Amphiphilic copolymers, process

Polymeric micelle formation occurs as a result of two forces. One is an attractive force that leads to the association of molecules while the other one, is a repulsive force, preventing unlimited growth of the micelles to a distinct macroscopic phase (Price, 1983 AstaLeva et al., 1993 Jones and Leroux, 1999). Amphiphilic copolymers form micellar structures through self-association of the insoluble segments when placed in a solvent that is selective for the other monomer (Kataoka et al., 1993 Jones and Leroux, 1999). The process of micellization for amphiphilic copolymers is similar to the process described for conventional hydrocarbon chain-based surfactants as described in the Lrst part of this chapter. 

The polymerization process of two monomers with different polarities in similar ratios is a difficult task due to the solubility problems. Using the miniemulsion process, it was possible to start from very different spatial monomer distributions, resulting in very different amphiphilic copolymers in dispersion [88]. The monomer, which is insoluble in the continuous phase, is miniemulsified in order to form stable and small droplets with a low amount of surfactant. The monomer with the opposite hydrophilicity dissolves in the continuous phase (and not in the droplets). As examples, the formation of acryl-amide/methyl methacrylate (AAm/MMA) and acrylamide/styrene (AAm/Sty) copolymers was chosen using the miniemulsion process. In all cases the synthe-... 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

Such process is described as resusdtable free radical polymerization [204]. It has been applied to the synthesis of poly(a-Me styrene)-b-poly(methyl methacrylate) copolymers from a-methyl styrene oligomers [206] and of amphiphilic copolymers containing polymethacrylic acid and PS sequences [205]. However, macroinitiation from styrene oligomers failed as a probable consequence of the too high stability of the end-groups. 

The self-assembly process of nonionic surfactants in aqueous media differs in several aspects from the mi-cellization of amphiphilic copolymers ... 

Matyjaszewski et al. first used initiators bearing an unsaturated group for the ATRP process of styrene. In 1998, they [318] used vinyl chloroacetate as the initiator for the ATRP of styrene. As VAc was unreactive towards styrene in radical copolymerization, vinyl chloroacetate was able to initiate the ATRP of styrene (Scheme 65). The resulting PS macromonomers, with molar mass ranging from 5 x 103 to 15 x 103 gmol, were copolymerized with N-vinylpyrrolidinone. The amphiphilic copolymers obtained were used as hydrogels. 

In Chapter 4, Massignani, Lomas and Battaglia review the fabrication processes used to form polymersomes, membrane-enclosed structures that are formed through self-assembly of amphiphilic copolymers. The resulting molecular properties, methods to control their size, loading strategies and applications of polymersomes are also detailed. 

Detailed overviews on the micellisation of amphiphilic copolymers in organic solvents are provided in the reviews of Riess [1], Gohy [2], Hamley [7] and Chu et al. [101]. Apparently, a wide range of styrene, acrylate or methacrylate and diene-based block copolymers have been investigated, while AB diblock and ABA triblock architectures have been systematically compared. One of the main conclusions is that the formation of micelles in organic solvents can generally be considered as an entropy-driven process. 

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