Pluronic-type copolymers

Very well known commercially available block copolymers showing micellar behavior are polymers of the Pluronic type. Pluronic-type copolymers are symmetric triblock copolymers with the central block of hydrophobic polypropylene... 

The group of Goldfarb and coworkers have in recent years explored how (spin-labeled) thermoresponsive triblock copolymers of the Pluronic -type (PEO-PPO-PEO, poly(ethylene oxide)-poly(propyleneoxide)-poly(ethyleneoxide)) can be used to build templates, e.g., for the formation of mesoporous frameworks [93, 94]. These structures bear great potential as carrier materials for catalysts and hence could aid societal needs in energy and sustainability. 

One of the few reports on the preparation of mesoporous sihca nanofibers with a soft template other than triblock copolymers of the Pluronic type deals with the synthesis of sihca nanofibers with high aspect ratios containing linear arrays of mesopores by a solution-induced self-assembly process (Fig. 23), as previously reported for thin-film configurations [184]. To this end, PS-fo-PEO diblock copolymers were employed as structure-directing agents in sol solutions containing toluene/ethanol mixtures. For a Dp-value of 35 nm, a single line of mesopores formed, for a Dp-value of 60 nm two parallel rows of mesopores were obtained [185]. 

Pluronic-type PEO-PPO-PEO triblock copolymers were anchored into bilayers of soybean lecithin to prepare sterically stabihzed vesicles (203-205). Vesicles with anchored pol5mier are reported to have an increased stability with respect to cation (Na+, Mn +)-induced flocculation. Also the addition of other hydropho-bically modified polymers to lipid vesicles lead to a stabilization of the vesicles (206). 

For triblock copolymers of the Pluronics type, the interaction is now well established. These BCPs behave very similarly when mixed with different surfactants. The behavior is qualitatively independent of the block lengths and also independent of the type of surfactant used. At high surfactant concentration, a complete decomposition of the polymer micelles is usually observed. In the final state, the mixed micelles only contain a single polymer molecule. This behavior is caused by the interaction of the hydrophobic PPO block with the alkyl tails of the different surfactants. It is also a common feature of these mixed systems that low surfactant concentrations lead to a decrease of the cmt of the different Pluronics. However, for other triblocks the available number of studies is rather small and it is not necessarily possible to transfer the outcome of the studies on the behavior of Pluronics to other systems. 

Exploiting ATRP as an enabling technology, we have recently synthesised a wide range of new, controlled-structure copolymers. These include (1) branched analogues of Pluronic non-ionic surfactants (2) schizophrenic polymeric surfactants which can form two types of micelles in aqueous solution (3) novel sulfate-based copolymers for use as crystal habit modifiers (4) zwitterionic diblock copolymers, which may prove to be interesting pigment dispersants. Each of these systems is discussed in turn below. 

The poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers were also used to gel the continuous aqueous phase. Poloxamers may be used as the secondary hydrophilic surfactant in the preparation of the w/o/w system, and the finished emulsion is then irradiated. The polymerisation reaction can be monitored by cone-and-plate viscometry. Fig. 9 shows the flow curve obtained for a water/isopropyl myristate/water emulsion as a function of the radiation dose. As the dose of y-irradiation is increased, the viscosity of the w/o/w emulsion increased up to a gel-point1. The gel-point of the emulsion is dependent on the type and concentration of poloxamer. In the example shown, prepared using a mixture of 5% (w/v) Pluronic F87 and 5% (w/v) Pluronic F88 in the external phase, the gel-point was reached at 4.2 (Fig. 9). Fig. 10 shows the changes in the properties of irradiated systems on storage. 

More recently, investigations of the solution behavior of block copolymers of the polyethylene oxide) (PEO)-poly(propylene oxide) (PPO) type have been extended to nonaqueous, polar solvent systems. The block copolymer Pluronic... 

Kim and his colleagues (51-55) designed ABA-type triblock polymers that, as do Pluronics, exhibit thermo-responsive gelation at body temperatme. As this book has devoted one chapter for these thermogeUing triblock copolymers and their applications in drug delivery systems, only limited information will be given here. 

The Pluronic and Tetronic surfactants (BASF) are examples of the polyoxyethylenated (PEO) polyoxypropylene (PPO) glycol type of nonionic surfactants. The Pluronic surfactants, are PEO-PPO-PEO triblock copolymers in which the PEO portion constitutes between 10 and 80% of the copolymer [92]. Pluronic F38 and F68 (both with HLB > 24) are typical examples of these surfactants used in emulsion polymerization formulations. The Tetronic surfactants are tetrafunctional block copolymers prepared from the addition of FPO and PEO to ethylene diamine, lypical examples of the Tetronic surfactants used in emulsion polymerization are Tetronic 707 and 908 (HLB > 24). The addition of these types of surfactants enhances freeze-thaw, shear and electrolyte stability, results in low foam formation and typically decreases the water sensitivity. 

Stable double emulsions, based on various block copolymers of polyethylene oxides and polypropylene oxides known as Pluronies, have been used. In a recent example. Cole and Whateley (84) have used complexes of Pluronic F127 PAA (polyaerylie aeid) in the internal aqueous phase. In the oil phase, Span 80 and Pluronie LlOl (5 wt %) were used. The outer interfaee was stabilized by xanthan gum (0.25 wt %) and Tween 80 (1 wt %). Theophylline and I-insulin (iodinated insulin) were ineorporated in the internal aqueous phase of the stabilized multiple emulsion, and the release rates were studied. The release rates were found to be related to the droplet sizes of the emulsion whieh were dependent on the partiele size of the pluronie F127 PAA eomplex in the internal aqueous phase and the type of the lipophilie surfaetant in the oil phase. The authors have used the eomplex between the poloxamer surfactant and PAA that occurred at pH 2 and at low molar ratio as a barrier for the release of active matter from the inner to the outer phase. 

The most convenient polymeric surfactants are those of the block and graft copolymer type. A block copolymer is a linear arrangement of blocks of variable monomer composition. The nomenclature for a diblock is poly-A-block-poly-B, and for a triblock it is poly-A-block-poly-B-poly-A. One of the most widely used triblock polymeric surfactants are the Pluronics (BASF, Germany) or Synperordc PE (ICI, U.K.), which consists of two poly-A blocks of poly(ethylene oxide) (PEO) and one block of poly(propylene oxide) (PPO). Several chain lengths of PEO and PPO are available. More recently, triblocks of PPO-PEO-PPO (inverse Pluronics) became available for some specific applications. 

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