Radiation-induced Polymerizations

The creation of active sites as well as the graft polymerization of monomers may be carried out by using radiation procedures or free-radical initiators. This review is not devoted to the consideration of polymerization mechanisms on the surfaces of porous solids. Such information is presented in a number of excellent reviews [66-68]. However, it is necessary to focus attention on those peculiarities of polymerization that result in the formation of chromatographic sorbents. In spite of numerous publications devoted to problems of composite materials produced by means of polymerization techniques, articles concerning chromatographic sorbents are scarce. As mentioned above, there are two principle processes of sorbent preparation by graft polymerization radiation-induced polymerization or polymerization by radical initiators. We will also pay attention to advantages and deficiencies of the methods. 

Other polymerization methods such as coordination polymerization," " surface electro-initiated emulsion polymerization," radiation-induced graft polymerization (RIGP)," surface thiol-lactam-initiated radical polymerization (TLIRP)" and LbL click grafting" have been employed to graft polymers from CNT surfaces. 

Photochemical and Radialion Initiatioii.— The topics of photochemical initiation of cationic polymerizations, radiation-induced initiation, and their inter-relationships have been reviewed recently. 

Strahlenpolymerisation radiation polymerization, radiation-induced polymerization, radiolytic polymerization... 

Solution polymerization of VDE in fluorinated and fluorochlorinated hydrocarbons such as CEC-113 and initiated with organic peroxides (99), especially bis(perfluoropropionyl) peroxide (100), has been claimed. Radiation-induced polymerization of VDE has also been investigated (101,102). Alkylboron compounds activated by oxygen initiate VDE polymerization in water or organic solvents (103,104). Microwave-stimulated, low pressure plasma polymerization of VDE gives polymer film that is <10 pm thick (105). Highly regular PVDE polymer with minimized defect stmcture was synthesized and claimed (106). Perdeuterated PVDE has also been prepared and described (107). 

The bismaleimide can then be polymerized by reaction with additional amine to form polyaininobismaleknide or by radiation-induced homopolymerization to form polybismaleimide (4). 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

The y-radiation-induced polymerization requires an extremely high purity reaction system. Trace amounts of water can terminate a cationic reaction and inhibit polymerization. Organic bases such as ammonia and trimethylamine also inhibit polymerization. The y-radiation-induced polymerization of a rigorously dried D obeys the Hayashi-WilHams equation for completely pure systems (150). 

An analogous mechanism should also produce polymers on irradiation of epoxies. Crivello s recent mechanistic suggestions [29] are consistent with the mechanisms given above. One can conclude that radiation-induced polymerization of epoxies can proceed via several mechanisms. However, further work is needed to determine the relative contributions of the different mechanisms, which might vary from one epoxy to another. As part of the Interfacial Properties of Electron Beam Cured Composites CRADA [37], an in-depth study of the curing mechanism for the cationic-initiated epoxy polymerization is being undertaken. 

III. KINETICS OF RADIATION-INDUCED POLYMERIZATION OF ACRYLAMIDE IN AQUEOUS SOLUTION... 

The advantage of using radiation-induced polymerization is that the polymer is homogeneous and free from any impurities. Moreover, the molecular weight of the formed polymer is controlled by varying doses and dose-rates. 

The reported values for the exponent of the dose-rate for the polymerization rate in gamma radiation-induced copolymerization of acrylamide with methyl chloride salt of A, A -dimethylaminoethyl methacrylate (DMAEM-MC) in aqueous solution was found to be 0.8 [16]. However, the dose-rate exponent of the polymerization rate at a lower dose-rate was found to be slightly higher than 0.5 for gamma radiation-induced polymerization of acrylamide in aqueous solution [45,62]. 

Radiation-induced graft polymerization may be carried out by the following procedures [69] ... 

Nakase, Y., Kurijama, I. and Odajima, A. Analysis of the Fine Structure of Poly(Oxyme-thylene) Prepared by Radiation-Induced Polymerization in the Solid State. Vol. 65, pp. 79-134. 

Yoshida, H. and Hayashi, K. Initiation Process of Radiation-induced Ionic Polymerization as Studied by Electron Spin Resonance. Vol. 6, pp. 401—420. 

These processes compete with radiation-induced crosslinking, scission and, for case (c), polymerization. 

Hoffman and his coworkers have done a lot of work on the apphcation of radiation-induced graft polymerization for medical apphcation. The hydrophilic polymers that have been used for radiation-induced grafting are Al-vinyl pyrohdone (NVP), 2-hydroxyethyl methacrylate (HEMA), acrylamide (AAm), acrylic acid (AAc), glycidyl methacrylate (GMA), ethyleneglycol dimethacrylate (EGDMA), and ethyl methacrylate (EMA) onto sihcone rubber were widely smdied. 

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