Water-Soluble Hydrophilic Polymers

Soluble matrix systems. The third matrix system is based on hydrophilic polymers that are soluble in water. For these types of matrix systems, water-soluble hydrophilic polymers are mixed with drugs and other excipients and compressed into tablets. On contact with aqueous solutions, water will penetrate toward the inside of the matrix, converting the hydrated polymer from a glassy state (or crystalline phase) to a rubbery state. The hydrated layer will swell and form a gel, and the drug in the gel layer will dissolve and diffuse out of the matrix. At the same time, the polymer matrix also will dissolve by slow disentanglement of the polymer chains. This occurs only for un-cross-linked hydrophilic polymer matrices. In these systems, as shown in Fig. 5.3, three fronts are formed during dissolution9-11 ... 

Amongst the various water-soluble or water-swellable polymers with high molecular weight used in hydrophilic matrices, hypromellose (hydroxypropylmethylcellulose [HPMC]) is the most commonly used polymer [5,13,37-40]. Other polymers have been studied and used on their own or in combination with HPMC to successfully modulate drug release. Examples include polyethylene oxide (PEO), with a recent review looking at its application in controlled release tablet systems [41]. Typical water-soluble hydrophilic polymers used may be classified based on their chemistry as follows ... 

A novel polymerized vesicular system for controlled release, which contains a cyclic a-alkoxyacrylate as the polymerizable group on the amphiphilic structure, has been developed. These lipids can be easily polymerized through a free radical process. It has been shown that polymerization improves the stabilities of the synthetic vesicles. In the aqueous system the cyclic acrylate group, which connects the polymerized chain and the amphiphilic structure, can be slowly hydrolyzed to separate the polymer chain and the vesicular system and generate a water-soluble biodegradable polymer. Furthermore, in order to retain the fluidity and to prepare the polymerized vesicles directly from prev lymerized lipids, a hydrophilic spacer has been introduced. 

In the most succinct sense, a hydrogel is simply a hydrophilic polymeric network cross-linked in some fashion to produce an elastic structure. Thus any technique which can be used to create a cross-linked polymer can be used to produce a hydrogel. Copolymerization/cross-linking free radical polymerizations are commonly used to produce hydrogels by reacting hydrophilic monomers with multifunctional cross-linkers. Water-soluble linear polymers of both natural and synthetic origin are cross-linked to form hydrogels in a number of ways ... 

Dry strength additives are usually water soluble, hydrophilic natural or synthetic polymers, the commercially most important of which are starch, natural vegetable gums and polyacrylamides. These polymers are often made in cationic form by the introduction of tertiary or quaternary amino groups into the polymer, and are therefore polyelectrolytes. They are thus also able to function to some extent as drainage and retention aids. 

Polyvinyl alcohol (PVA), which is a water soluble polyhidroxy polymer, is one of the widely used synthetic polymers for a variety of medical applications [197] because of easy preparation, excellent chemical resistance, and physical properties. [198] But it has poor stability in water because of its highly hydrophilic character. Therefore, to overcome this problem PVA should be insolubilized by copolymerization [43], grafting [199], crosslinking [200], and blending [201], These processes may lead a decrease in the hydrophilic character of PVA. Because of this reason these processes should be carried out in the presence of hydrophilic polymers. Polyfyinyl pyrrolidone), PVP, is one of the hydrophilic, biocompatible polymer and it is used in many biomedical applications [202] and separation processes to increase the hydrophilic character of the blended polymeric materials [203,204], An important factor in the development of new materials based on polymeric blends is the miscibility between the polymers in the mixture, because the degree of miscibility is directly related to the final properties of polymeric blends [205],... 

Water soluble polymers are well represented in the human environn nt and in food. Thus, our very existence constitutes solid proof of the lack of the physiological effects of many of these compounds. Nevertheless, some water soluble synthetic polymers, even at very low concentrations, influence enzymatic processes that form the basis of the physiology of the body. The reason for a general lack of bioactivity of synthetic polymers on the organism s level is the inability of polymers to penetrate to the location where the body s basic biochemical processes occur. The human body s most prevailing component is water (>fi)%). However, this body of water is not a continuous phase, it is subdivided by lipid membranes into spaces of microscopic size. Lipids constitute about 15% of body weight and a considerable portion of that amount is used to form and maintain cellular membranes, a structural element of the body that diminishes the mobility of hydrophilic polymers in organisms. 

