Supercritical carbon dioxide block copolymers

De Simone et al. synthesized poly(fluoroalkyl acrylate)-based block copolymers for use as lipophilic/C02-philic surfactants for carbon dioxide applications [181]. The particle diameter and distribution of sizes during dispersion polymerization in supercritical carbon dioxide were shown to be dependent on the nature of the stabilizing block copolymer [182]. 

Londono, J. D. Dharmapurikar, R. Cochran, H. D. Wignall, G. D. McClain, J. B. Betts, D. E. Canelas, D. A. DeSimone, J. M. Samulski, E. T. Chillura-Martino, D. Triolo, R. The Morphology of Block Copolymer Micelles in Supercritical Carbon Dioxide by Small-Angle Neutron and X-ray Scattering. 

Colina, C.M., Hall, C.K., and Gubbins, K.E., Phase behavior of PVAC-PTAN block copolymer in supercritical carbon dioxide using SAFT, presented at the 9th International Conference on Properties and Phase Equilibria for Product and Process Design, Kurashiki, Japan, May 20-25, 2001, 2001. 

Duxbury Christopher, J., Wang, W., de Geus, M., Heise, A., and Howdle Steven, M. (2005) Can block copolymers be synthesized by a single-step chemoenzymatic route in supercritical carbon dioxide J. Am. Chem. Soc., 127 (8), 2384-2385. 

Heise, Palmans, de Geus, Villarroya and their collaborators (17,41,42) have been working on a chemoenzymatic cascade synthesis to prepare block copolymers. They combine enzymatic ring-opening polymerization (eROP) and atom transfer radical polymerization (ATRP). The synthesis of block copolymers was successful in two consecutive steps, i.e., eROP followed by ATRP. In the one-pot approach, block copolymers could be obtained by sequential addition of the ATRP catalyst, but side reactions were observed when all components were present from at the onset of reactions. A successful one-pot synthesis was achieved by conducting the reaction in supercritical carbon dioxide. 

We investigated the chemoenzymatic synthesis of block copolymers combining eROP and ATRP using a bifunctional initiator. A detailed analysis of the reaction conditions revealed that a high block copolymer yield can be realized under optimized reaction conditions. Side reactions, such as the formation of PCL homopolymer, in the enzymatic polymerization of CL could be minimized to < 5 % by an optimized enzyme (hying procedure. Moreover, the structure of the bifunctional initiator was foimd to play a major role in the initiation behavior and hence, the yield of PCL macroinitiator. Block copolymers were obtained in a consecutive ATRP. Detailed analysis of the obtained polymer confirmed the presence of predominantly block copolymer structures. Optimization of the one-pot procedure proved more difficult. While the eROP was compatible with the ATRP catalyst, incompatibility with MMA as an ATRP monomer led to side-reactions. A successfiil one-pot synthesis could only be achieved by sequential addition of the ATRP components or partly with inert monomers such as /-butyl methacrylate. One-pot block copolymer synthesis was successful, however, in supercritical carbon dioxide. Side reactions such as those observed in organic solvents were not apparent. 

ZHA ZMng, Y., Gangwani, K.K., and Lemert, R.M., Sorption and swelling of block copolymers in the presence of supercritical carbon dioxide, J. Supercrit Fluids, 11, 115, 1997. 

LAC Lacroix-Desmazes, P., Andre, P., Desimone, J.M., Ruzette, A.-V., and Boutevin, B., Macromolecular surfactants for supercritical carbon dioxide applications Synthesis and characterization of fluorinated block copolymers prepared by nitroxide-mediated radical polymerization (experimental data by P. Lacroix-Desmazes), J. Polym. Sci. Part A Polym. Chem., 42, 3537, 2004. 

In ATRP, polymeric halides are necessary in order to prepare AB block copolymers. This can be achieved either by reactivating the active sites of an isolated macroinitiator (dormant homopolymer) or by in situ addition of a second monomer (sequential monomer addition).This technique can be carried out in environmentally friendly reaction media, such as water, supercritical carbon dioxide, or even ionic liquids. The main advantage of this polymerization type is the fact... 

