Block copolymers

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Chapter: Pharmaceutical Drugs and Dosage: Pharmaceutical polymers

Block copolymers consist of two or more repeating units in a specific pattern.

Block copolymers

Block copolymers consist of two or more repeating units in a specific pat-tern. The different repeating units of a block copolymer differ in chemical structure and physicochemical properties. For example, in a block copo-lymer of the type AAABBBAAA, in which A is water-soluble repeating unit and B is water-insoluble repeating unit, the insoluble parts tend to colocalize and aggregate in solution. Poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) (PEO-PPO-PEO) block copolymers (Figure 11.5), com-mercially known as Pluronic® or Poloxamer, exhibit such properties and are used as nonionic surfactants. In addition, aqueous solutions of some Poloxamers exhibit temperature-induced phase transitions from solution to gel, when the polymer concentration is above a critical value.

Figure 11.5 Chemical structure of poly(ethylene oxide-co-propylene oxide-co-polyethylene oxide) (PEO-PPO-PEO) (commercially known as Pluronic and poloxamer).

Block copolymer micelles are of great interest due to the following reasons:

·           Hydrophobic drugs can be physically entrapped in the core of block copolymer micelles and transported at concentrations that exceed their intrinsic water solubility. An important property of micelles is their ability to increase the solubility of materials that are normally insoluble or only slightly soluble in the dispersion medium used.

·           Hydrophilic blocks, which are often composed of PEO or PEG, can form a tight shell, or corona, around the micellar core. Diblock copo-lymer micelles with a PEO corona resist protein adsorption and cell adhesion. This helps prevent recognition by the phagocytotic reticu-loendothelial system (RES) cells, which is a rapid metabolism and elimination pathway for sensitive drug delivery systems.

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