Polymers for Inert Matrices

Ethylcellulose is an ethyl ether of cellulose, prepared by ethylation of the alkali cellulose, whose most important property is to be insoluble in water. Different ethylcelluloses are commercially available varying both the ethoxyl content (45.0-49.5%) (Ethocel , by Dow Chemical Company) and viscosity values (10-100 cP). As a general rule, viscosity increases as the molecular weight (MW) of polymer increases. The lower viscosity grades of this polymer are more compressible and compactible, leading to harder tablets and, consequently, to lower drug dissolution rates [14-16], This increase in com-pactibility has been explained based on the fact that the polymers with lower MW show a less ordered structure [14]. 

Moreover, polymethacrylic acid has been successfully blended with other polymers, both physical and chemically, by copolymerization. For example, physical mixtures with native starches, which are not suitable for delayed drug release, have been reported to be useful as drug delivery systems and to guarantee the insolubility of dosage forms in the acidic medium [17]. In relation to chemical blends, methacrylic monomers are especially indicated for the modification and improvement of natural biopolymers, since they have unique characteristics such as copolymerization ability, stability, and efficacy, which compensate for its high cost [18-21]. Grafting of methacrylic monomer on carbohydrate backbone causes important modifications on the physicochemical properties of the final product such as lower crystallinity and density, higher hydrophobicity 

Polyvinyl acetate (PVAc) is a homopolymer synthesized from vinyl acetate monomer using a free radical polymerization procedure [25]. This water-insoluble polymer is mostly employed blended with other polymers. The most famous association is the physical mixture of PVAc and polyvinyl pirrolidone (PVP), commercially known as Kollidon SR by the chemical company BASF. Kollidon SR consists of 80% PVAc, 19% of PVP and 1% of sodium lauryl sulfate and silica as stabilizers (BASF). The PVP is added as water-soluble polyamide which forms pores into the matrix tablet allowing drug diffusion. Kollidon SR has been shown to be a suitable pH-independent excipient to form matrix tablets, controlling drug release by both diffusion and erosion mechanisms [24]. 

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Polymers for inert matrices ethylcellulose, amino-methacrylate (Eudragit RS), polyvinyl acetate/ polyvidone (mixture, no co-polymer)... 

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