Copolymerization and Crosslinking

We have considered so far only copolymerization of bifunctional vinyl monomers. However, if any of the monomers in the copolymerization is a divinyl compound 

We have considered so far only copolymerization of bifimctional vinyl monomers. However, if any of the monomers in the copolymerization is a divinyl compound or any other ole rue monomer with functionabty greater than 2, a branched polymer can result and, furthermore, the growing branches can intercormect to form an in nite crosslinked network known as gel . It is useful to be able to predict the conditions imder which such gel formation will occur. 

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Copolymerization and crosslinking processes are very important in the paint and coating industry. The theoretical description of such processes is helpful in understanding the essential parameters in these complex operations and in improving process conditions and product specifications with a minimum of time-consuming systematic experiments. 

Functionalized biopolymers have successfully taken over the era of synthetic polymers because they are cost effective, biodegradable, environmentally friendly and efficient. These biopolymers, after graft copolymerization and crosslinking, can be used for the sustained release of drugs. Based on relative rates of diffusion (Rjjff) and rate of polymer relaxation (R,, ), the diffusion of drugs depends upon the value of Diffusion Exponent (n). If n < 0.5, the R < R, . and if n a 1.0, the R. > whereas in case of n> 0.5 and < i.o, ... 

The physical and mechanical properties of chitosan can be amefiorated by using graft copolymerization and crosslinking. Chitosan forms aldimines and ketimines with aldehydes and ketones, respectively. Upon hydrogenation with simple aldehydes, chitosan produces A-alkyl chitosan [60]. The physicochemical and biological properties [61] as well as conformational structures [62] of chitosan are very effective for biomedical applications. 

Tsuchida and co-workers (97) tried similar crosslinking with branched and linear poly(ethylene imin ) and W th poly(vinylimidazole) in the presence of Cu or Co ion templates, but no enhancement of selectivity towards the same metal ions was achieved. The same was true when they prepared a polymer from a monomeric complex (98). In this case the metal-1-vinylimidazole complex (from Ni, Co or Zn as the template) was copolymerized and crosslinked... 

Thermal Free Radical Copolymerization and Crosslinking of Allyl Xylok wilh4,4 -Bismaleimidodiphenyl methane (from ref [143])... 

Although PANI exhibits good conducting properties, its poor biocompatibility and dispersibility in aqueous media delayed its use in biomedical applications. However, recent studies have demonstrated its biocompatibility and revived interest in this CP for tissue engineering applications [248]. To enhance biocompatibility and dispersibility, copolymerization and crosslinking with other molecules have been done. Modification has also been done to improve the mechanical properties of aniline for electrospun fibers (Fig. 13.23) [249]. Functionalization of PANI has also been done with biomolecules with tryptophan and NGF. Adhesion and growth of cells on unmodified PANI is still debatable. 

A series of copolymers of acrylic acid (AAc) and acrylamide (AAm), cross-linked with Bis, were either prepared or, as in the case of CO(AAc/AAm)V, purchased. Those synthesized in the laboratory were produced from freshly distilled AAc and AAm recrystallized from chloroform. The solution copolymerization and crosslinking, when carried to approximately 5% conversion, yielded an insoluble gel that was subsequently broken in a blender and forced through a 20-mesh screen, washed in 0.5N HCl, washed in distilled water, freeze-dried, and stored in a desiccator under vacuum. By controlling both the pH of the reaction medium and the feed ratio of the two monomers, the series of copolymers listed in Table II was prepared, the analysis being by the titration, with O.IN NaOH, of a suspension of the copolymer in O.IN HCl to determine the AAc content of the copolymer. 

To determine the crosslinking density from the equilibrium elastic modulus, Eq. (3.5) or some of its modifications are used. For example, this analysis has been performed for the PA Am-based hydrogels, both neutral [18] and polyelectrolyte [19,22,42,120,121]. For gels obtained by free-radical copolymerization, the network densities determined experimentally have been correlated with values calculated from the initial concentration of crosslinker. Figure 1 shows that the experimental molecular weight between crosslinks considerably exceeds the expected value in a wide range of monomer and crosslinker concentrations. These results as well as other data [19, 22, 42] point to various imperfections of the PAAm network structure. 

The synthesis of elastomers by step, chain, and ring-opening polymerizations is reviewed. These reactions are characterized as to the process variables which must be controlled to achieve the synthesis and crosslinking of an elastomer of the required structure. Both radical and ionic chain polymerizations are discussed as well as the structural variations possible through copolymerization and s tereoregularity. 

Fig. 34. Schematic picture of cyclization (a), multiple crosslinking (b), and crosslinking (c) in radical crosslinking copolymerization.

Moreover, in-situ copolymerization approaches of polymerizable chiral cin-chonan carbamate selectors have also been shown to be viable straightforward routes to enantioselective separation media. In one approach, polymethacrylate-type monoliths have been fabricated by copolymerization of functional monomers and crosslinker in presence of porogenic solvents [80-85]. They have been utilized mainly for CEC (and will be described in detail later) while they turned out to be less suitable for HPLC application because of a low crosslinking degree. 

As the amount of polymer in the system increases, the probability of interaction of growing free radicals with polymeric (inactive) species either by transfer or, sometimes, by copolymerization through residual double bonds must increase. Because such reactions automatically give rise to branched and crosslinked species, reliable experimentation aimed at studying these processes is difficult to achieve. This follows from the fact that the region of the reaction in which study is essential is precisely that region in which there are rapid changes in the characteristics of the polymer produced, particularly in solubility. This is of... 

The competition between the phase separation and crosslinking processes opens up the possibility of controlling pattern formation and the structure of the resulting material by varying the reaction temperature and the blend composition. There is considerable effort to modify the useful properties of elastomers by introducing a coagent that is copolymerized with the elastomer and contributes positively... 

For covalent imprinting, the template molecule is copolymerized into the polymer network with the monomer and crosslinker. Instead of extracting... 

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