Polymer surface hydrolysis

Convincing evidence for a surface erosion process is shown in Fig. 8, which shows the concomitant release of the incorporated marker, methylene blue, release of the anhydride excipient hydrolysis product, succinic acid, and total weight loss of the device. According to these data, the release of an incorporated drug from an anhydride-catalyzed erosion of poly (ortho esters) can be unambiguously described by a polymer surface erosion mechanism. 

Polymer properties, influence of ions, 258 Polymer surface reactions, kinetics, 322-323 Polymer transformation reactions configurational effect, 38 conformational effects, 38 hydrolysis of polyfmethyl methacrylate), 38 neighboring groups, 37-38 quaternization of poly(4-vinyl pyridine), 37-38 Polymerization, siloxanes, 239... 

The mode of action of the antifouling polymers thus conforms to the bulk abiotic bond cleavage model. All the controlling factors, viz., diffusion of water into the polymer matrix, hydrolysis of the tributyltin carboxylate, diffusion of tributyltin species from the matrix to the surface, phase transfer of the organotin species, and its migration across the boundary layer, are analyzed. It is found that the transport of the mobile tributyltin species in the matrix is the rate limiting factor. 

Hydrolases in Polymer Chemistry Part III Synthesis and Limited Surface Hydrolysis of Polyesters and Other Polymers... 

Table 1 Monitoring enzymatic surface hydrolysis of polymers...

Guebitz GM, Cavaco-Paulo A (2008) Enzymes go big surface hydrolysis and functionalisation of synthetic polymers. Trends Biotechnol 26 32-38... 

Chapter 5 shows that the application of hydrolytic enzymes is a powerful yet mild strategy to directly improve polymer surface properties (i.e. hydrophilicity) or activate materials for further processing. The surface hydrolysis of polyamides (PA), polyethyleneterphthalates (PET) and polyacrylonitriles (PAN) is discussed, as well as the mechanistic details on the enzymatic surface hydrolysis. The mechanistic data, combined with advances in structural and molecular biology, help to explain different activities of closely related enzymes on polymer surfaces. 

It is customary to stop the hydrolysis of PVAc before all the acetyl groups are removed. Thus the commercial product, with a degree of hydrolysis of about 88%, is readily soluble in water but is resistant to less polar solvents, such as benzoie and gasoline. PVA fibers (Kuralon) are strong and insoluble in water because of a surface treatment with formaldehyde which reacts with the surface hydroxyl groups to produce polyvinyl formal on the polymer surface. 

Figure 3.21 Bioerodable polymers can be used for the controlled release of pharmaceutical molecules (black clipscs). Ideally, hydrolysis of the implanted matrix polymer should occur at the polymer surface only so that the drug molecules are released at a constant rate—a so-called zero-order release profile.

The results showed that all batch polymerizations gave a two-peaked copolymer compositional distribution, a butyl acrylate-rich fraction, which varied according to the monomer ratio, and polyvinyl acetate. All starved semi-continuous polymerizations gave a single-peaked copolymer compositional distribution which corresponded to the monomer ratio. The latex particle sizes and type and concentration of surface groups were correlated with the conditions of polymerization. The stability of the latex to added electrolyte showed that particles were stabilized by both electrostatic and steric stabilization with the steric stabilization groups provided by surface hydrolysis of vinyl acetate units in the polymer chain. The extent of this surface hydrolysis was greater for the starved semi-continuous sample than for the batch sample. 

More recently, Chen et al. described a surface modification whereby the polymer poly(Ar,Ar-dimethyl-Af-(ethoxycarbonylmethyl)-Ar-[2/-(methacryloyloxy)ethyl]-ammonium bromide) was grafted from a surface via ATRP [136], The cationic polymer effectively kills E. coli and is subsequently converted into a zwitterionic polymer by hydrolysis of the head group (Fig. 9). It then repels all attached cells dead or alive. This is the first example of a surface that can kill microbes on contact and repels them after that. The only downside of this elegant system is that it will eventually exhaust and turn into a more or less effective repelling surface. 

Cordes and co-workers 191 found that the alkaline hydrolysis of p-nitrophenyl hexanoate is subject to catalysis by polyvinylpyridine-based polysoaps. For example, k bs is increased from 0.1 mm to 1.4 mm in the presence of 5 x 10 7 M 38% polysoap (23) (the same material used in the Strauss work). With 5 x 10-7 M polymer having a 15 % dodecyl content, the rate is increased only 3 times above background. The simplest rationale for the kinetics invokes both hydrophobic and electrostatic forces. Thus, dodecyl chains on the polymer hydrophobically bind p-nitrophenyl hexanoate to the polymer surface. Since the polymer possesses a high density of cationic nitrogens, hydroxide ions also accumulate at the polymer surface where they catalyze the hydrolysis of bound ester. Addition of nitrate ion to the aqueous reaction... 

Adherent, conformal LPD ceramic film either crystalline (Method 2) or amorphous (Method 1) can be obtained on polyimides. This is in contrast to silanol-bearing surfaces (good for Method 1 only) or sulfonated surfaces (good for Method 2 only). The fact that the polyimides accommodate both titania preparations may be due to the partial hydrolysis of the polymer surface under oxide deposition conditions. This provides a mix of carboxylic acid and amide sites that anchor the titania by a combination of coulombic and chelation-based effects.22 An important lesson of this work is that the interaction of the polymer surface with the deposition solution may create oxide film anchoring sites. This does not negate activating the polymer surface. It recognizes that the polymer surface can react further under the deposition conditions. 

An extensive study of the catalytic hydrolysis of peptides and proteins by Dowex-SO was conducted by Deathrage and co-wodcers (8-11). They found that, in general, the ion exchange resin is superior as add catalyst to HQ [Eq. (3—1)]. For exanq>le, most dipeptides are hydrolyzed about 100 times faster. Ihis was attributed to binding of protonated free amino groups to the polyanion and concentration of the hydrogen ion at the polymer surface. 

Silicic acid esters, Si(OR)4, are produced by the reaction of SiCl4 with the appropriate alcohols. The most important representative of this group is tetraethoxysilane (tetraethyl orthosilicate) Si(OC2H5)4, which is used directly, or after hydrolysis to ethylpolysilicates, as a binder for ceramic pastes, for inorganic zinc dust paints (corrosion protection), for the surface treatment of glass and for the modification of silicates. Silicic acid esters are further used for rendering polymer surfaces scratch-resistant. 

Alternatively, hyperbranched polymers can be prepared in situ on the monolayer surface. Treatment of the acid-terminated layer with chloroformate and amine-terminated poly(f-butyl acrylate) produces ester groups on the surface. Hydrolysis of the ester functions with tosic acid gives carboxyl groups. The whole procedure is then repeated several times... 

Thus, considerable effort has been devoted to the development of polymers that are able to erode by a well controlled surface hydrolysis process. Dominant among these are polyanhydrides [2] and poly (ortho esters) [3]. 

This process is schematically shown in Fig. 8 where it has been analyzed in terms of the movement of two fronts, Vl5 the movement of a hydrating front and V2, the movement of an erosion front [29]. Clearly, the ultimate behavior of a device will be determined by the relative movement of these two fronts. If Vi > V2, the thickness of the reaction zone will gradually increase and at some time, the matrix will be completely permeated by water. At that point, all ortho ester linkages will hydrolyze at comparable rates and bulk hydrolysis will take place. However, if Vx = V2, then hydrolysis is confined to the surface layers and only surface hydrolysis will take place. In this latter case, rate of polymer erosion will be completely determined by the rate at which water intrudes into the polymer. 

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