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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