Reprinted from H. Yokoyama, L. Li, T. Nemoto, Tunable nanocellular polymeric monoliths using fluorinated block copolymer templates and supercritical carbon dioxide. Advanced Materials, 16 (17) (2004) 1542-1546 with permission from John Wiley and Sons. 

Reproduced from T. Shinkai, et al.. Ordered and foam structures of semifluorinated block copolymers in supercritical carbon dioxide. Soft Matter, 8 (21) (2012) 5811 with permission of the Royal Society of Chemistry. 

L. Li, et al.. Facile fabrication of nanocellular block copolymer thin films using supercritical carbon dioxide. Advanced Materials 16 (2004) 1226-1229. 

R. Zhang, et al.. Thermally robust nanocellular thin films of high-Tg semifluorinated block copolymers foamed with supercritical carbon dioxide. Soft Matter 7 (8) (2011) 4032. 

In a carbon dioxide supercritical fluid, polystyrene can be polymerized by such a dispersion polymerization, using polystyrene-h/oc -poly(l,l-dihydroperfluorooctyl acrylate), FOA, as the stabilizer. The perfluorinated moiety, soluble in supercritical carbon dioxide, provides the steric stabilization. Figure 14.14 (51) illustrates the particles of polymer produced in this way. With increasing molecular weight of the stabilizing block copolymer, the particles become smaller and more uniform. 

PMMA is mostly homo- or copolymerized in aliphatic hydrocarbon dispersions, using different rubbers, polysiloxanes, long-chain polymethacrylates, or different block and graft copolymers as stabilizers. An interesting variant of the dispersion polymerization of acrylates is carried out in supercritical carbon dioxide [45,46]. Transition-metal-mediated living radical suspension polymerization is discussed in Ref. [47]. Common radical initiators are described in Refs. [48] and [49]. The entire field is reviewed extensively in Ref. [50]. 

As a solventless route to block copolymers the application of supercritical carbon dioxide in the SFRP process was investigated. This offers additional potential for providing a higher complexity of macromolecular structures in the absence of organic solvents. Due to the increased diffusivity of monomer dissolved in the supercritical CO2 and the plasticization of the polymer, the rate of polymerization of the second block can be increased, and thereby, a one pot synthesis of block copolymers becomes possible [282,283]. 

As an enviromnentally friendly alternative to organic solvent, the use of supercritical carbon dioxide has recently attracted considerable interest. It offers additional advantages as low solution viscosity and the fact of being effectively chemical inert. Fluorinated methacrylates were successfully polymerized in supercritical carbon dioxide and the Hving nature was examined by low PDIs and the synthesis of block copolymers [323]. 

Tai, H. Y., Popov, V. K, Shakesheff, K. M. Howdle, S. M. (2007). Putting the fizz into chemistry applications of supercritical carbon dioxide in tissue engineering, drug delivery and synthesis of novel block copolymers. Biochemical Society Transactions 35 516-521. 

In this chapter, we focus on the fact that fluorine-containing polymers are in a rare class of polymers that have a high afflnity to supercritical carbon dioxide (O Neill et al, 1998). We describe the polymerization of fluorine-containing block copolymers and their use to selectively localize supercritical carbon dioxide in fluorine-containing domains for introducing nanocellular and nanoporous structures. 

As described in the Introduction, perfiuoroalkylated polymer segments play an important role in nanofabrication under supercritical carbon dioxide conditions. It should be mentioned that those nanocellular structures can be defined by the segment ratio and block lengths of perfluorinated polymer segments. Therefore, some well-defined architectural features of block copolymers, such as controlled composition, predictable molecular weight, and narrow molecular-weight distribution, are essential for this study. 

We first focus on fluorine-containing block copolymers that form spherical domains of a fluorine-containing block since they provide well-ordered and densely packed spherical nuclei for nanobubbles of carbon dioxide, which lead to closed nanocells. Such spherical domains of fluorine-containing blocks are expected to work as a template to provide nanocells, which are cells on a nanometer scale and much smaller than conventional microcells using homopolymers and supercritical carbon dioxide. In order for spherical domains to work as nuclei for nanobubbles, an affinity of fluorine-containing domains to carbon dioxide higher than that of the other block domains is essential. 

---