Polymer hydrolysis effect

The many circumstances leading to the Henri equation for enzyme conversion of soluble substrates are first noted, followed by some kinetic forms for particulate and polymer hydrolysis. Effects common to immobilized enzyme systems are summarized. Illustrative applications discussed Include metabolic kinetics, lipid hydrolysis, enzymatic cell lysis, starch liquefaction, microenvironment influences, colloidal forces, and enzyme deactivation, all topics of interest to the larger themes of kinetics and thermodynamics of microbial systems. 

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. 

Several assumptions were made in using the broad MWD standard approach for calibration. With some justification a two parameter equation was used for calibration however the method did not correct or necessarily account for peak speading and viscosity effects. Also, a uniform chain structure was assumed whereas in reality the polymer may be a mixture of branched and linear chains. To accurately evaluate the MWD the polymer chain structure should be defined and hydrolysis effects must be totally eliminated. Work is currently underway in our laboratory to fractionate a low conversion polydlchlorophosphazene to obtain linear polymer standards. The standards will be used in polymer solution and structure studies and for SEC calibration. Finally, the universal calibration theory will be tested and then applied to estimate the extent of branching in other polydlchlorophosphazenes. 

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. 

Summarising, the paint-seawater mechanism includes the following rate-influencing steps hydrolysis and erosion of the active TBT-polymer binder, effective diffusion in the leached layer of dissolved pigment species and TBTC1,... 

Increasingly sophisticated catalytic domains have been synthesized and used as adducts to the framework polymers. These synthetic macromolecules show substantially enhanced catalytic effects on hydrolytic reactions, decarboxylation, Schiffbase hydrolysis, aromatic nucleophilic substitution, and oxidation [63-69]. Several of these synthetic polymers are effective peptidases and nucleases. 

Szabo (1979) reported the increases in the viscosity of AM/AMPS copolymer solntion when NaOH was added. All these observations are probably related to early time hydrolysis effect. Adding alkali also increases electrolytes, which shonld decrease polymer solution viscosity. Even without alkali, hydrolysis will occur. Therefore, for the long term, the effect used to increase hydrolysis will become less important than the salt effect, and the polymer viscosity will decrease. These statements are consistent with those reported by Flournoy et al. (1977) in that the apparent viscosity was very dependent on pH, with the maximum apparent viscosity occurring at a pH of about 6 to 10 for polyacrylamide and at a pH of abont 4 to 9 for polysaccharide. Considering the aging effect, the relationship between polymer viscosity and pH or alkali becomes more complex this issue is discussed in more detail in Section 11.2. 

Alkali has two main effects on polymer viscosity increased salt effect and increased hydrolysis effect. The former decreases viscosity, whereas the latter increases viscosity. The final viscosity increases or decreases depends on the balance of the two effects. In general, the salt effect is greater. Thus, polymer viscosity decreases with alkaline concentration. 

FIGURE 11.5 Effects of polymer hydrolysis, type of alkali, and contact time with crude on IFT, 3315 mg/L TDS water, 60°C, 0.50 mg KOH/g acid number. The NaOH concentration was 1%, whereas the Na2C03 concentration was 3%. Source Data from Sheng et ah (1993). 

Figure 13.20 shows the polymer (HPAM) effect on 0/W emulsion stability. In low polymer concentrations, the stability was not very sensitive to the concentration. When the concentration was above 150 mg/L, the stability was significantly improved. 0/W emulsion stability is controlled by the strength of the water film between oil droplets. The existence of polymer in water significantly increases the water film s strength and water phase viscosity. Therefore, HPAM has a significant effect on 0/W emulsion stability. It has also been observed that emulsion stability increased with polymer hydrolysis (Kang, 2001). 

Unlike the influence of morphology on mechanical behavior, the effect on biological behavior is a less mature area of endeavor. For absorbable polymers, the effect of crystalline structure on diffusion and reaction rates provides insight. The relative amount of crystalline phase influences the rate of diffusion of water into a hydrolytically unstable polymer. Furthermore, the rate of hydrolysis of a given ester group in the polymeric chain will depend on whether the group resides in a self-protecting crystal or whether it exists in an unprotected, easily accessed, amorphous phase. 

Zhang Y, Zale S, Sawyer L et al (1997) Effects of metal salts on poly (oL-lactide-co-glycolide) polymer hydrolysis. J Biomed Mater Res 34 531-538... 

Condensation polymers particularly effective as encapsulating materials are listed in Table 11. The six polyesters cited in the table have all demonstrated hydrolytic sensitivity in and on the human body. Equally important, the hydrolysis by-products of these polyesters are biocompatible. This means that as the encapsulated material is released and the encapsulation particle is broken down, the by-products can be processed by the body s waste disposal systems. Research efforts continue to explore new combinations of these esters to create new and more useful polymer properties (180). 

Leonhardt et al were able to show a specific hydrolytic effect when treating nitrophenyl esters with an MIP imprinted with pyridine derivatives of N-boc-amino acids using DVB and 4(5)-vinylimidazole as monomers in combination with chelated Co ions. Compared to the control polymer, hydrolysis was accelerated by a factor of 4 to 5, while a comparison with MIP imprinted with other pyridine derivatives of N-boc-amino acids only gave an acceleration of the reaction by a factor of 2 to 3. Robinson et al used phosphonates as TSA and showed a catalytic effect of an MIP imprinted with p-nitrophenylmethyl phosphonate, using 4(5)-vinylimidazole and Co ions, on the hydrolysis of p-nitrophenol acetate. The authors— aware of the fact that imidazole containing polymers in general exhibit catalytic effects—could nevertheless demonstrate that the imprinted specimens were of 60% higher activity than the control polymers. 

Systematic studies of the acid-catalysed hydrolysis of esters in the presence of sulphonated polystyrene showed an increase in the value of q with decreasing solubility of the esters in water [17, 22]. Correspondingly, for soluble macromolecular acids the values of q have been found to decrease with increasing degree of sulphonation of the polymer. These effects are consistent with hydrophobic interaction between resin and substrate and its effect on the value of Ar. The importance of hydrophobic interaction can be reduced if, for example, acetone is added to the solvent. Such solvent changes have the opposite effect on hydrophilic substrates [26]. 

Y. Zhang, S. Zale, L. Sawyer and H. Bernstein, "Effects of metal salts on poly(DL-lactide-co-glycolide) polymer hydrolysis", J. Biomed. Mater. Res., 34, 531-538 (1997). 

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