Photografting-photoinitiation

The photografting and photocrosslinking processes in which the photoinitiation is carried out by pre-existing polymer in the presence of vinyl monomer. The pre-existing polymer may itself be photoactive or, with the presence of photoinitiator, can interact with a polymer to produce a radical on the backbone of the polymer chain. 

Bamford and Mullik [62] have succeeded in photografting a vinyl monomer onto a styrene-MMA copolymer using the Mn2(CO)io/C2F4 photoinitiating system in acetic acid. The following scheme was reported for this process ... 

Since the pioneering work by Oster et al. in the 1950 s (3,A) where monomers were photografted with UV light onto polymers blended with photoinitiators, only few researchers have more recently followed up this work. The interest has been focused on the use of high energy radiation. 

Combinations of benzophenone and tertiary amine groups, one of which was anchored to a polymer matrix, have also been checked in the photoinduced polymerization and photografting of 2-ethylhexyl methacrylate (EHMA) [52]. Indeed, poly(VBP-co-St)s, in the presence of 7V,Af-diethylaniline (DEA), are found to display lower photoinitiation activity and photografting efficiency than the corresponding copolymers of styrene with 4-(Af,Af-diethylamino) styrene [poly(St-co-DEAS)] in the presence of benzophenone (BP), as revealed by conversion and fractionation data (Table 10). 

Anthraquinone-2,7-disulfonic acid was used as the photoinitiator to photograft acrylonitrile, methyl methacrylate, styrene and vinyl acetate upon cellulose. The radical site formed on the cellulose chain initiated polymerization of quantities of monomers of up to three-and-one-half times the weight of the cellulose 18>. 

Before proceeding with the photoinduced grafting methods, a brief introduction to photoinitiation is needed for the realization of photografting. However, the reader is encouraged to refer to comprehensive reviews of photoinitiated polymerization [5-8]. 

Another interesting case is the self-initiation of some acrylic and styrenic monomers upon UV irradiation. Since this process is quite slow, a photoinitiator is normally required. Recent findings on the self-initiation of maleic anhydride, [ 10] styrene, [11] and anumber of acrylic monomers [12] have demonstrated that photopolymerization and photografting could possibly be achieved without using photoinitiators or sensitizers. 

B. Ranby, Photoinitiated Modifications of Synthetic Poiymers Photocrosslinidng and Surface Photografting, in N.S. Allen, M. Edge, I.R. Bellobono, E. Selli (eds.), Current Trends in Polymer Photochemistry, Horwood, New York (1995), Chapter 2, p. 23. 

The photoinitiator may be loaded on the membrane surface by adsorption or may be dissolved in the monomer solution. For example, PET nucleopore membranes were modified using BP dissolved in the monomer solution following UV irradiation of the solution and the immersed membrane (Yang and Yang 2003). It was shown that photografting occurred mainly on the top membrane surface rather than in the membrane pores. This approach is relatively simple however, its main drawback is a low local concentration of BP on the membrane surface because BP moves to the membrane surface only by diffusion. This results in a low grafting efficiency. High bulk BP concentration may cause a side reaction, such as homopolymerization. In addition, the use of monomers that do not have a common solvent with BP is limited BP is almost insoluble in water. 

UV photografting of poly(methacrylic acid) (PMAA) on the surface of PHBHV fibers, and complexation of in-situ photogenerated silver nanoparticles (Ag NPs) (Fig. 6.10). The photografting process is conducted through a photoinduced free-radical process employing a ketone-based photoinitiator in aqueous medium [39]. 

RAn 98] RAnby B., Photoinitiated modifications of polymers photocrosslinking, surface photografting and photolamination . Materials Research Innovations, vol. 2, pp. 64-71, 1998. 

I consider it also important to bring into our discussion the subject of photopolymerization, or as it is more correctly described, photoinitiation. Photoinitiation not only lends itself to the preparation of photopolymers from monomers but makes possible photografting of monomers to existing polymers. Thus, we can readily modify solution or other physical properties by means of photografting. 

The classical example of photografting was carried out by Oster and Shibata [12] who grafted hydrophilic acrylamide units upon natural rubber by means of benzo-phenone photoinitiation. 

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