Polymers hydrolysis

The metabohc rate of poly(ester—amide) where x = Q has been studied in rats using carbon-14 labeled polymer. This study indicates that polymer degradation occurs as a result of hydrolysis of the ester linkages whereas the amide linkages remain relatively stable in vivo. Most of the radioactivity is excreted by urine in the form of unchanged amidediol monomer, the polymer hydrolysis product (51). 

Poly(vinyl alcohol) used to manufacture the poly(vinyl acetal)s is made from poly(vinyl acetate) homopolymer (see Vinyl polymers, vinyl alcohol polymers Vinyl POLYMERS, vinyl acetate polymers). Hydrolysis of poly(vinyl acetate) homopolymer produces a polyol with predominandy 1,3-glycol units. The polyol also contains up to 2 wt % 1,2-glycol units that come from head-to-head bonding during the polymeri2ation of vinyl acetate monomer. Poly(vinyl acetate) hydrolysis is seldom complete, and for some appHcations, not desired. For example, commercial PVF resins may contain up to 13 wt % unhydroly2ed poly(vinyl acetate). Residual vinyl acetate units on the polymer help improve resin solubiHty and processibiHty (15). On the other hand, the poly(vinyl alcohol) preferred for commercial PVB resins has less than 3 wt % residual poly(vinyl acetate) units on the polymer chain. 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Initial polymer hydrolysis products are the diol or mixture of diols used in the reaction with the diketene acetal, and pentaerythritol dipropionate, or diacetate if 3,9-bis(methylene-2,4,8,10-tetraoxaspiro-[5,5Jundecane) was used. These pentaerythritol esters hydrolyze at a slower rate to pentaerythritol and the corresponding aliphatic acid (13). 

The rate of polymer erosion in the presence of incorporated anhydride and release of an incorporated drug depends on the pK of the diacid formed by hydrolysis of the anhydride and its concentration in the matrix (20). This dependence is shown in Fig. 7 for 2,3-pyridine dicarboxylic anhydride and for phthaUc anhydride. In this study, methylene blue was used as a marker. The methylene blue release rate depends both on the pK and on the concentration of diacid hydrolysis product in the matrix. However, at anhydride concentrations greater than 2 wt%, the erosion rate reaches a limiting value and further increases in anhydride concentration have no effect on the rate of polymer hydrolysis. Presumably at that point Vj, the rate of water intrusion into the matrix, becomes rate limiting. 

The explanted devices were also examined by scanning electron microscopy and the results shown in Fig. 21 (18). The pictures clearly show a progressive diminution of a central uneroded zone and the development of voids around the periphery of the rod-shaped device. The presence of voids suggest that once erosion starts, generation of hydrophilic degradation products at that location accelerates further polymer hydrolysis. 

Kinetic Studies. The pioneering work of Hierl et al. (8) and Delaney et al. (9) had established that hydrolysis of jr-nitro-phenylcarboxylates was an excellent means of observing the nucleophilic catalysis by 4-(dialkylamino) pyridine functionalized polymers. Hydrolysis of p-nitrophenylacetate in a buffer at pH 8.5 showed that the polymer was a slightly better catalyst than the monomeric analog PPY (Table II). However, preliminary results indicate that the polymer bound 4-(dialkylamino) pyridine is more effective as a catalyst than the monomeric analog in the hydrolysis of longer carbon chain p-nitrophenylcarboxylates, such as p-nitrophenylcaproate. 

Assay of Enzymatic Hydrolysis of Synthetic Solid Polymers. Hydrolysis of solid polymers was measured by the rate of their solubilization, and the measurement process does not necessarily involve complete hydrolysis into the constituent parts. The rate was determined by measuring the water-soluble total organic carbon (TOC) concentration at 30 °C in the reaction mixture using a Beckman TOC analyzer (Model 915-B). In the substrate and enzyme controls, enzyme or substrate was omitted from the reaction mixture. 

Middleboe, M., M. Sondergaard, Y. Letarte, and N. H. Borch. 1995a. Attached and free-living bacteria Production and polymer hydrolysis during a diatom bloom. Microbial Ecology 29 231-248. 

Most analysis methods for the determination of carbohydrates in biomass incorporate a two-stage acid hydrolysis to separate individual polymers and hydrolyze them to simple compounds that can be readily analyzed by chromatographic or spectroscopic techniques. The first stage subjects the biomass sample to a concentrated acid that disrupts the noncova-lent interactions between biomass polymers. A second, more dilute stage follows, which is optimized for complete polymer hydrolysis and minimized degradation of monomeric sugars. Failure to remove nonstructural materials may result in incomplete hydrolysis of... 

The liquid portion of biomass-derived process samples may also contain carbohydrate degradation products, such as 5-(hydroxymethyl)-2-furaldehyde (HMF), levulinic acid, and furfural, as well as other components of interest, such as organic acids and sugar alcohols. Portfolio methods are available for the quantitative measurement of these degradation products and byproducts of polymer hydrolysis. 

Mineralization of organic macromolecules is initiated by extracellular enzymes because bacteria are unable to hydrolyze substrates that are much larger than about 600 Da (Weiss et al., 1991). Not all bacteria are capable of synthesizing these enzymes, as is often the case with those responsible for terminal decomposition and some intermediary metabolisms. As a result, these terminal organisms depend heavily on the activities of other bacteria for substrates. It is clear that polymer hydrolysis occurs since these compounds are required to support microbial activities in sediments, but some studies have failed to detect polymer hydrolysis potentials sufficient to support in situ rates of metabolism (Arnosti, 1998). Such studies underscore the difficulties of examining hydrolytic processes. 

---