There are two methods of forming a chemically crosslinked network; one alluded to in the preceding pages is the formation of "inadvertent" chemical bonds between multifunctional molecules to form a network. The other is deliberate formation of a network by adding a multifunctional crosslinking agent to a CRP.(6)
The other approach is the creation of physical crosslinks formed by phase separation of a segmented copolymer. Many workers have employed CRP to prepare segmented copolymers capable of phase separation. In the following discussion two examples of physically crosslinked systems, a hydrogel and a supersoft elastomer, are discussed in addition to chemically crosslinked systems and examples of degradable crosslinked gels.(7,8)
An early example of a physically crosslinked gel that targeted a known application was the preparation of a phase seperable graft copolymer that functioned as a hydrogel.(9) Well defined polystyrene macromonomers were prepared by ATRP using vinyl chloroacetate as an initiator. Since styrene and vinyl chloroacetate do not radically copolymerize, no branching or incorporation of the initiator into the backbone was observed and linear vinyl terminated polystyrene was prepared. Macromonomers of several different MW were prepared and copolymerized in a standard free radical polymerization with N-vinylpyrrolidinone, in varying feed ratios, in order to produce poly(NVP-g-Sty) graft copolymers. The macromonomers used were of sufficiently high molecular weight to undergo phase separation and form physical crosslinks when the graft copolymers were dispersed in solvents, such as water, which favor the hydrophilic NVP preventing the copolymer from dissolving and allowing it to swell. These recyclable thermoplastic materials formed hydrogels with swell-abilities in water exceeding 95%, depending on the amount of styrene that was incorporated into the copolymer.(9,10)
In a later example, examining the properties of a chemically crosslinked hydrogel, the benefit of a CRP was seen when gels formed by ATRP copolymerization of 2-(N,N-dimethylamino)ethyl methacrylate (DMAEMA) and ethylene glycol dimethacrylate (EGDMA) using ethyl 2-bromoisobutyrate as the initiator were analyzed, via a comparison to Flory's gelation theory.(11) The copolymers were found to be more homogeneous than similar hydrogels prepared by conventional free-radical polymerization methods.(12) This observation is consistent with the work of Fukuda using Nitroxide Mediated Polymerization (NMP) to form well defined polystyrene gels.(13)
A hybrid approach to hydrogel formation was developed when PEG-b-PNIPAM block copolymers were synthesized by ATRP of NIPAM using a PEG macro-initiator. The block copolymer is soluble in water at 25 °C but phase-separates to form micelles when the temperature is raised to 50 °C, i.e. above the LCST of the PNIPAM block in the PEG-b-PNIPAM block copolymer.(14) A small amount of N,N'-ethylenebisacrylamide can be added as cross-linker to the reaction system to prepare stable hydrogel nanoparticles in water at room temperature. The size of nanoparticles is controlled by the composition of the mixed solvent.(14) Thermo-reversible hydrogel networks are of interest as biocompatible scaffolds with switchable properties between room temperature and physiological temperature. Targeted applications include regenerative medicine, cell engineering, transdermal patches, and implants. Materials based on P(MEO2MA-co-OEGMA) were synthesized to demonstrate that oligo(ethylene glycol) methacrylates constitute a unique platform for preparing bio-relevant thermogels with optimal properties under near physiological conditions.(15)
An example of post polymerization photo-crosslinking was examined by preparation of well-defined copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA) and benzophenone methacrylate (BPMA).(16) The MW of the copolymers was held close to 30 000 g/mol, while the BPMA content varied from 2.5 to 10 mol %. The copolymers with a low content of BPMA (2.5 and 5 mol %) exhibited a sharp thermal transition at 33-36 °C in aqueous solution. The hydrogel was immobilized and patterned on a silicon wafer via UV treatment of the spin-coated polymer layer using a photo-mask technique. The thermo-responsive behavior of the patterned polymer gel was quantitatively investigated by variable temperature in situ contact mode atom force microscopy, which revealed the presence of two lower critical solution temperature regions. One region was between 25 and 30 °C, corresponding to the topmost layer of the hydrogel film, and the other region, around 40 °C, corresponded to the bulk of the hydrogel. Concurrent lateral force microscopy measurements revealed that, just above the transition temperature, the bulk region exhibited enhanced friction properties.
AGET ATRP of di(ethylene glycol) methyl ether methacrylate (M(EO)2MA) was successfully conducted in miniemulsion at 65 0C. The reaction system was stable without diffusion of monomer and polymer into the aq. phase because the monomer is water-insoluble and PM(EO)2MA becomes hydrophobic above 25 0C. The polymerization was well-controlled when hydrazine, a mild water-soluble reducing agent, was employed to activate the catalyst complex yielding PM(EO)2MA homopolymer with narrow molecular weight distribution (Mw/Mn = 1.2-1.6). Using this technique, well-defined PM(EO)2MA microgels were prepared with degradable disulfide cross-linker. The microgels became magnetic after physical loading oleic acid-coated Fe3O4 nanoparticles, which could not diffuse out of the microgels due to their hydrophobicity. Thermally responsive and drug loading-releasing behavior of the magnetic microgels was studied using Rhodamine B as a model for hydrophilic drugs. The drug releasing behavior can be well-controlled by both temperature and addition of reducing agent, indicating that the PM(EO)2MA magnetic microgels could find potential application for controlled targeted drug delivery.(17)
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