Graft polymers free-radical grafting

One universally applicable method of grafting (with respect to substrate) is by chain transfer. A radical P (polymer free radical) or R (initiator free radical) abstracts, for example, an H or Cl atom and forms a macroradical, which initiates the polymerization of the added monomer ... 

The transfer constants of the polymer free radicals P are relatively low, however, so that the graft yield will be very small. The macroradicals are therefore formed through an addition of initiator radicals R. ... 

Remember from Sec. 1.3 that graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone. These can be prepared by introducing a prepolymerized sample of the backbone polymer into a reactive mixture—i.e., one containing a source of free radicals—of the side-chain monomer. As an example, consider introducing polybutadiene into a reactive mixture of styrene ... 

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

Polymers ndResins. / fZ-Butyl peroxyneopentanoate and other peroxyesters of neopentanoic acid can be used as free-radical initiators for the polymeri2ation of vinyl chloride [75-01-4] (38) or of ethylene [74-85-1]. These peresters have also been used in the preparation of ethylene—vinyl acetate copolymers [24937-78-8] (39), modified polyester granules (40), graft polymers of arninoalkyl acrylates with vinyl chloride resins (41), and copolymers of A/-vinyl-pyrrohdinone [88-12-0] and vinyl acetate [108-05-4] (42). They can also be used as curing agents for unsaturated polyesters (43). 

Type AD-G is used in an entirely different sort of formulation. The polymer is designed for graft polymerisation with methyl methacrylate. Typically, equal amounts of AD-G and methyl methacrylate are dissolved together in toluene, and the reaction driven to completion with a free-radical catalyst, such as bensoyl peroxide. The graft polymer is usually mixed with an isocyanate just prior to use. It is not normally compounded with resin. The resulting adhesive has very good adhesion to plasticised vinyl, EVA sponge, thermoplastic mbber, and other difficult to bond substrates, and is of particular importance to the shoe industry (42,43). 

Mention may finally be made of graft polymers derived from natural rubber which have been the subject of intensive investigation but which have not achieved commercial significance. It has been found that natural rubber is an efficient chain transfer agent for free-radical polymerisation and that grafting appears to occur by the mechanism shown in Figure 30.8. 

MMA onto cellulose was carried out by Hecker de Carvalho and Alfred using ammonium and potassium persulfates as radical initiators [30]. Radical initiators such as H2O2, BPO dicumylperoxide, TBHP, etc. have also been used successfully for grafting vinyl monomers onto hydrocarbon backbones, such as polypropylene and polyethylene. The general mechanism seems to be that when the polymer is exposed to vinyl monomers in the presence of peroxide under conditions that permit decomposition of the peroxide to free radicals, the monomer becomes attached to the backbone of the polymer and pendant chains of vinyl monomers are grown on the active sites. The basic mechanism involves abstraction of a hydrogen from the polymer to form a free radical to which monomer adds ... 

In its simplest form the direct grafting method involves the irradiation of polymeric substrate in the absence or presence of oxygen. Graft copolymerization of the monomer to the polymer is then initiated through the free radicals generated in the latter. The reaction can be schematically written as ... 

The radiation sensitivity of a substrate is measured in terms of its GR value or free radical yield, which is the number of free radicals formed per 100 eV energy absorbed per gram. The highest grafting yields will occur for polymer monomer combinations in which the free radical yield of the polymer is much greater than for the monomer. It also follows that the grafting yield will increase at a lower monomer concentration. 

If polyvinyl alcohol is used as the reducing agent and the oxidation is conducted in the presence of vinyl monomer, grafting occurs. This method of grafting yields substantially pure graft copolymers since the free radicals are formed exclusively on the polymer backbone. 

Grafting presents a means of modifying the cellulose molecule through the creation of branches of synthetic polymers, which impart to the cellulose certain desirable properties without destroying the properties of cellulose. The polymerization of vinyl monomers may be initiated by free radicals or by certain ions. Depending on the monomer, one or the other type of initiation may be preferred. The grafting process depends on the reactivity of the monomer used, the type of initiation, and cellulose accessibility [1,2]. 

Grafting reactions onto a polymer backbone with a polymeric initiator have recently been reported by Hazer [56-60]. Active polystyrene [56], active polymethyl methacrylate [57], or macroazoinitiator [58,59] was mixed with a biopolyester polyhydroxynonanaate [60] (PHN) or polybutadiene to be carried out by thermal grafting reactions. The grafting reactions of PHN with polymer radicals may proceed by H-abstraction from the tertier carbon atom in the same manner as free radical modification reactions of polypropylene or polyhy-droxybutyratevalerate [61,62]. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

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