Photo-copolymerization

Thin-film MIP composite membranes (cf. Scheme lb), imprinted for theophyUin and caffein, had been prepared by Hong et al. [99], using photo-copolymerization of a MAA/EDMA mixture on top of an asymmetric 20 nm pore size alumina membrane. Additional gas permeation studies suggested that the membranes were defect ( pinhole ) free. 

The photo copolymerization of N-vinylpyrrolidone with methyl methacrylate in the presence of zinc chloride was mentioned in passing in Section 3F above [78]. While the thermal copolymerization of these two monomers does not take place in vacuoy it does take place in the presence of oxygen, after an induction period. Thus oxygen seems to participate in the formation of an active species. The rate is increased in the presence of zinc chloride. It has been postulated that a charge-transfer polymerization process is involved here [78]. 

The initial methods to fabricate GI preforms took advantage of the difference in monomer reactivity ratios in copolymerization reactions. These methods are of two types photo-copolymerization [4-6] and interfacial-gel copolymerization [7, 8]. 

G1 preforms can also be fabricated by an interfacial-gel polymerization technique. The principle is basically the same as that of the photo-copolymerization method discussed above, except for the mechanism that forms the initial gel phase. In this method, the core solution (the monomer) is placed in a polymer tube rather than in a glass tube. The gel phase in the photo-copolymerization method is referred to as a prepolymer with a conversion of less than 100%, whereas in this method the gel phase comprises the polymer layer on the inner wall of the tube swollen by the core monomer. The reaction is carried out under UV irradiation or heating. 

Table 5.1 Categories of possible monomers for photo-copolymerization and interfacial-gel copolymerization processes.

In this method, nonreactive compounds are employed as the high-refractive-index component [11]. For example, MM A and bromobenzene (BB), which have higher refractive indices than PMMA, can be utilized as the monomer and the nonreactive compound, respectively. The fabrication procedure is the same as in the photo-copolymerization and interfacial-gel polymerization methods. However, the principle of formation the GI profile is different. In contrast to the previous methods that use the difference in the monomer reactivity ratios, in this method the difference in the molecular size is important. Because the molecular size of MM A is smaller than that of BB, MM A more easily diffuses into the gel phase. Thus, BB molecules are concentrated into the middle region to form the GI profile as the polymerization progresses. The mechanism is schematically described in Figure 5.11. 

In a GI fiber, the refractive index varies radially and hence increases the signal bandwidth. So the GI fiber is expected to be a high-speed data link optical signal transmission medium. Concerning GI plastic rods with the required quadratic refractive index profile, two-step copolymerization and photo-copolymerization techniques have been reported. The two-step copolymerization technique was usually applied for thermosetting resins such as CR-39 resin as a mother rod. It is difficult to draw this GI plastic rod into optical fibers. 

Instead of using thermal energy to trigger the hydrogen abstraction mechanism, photo-induced reactions can be also be used to successfully crosslink acrylic PSAs [74-76], In this case, photoactive compounds, such as for example those containing benzophenone, anthraquinone or triazine nuclei are compounded with the polymer or copolymerized as one of the monomers. After drying, the adhesive... 

Graft Copolymerization of Vinyl Monomers Onto Macromolecules Having Active Pendant Group via Ceric Ion Redox or Photo-Induced Charge-Transfer Initiation... 

Therefore, the graft copolymerization of vinyl monomers onto macromolecules having active an pendant group can be achieved either by redox initiation with a Ce(IV) ion or by photo-induced charge-transfer initiation with BP, depending on the structure of the active groups. 

Yang H, Lazos D, Ulbricht M. Thin, highly crosslinked polymer layer synthesized via photo-initiated graft copolymerization on a self-assembled-monolayer-coated gold surface. J Appl Polym Sci 2005 97 158-164. 

Fig. 5 Fluorescence micrograph of siuface photo-graft-copolymerized with (W-dimethyl-amino) propyl acrylamide methiodide (DMAPAAmMel) by UV irradiation through the stripe-patterned projection mask and the neutral-density filter and subsequently stained with rose bengal, and the three-dimensional image, b of the distribution of the florescence intensity in the area shown in a...

At first sight, benzophenone compounds appear to be the most efficient photo-initiator for the copolymerization reaction, whereas phenylacetophenone derivatives do not rank among the best. 

Graft Copolymerization. Graft copolymers of cellulose and cellulose derivatives were prepared in a quartz reactor containing 0.5 g oven dried sample, 10 ml monomer and 100 ml water at 45°C. The grafting mixtures or solutions were photo-irradiated with the ultraviolet light of X > 254 nm and X >... 

The photo-induced charge-transfer copolymerization entity consists of a donor monomer and an acceptor monomer, without initiator. Reported combinations usually consist of an aryl vinyl monomer as the donor component and a 1,2-disubstituted vinyl monomer as the acceptor component. 

Generally, the photo-induced charge-transfer copolymerization follows a radical mechanism and produces alternating copolymers. Accompanying cyclobutane formation is usually observed, which provides information for understanding the mechanism. 

A charge-transfer complex (CT complex or CTQ is involved in all thermal charge-transfer polymerizations and in most of the photo-induced charge-transfer copolymerizations. Mixing of a donor (D), such as VCZ, with an acceptor, such as tetracyanoethylene (TCNE), immediately produces color [31] due to the formation of a CT complex ... 

Not much attention has been paid to the two different kinds of excitation in photo-induced copolymerizations. A number of authors do not mention which species was excited, one of the partners or the CT complex. However, there are investigators who have made sure to excite the CT complex only by selecting the wavelength of the incident light [23]. Neither St nor FN absorbs 365-nm light,... 

Spontaneous thermal copolymerization is usually observed in strong donor (D)/strong acceptor (A) pairs, for example a-MSt/MAn, because only in these pairs is the charge-transfer interaction strong enough to produce initiating radicals. On the other hand, photo-induced copolymerizations encompass a wider range of donor/acceptor combinations. 

Table 3. Initial Rp (%/min) of Photo-Induced Copolymerizations of D/A pairs ...

The initiation mechanism of photo-induced charge-transfer copolymerization of donor-acceptor monomer pairs is clarified by integrating the results from organic chemistry and polymer chemistry. A reasonable suggestion for the initiating species in certain cases may be a tetramethylene 1,4-diradical. The excited complex of the donor/acceptor monomers undergoes multiple follow-up reactions to produce the... 

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