In an inverse emulsion polymerization, a hydrophilic monomer, frequently in aqueous solution, is emulsified in a continuous oil phase using a water-in-oil emulsifier and polymerized using either an oil-soluble or water-soluble initiator the products are viscous latices comprised of submicroscopic, water-swollen, hydrophilic polymer particles colloidally suspended in the continuous oil phase. The average particle sizes of these latices are as small as 0.05 microns. The technique is applicable to a wide variety of hydrophilic monomers and oil media. The inverse emulsion polymerization of sodium p-vinylbenzene sulfonate initiated by both benzoyl peroxide and potassium persulfate was compared to the persulfate-initiated polymerization in aqueous solution. Hypotheses for the mechanism and kinetics of polymerization were developed and used to calculate the various kinetic parameters of this monomer. 

In an inverse emulsion polymerization, an aqueous solution of a hydrophilic monomer is emulsified in a continuous hydrophobic oil phase using a water-in-oil emulsifier. The polymerization is initiated with either oil-soluble or water-soluble initiators. Figure 2 shows a schematic representation of this system. The formation of micelles is uncertain, but is portrayed speculatively. The hydrophilic part of the emulsifier molecule is oriented toward the hydrophilic dispersed phase and the hydrophobic part toward the hydrophobic continuous phase. The initiation of polymerization proceeds by a mechanism analogous to that of the conventional system and submicroscopic particles of water-swollen hydrophilic polymer are generated in the continuous oil phase. 

The selection of polymer is critical. If too water soluble, the polymer will not adsorb very well on the droplet surface. If not hydrophilic enough, the polymer will lie flat on the surface, so that van der Waals attraction can again take place. 

Free-radical and reagent absorption. Free radicals, monomers, and other reagents are transported into the polymer particles. The free radicals are likely to be oligomers because a hydrophilic ion-radical would remain in the aqueous phase. Monomers and other reagents can diffuse from the monomer droplets to the particles if they possess adequate water solubility. Dissolved polymer and other water-insoluble ingredients would remain in the droplets. 

Methyl methacrylate (MMA) and sodium styrene sulfonate (SSNa) are water-soluble. These polymers behave like a low MW surfactant as they form micelles in aqueous solution in which the hydrophobic part is directed towards the centre and the hydrophilic part is situated on the periphery of the micelle. Owing to such features, amphiphilic block copolymers have wide-ranging applications in drugs, pharmaceuticals, coatings, cosmetics and paints. They also exhibit very high antibacterial activities. Oikonomou and co-workers used ATRP to prepare amphiphilic block copolymers, consisting of polymethyl methacrylate (PMMA) and poly (sodium styrene sulfonate) (PSSNa) blocks [18]. The synthesis methods are described below. 

Aqueous SEC was first reported in 1959 by Porath and Flodin [1]. They separated proteins and salts according to molecular size by using cross-linked dextran gels. Since then it has been widely employed, especially in the field of biochemistry, for various purposes such as purification of proteins and nucleic acids, estimation of molecular masses of proteins and determination of molecular mass distributions of polysaccharides. In addition, it has been a powerful tool for the determination of molecular mass distributions of water-soluble synthetic polymers since high-performance aqueous SEC was realized in 1978 by the development of semirigid microparticulate macroporous supports based on hydrophilic synthetic polymers [2-4]. 

Polyphosphazenes that bear -D-glucosyl side groups cosubstituted with methyl-amine, alkoxy or aryloxy side groups have been symthesized (Allcock et al, 1991) (Figure 22) resulting in a range of polymers with varying water solubility, hydrophilic-ity or hydrophobicity. 

Non-ideal transport becomes evident if the permeating molecule plasticizes the membrane. Plasticization is normally induced by the high solubility of the permeating molecule in the membrane which results in swelling of the membrane. Both diffusion coefficients and solubility coefficients may be concentration dependent. Examples of such plasticization include CO2 in cellulosics [15], organics such as toluene in silicones [16], and water in hydrophilic polymers. 

Poly(aspartic acid) (PAA) is another promising biomaterial synthesized from aspartic acid. It is a highly water-soluble ionic polymer which is readily degraded by lysosomal enzymes. It can be readily prepared as a hydrogel for various biomedical applications, and it has also been copolymerized with PLA, PCL, and PEG to improve its mechanical performance and reduce its hydrophilicity and rate of degradation. 

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