolymers copolymerized with SA prepolymers by melt condensation (49). Similarly, di-, tri-, and brush copolymers of poly(ethylene glycol) (PEG) with poly(sebacic anhydride) (PSA) have been prepared by melt copolymerization of carboxylic acid terminated PEG (50). 

Method of synthesis polyanhydrides can be prepared by melt condensation polymerization in which dicarboxylic acid monomer reacts with excess of acetic anhydride ... 

Cortisone acetate has been incorporated into several polyanhydrides (15). The rates of release of cortisone acetate from microcapsules of poly(terephthaUc acid), poly(terephthaUc acid-sebacic acid) 50 50, and poly(carboxyphenoxypropane-sebacic acid) 50 50 are shown in Fig. 8. These microcapsules were produced by an interfacial condensation of a diacyl chloride in methylene chloride with the appropriate dicarboxylic acid in water, with or without the crosslinking agent trimesoyl chloride. This process produces irregular microcapsules with a rough surface. The release rates of cortisone acetate from these microcapsules varied correspondingly with the rate of degradation of the respective polyanhydrides. It can be expected that the duration of release of cortisone acetate from solid microspheres, such as those produced by the hot-melt process, would be considerably longer. 

Scheme 2. Polyanhydride synthesis via melt polycondensation involves first the formation of oligomeric acetylated prepolymers, followed by condensation under vacuum. Acetic acid is formed as a byproduct of the second reaction.

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