Free radical reactions, graft

The presence of lignin, resins or other extractives in the fibers may interfere with the initiation or polymerization reactions, e.g. by termination or chain transfer of free radical reactions from phenolic groups. In some cases, lignin has no adverse effect and may even be grafted . 

The dose rate affects both the yield and chain length of the grafted material. Air has a detrimental effect on grafting since it inhibits the reaction, which is consistent with other radiation-induced free radical reactions. Increasing the temperature of fhe graffing sysfem increases the yield. This is very likely because raising the temperature increases the diffusion rate of the monomer into the substrate. ... 

A graft terpolymer of lignin has been made by free radical reaction of 2-propenamide and 2,2-dimethyl-3-amino-4-oxohex-5-ene-l-sulfonic acid in... 

A graft or block copolymer of cellulose is defined here as a combination of cellulose and polymer that is difficult to separate by solvent extraction without first degrading the cellulose. Furthermore, only proposed mechanisms and reaction methods of free-radical initiated graft and block polymerizations of vinyl monomers with cellulose are discussed. 

Photochemical and photophysical processes in cellulose and related compounds have received considerable attention during the last decades, resulting in research work concerned with the improvement of cellulosic materials via physical and chemical modifications. One method was to apply a copolymer between the cellulose and a synthetic polymer which are generally grafted by free radical reactions. 

Effect of Peroxides. In addition to benzoyl peroxide, lauryl-, acetyl-, 2,4-dichlorobenzoyl-, and methyl ethyl peroxide, and tert-hvXy hydroperoxide were studied and gave satisfactory results. The effectiveness of the peroxide is relatively independent of the half-life of the peroxide (Table X). By contrast, the catalyst AIBN is much less satisfactory, as found for methylvinylpyridine and acrylonitrile. The difference between these peroxides and AIBN suggests that the AFR polymer is not formed by a simple uncatalyzed free radical system which would give a graft polymer or a simple mixture of polypropylene and polyacrylate. It is well known that for the polymerization of acrylates AIBN is at least as good if not better than peroxide in initiating the free radical reaction (2). 

The modification of the properties of cotton cellulosic textile products, through free radical-initiated graft copolymerization reactions with vinyl monomers, has been investigated at the Southern Laboratory for a number of years (6, 9). In this chapter, we summarize the basic mechanisms and principles involved in free radical reactions of cellulose, initiated by high energy radiation, ceric ion in acidic solution, and aqueous solutions of ferrous ion and hydrogen peroxide. Some of the properties of fibrous cotton cellulose graft copolymers are also presented. 

Polymer grafting involves introducing functional groups (typically via free-radical reactions) in to a matrix polymer. The types of molecular architecture achievable by... 

Better control of grafting and less homopolymer formation is achievable in ionic reactions than can be obtained in free-radical reactions. Anionic grafting via backbone initiation has been demonstrated (22) with caprolacteim on macromolecular ester sites of styrene/methyl methacrylate copolymers. Cationic grafting of isobutylene onto poly(vinyl chloride) with the aid of aluminum alkyl has been carried out by J. P. Kennedy (23). 

Graft Copolymers. In graft copolymerization, a preformed polymer with residual double bonds or active hydrogens is either dispersed or dissolved in the monomer in the absence or presence of a solvent. On this backbone, the monomer is grafted in free-radical reaction. Impact polystyrene is made commercially in three steps first, solid polybutadiene rubber is cut and dispersed as small particles in styrene monomer. Secondly, bulk prepolymerization and thirdly, completion of the polymerization in either bulk or aqueous suspension is made. During the prepolymerization step, styrene starts to polymerize by itself forming droplets of polystyrene with phase separation. When equal phase volumes are attained, phase inversion occurs. The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. R. L. Kruse has determined the solubility parameter for the phase equilibrium. 

In another study, the monoacrylate 35 was grafted onto PP in the presence of tris(acryIoyhnethyl)propane (36) as coagent.Again close to 100% grafting yield was obtained. This was so despite the fact that 35 was anticipated to be an inhibitor of free radical reactions (in fact, phenols are poor inhibitors of (meth)acrylate polymerization - Section 5.3.4). The tris-acrylatc 36 and related species have previously been used for producing crosslinked/branched xhe structure... 

Because the free radical initiated graft reaction can also lead to the cross-linking of polyethylene, copolymers of ethylene and with acrylic acid (184,185), glycidyl methacrylate (184,186), methacrylic acid and 10-undecenoic acid (187-189) were synthesized to compatibilize polyethylene/polyamide blends. The poly (ethylene-co-methacrylic acid) ionomers neutralized by sodium (184) and zinc (45,118,190-192) has also used as compatibilizers. High energy irradiation, used to modify the surface of fibers or films at beginning, was also used to compatibilize the polyethylene/polyamide blends (193-196). 

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. 

In comparison with bulk polymerization, for the free radical reaction of VK in the presence CNTs, PVK was reacted directly with MWNTs at 70 °C in DCB with azo-bis-izobutyronitrile (AIBN) as the radical initiator [199]. After purification, deep-grey products, which can be dissolved in common solvents such as chloroform and 1,2-dichlorobenzene (DCB), were obtained. It was confirmed that PVK was grafted onto the surface of CNTs by FTIR and Raman spectroscopy, CPS, TGA, TEM, and UV-VIS spectra [199]. 

H.X. Wu, X.Q. Qiu, R.F. Cai, and S.X. Qian, Poly(N-vinyl carbazole)-grafted multi-walled carbon nanotubes S3mthesis via direct free radical reaction and optical bmiting properties, Appl. Surf. Sci., 253, 5122-5128 (2007). 

Graft Copolymerization by Conventional Free Radical Reactions... 

PVK-grafted multi-walled carbon nanotubes have been synthesized by a free radical reaction [23], The... 

---