Radical-Induced Grafting Processes

Radical induced grafting may be carried out in solution, in the melt phase, or as a solid state process. This section will focus on melt phase grafting to polyolefin substrates but many of the considerations are generic. The direct grafting of monomers onto polymers, in particular polyolefins, in the melt phase by reactive extrusion has been widely studied. Most recently, the subject has been reviewed by Moad and by Russell. More details on reactive extrusion as a technique can be found in volumes edited by Xanthos, A1 Malaika and Baker et The process most often involves combining a frcc-radical initiator (most commonly a peroxide) and a monomer or macromonomer with the polyolefin as they are conveyed through the extruder. Monomers commonly used in this context include MAII (Section 7.6.4.1), malcimidc derivatives and malcatc esters (Section 

(meth)acrylic acid and inelh)acrylate esters (Section 7.6.4.3), S, AMS and derivatives (Section 7.6.4.4), vinylsilanes (Section 7.6.4.5) and vinyl oxazolines (Section 7.6.4.6). 

A major issue is the control of the side reactions that accompany grafting. These reactions include radical-induced degradation of the substrate by cross-linking and/or chain scission and homopolymerization of the graftee monomer. 

A major challenge is then to devise conditions so as to maximize grafting and minimize or control these side reactions. Some discussion of many of these parameters is provided in the reviews mentioned above. It is significant that many recent publications and patents in the area of reactive extrusion relate, not to the development of new reactions or processes, but to the selection of operating parameters. 

It is also necessary to select the initiator according to the particular monomer(s) and the substrate. Factors to consider in this context, aside from initiator half-lives and decomposition rates, are the partition coefficient of the initiator between the monomer and polyolefin phases and the reactivity of the monomer vs- the polyolefin towards the initiator-derived radicals. 

---

With a few exceptions, most graft copolymers from free radical induced grafting processes usually produce not only the desired graft copolymers, but also homopolymers and other side reactions. Consequently, the exploration and detailed characterization of grafts produced by free radical methods is often cumbersome. 

With a history of more than 25 years, the free radical-induced grafting of MAH onto polyolefin substrates is one of the most studied polyolefin modification processes.29 "29, 302 The process has been carried out in the melt phase, in various forms of extruders and batch mixers, and there are numerous patents covering various aspects of the process. It has also been carried out successfully in solution and in the solid state. The materials have a range of applications including their use as precursors to graft copolymers, either directly, or during the preparation of blends.297... 

Ionizing radiation is unselective and has its effect on the monomer, the polymer, the solvent, and any other substances present in the system. The radiation sensitivity of a substrate is measured in terms of its G value or free radical yield G(R). Since radiation-induced grafting proceeds by generation of free radicals on the polymer as well as on the monomer, the highest graft yield is obtained when the free radical yield for the polymer is much greater than that for the monomer. Hence, the free radical yield plays an important role in grafting process [85]. 

As effective as these surface modification processes might be, they present limitations in terms of the extent to which the surfaces of polymers can be modified. Plasma-induced grafting offers another method by which chemical functional groups can be incorporated. In this process, free radicals are generated on the surface of a polymer through the use of an inert gas plasma. Because of the nonreactive nature of the inert gas plasma, surface chemical modification of the polymer does not occur. If the polymer surface that has been... 

Radiation-induced grafting is a process where, in a first step, an active site is created in the preexisting polymer. This site is usually a free radical, where the polymer chain behaves like a macroradical. This may subsequently initiate the polymerization of a monomer, leading to the formation of a graft copolymer structure where the backbone is represented by the polymer being modified, and the side chains are formed from the monomer (Fig. 1). This method offers the promise of polymerization of monomers that are difficult to polymerize by conventional methods without residues of initiators and catalysts. Moreover, polymerization can be carried out even at low temperatures, unlike polymerization with catalysts and initiators. Another interesting as-... 

The most widely used method for chemical initiation for graft copolymerization on polysaccharides has been with ceric salts such as CAN or ceric ammonium sulfate. Free radical sites are generated on a polymeric backbone by direct oxidation through ceric metal ions (e.g., Ce "). The ceric ion with low oxidation potential is the proper choice for the reaction. The proposed mechanism for such processes has been depicted by an intermediate formation of metal ion polymer complex (chelate type) [17, 21-24]. Such a complex formation is not restricted to all polymers. The plausible mechanism for ceric ion induced graft copolymerization by direct oxidation method is shown in Scheme 3.1. A series of four grades of Ag-g-PAM copolymers have been synthesized by the conventional method. 

In this work, researchers employed attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) (Figure 4.6) analysis to support their posmlation on UV-initiated grafting. In their proposed mechanism, the grafting process of the PES membrane started with the absorption of UV-light by phenoxy phenyl sulfone chromophores in the backbone of the polymer chain. Two radical sites were produced at the end of each polymer chains as a result of hemolytic cleavage of carbon-sulfur bond (in sulfone linkage). The produced radicals (aryl and sulfonyl) thus induced the polymerization of acrylic acid at the reactive sites of the radicals. Sulfonyl radical... 

---