Polymer graft

Graft copolymers can be prepared by radical grafting of a polymerisable monomer A onto a reactive polymer backbone B. As a result of the grafting reaction, a complex product is obtained comprising the graft copolymer AB, residual ungrafted polymer backbone B and homopolymer A. 

The condensation grafting of polyethylenimine (PEI) onto poly(acrylamide-co-acryhc acid) (PAM-co-AA) was studied by FTIR spectroscopy. The reaction mechanism, which proceeded by a two-step reaction, involved the conversion of AA to acid chloride (AC) using thionyl chloride, followed by the condensation of AC onto PAM and with amine onto PEI to form the graft copolymer (125). 

The grafting of PP films by PAA was demonstrated to start from the interface between the PP and the polymerisation solvent. The results suggested that it should be possible to control the location of grafting both by the content of antioxidants and by their extraction (322). 

LDPE was functionalised in the bulk through dicumyl peroxide-initiated grafting of dibutyl maleate and vinyltrimethoxysilane in the temperature range from 140 to 200 °C (327). 

The structure of EPDM grafted onto polyamide 66 was analysed by FTIR spectroscopy. Evidence indicated the presence of cyclic imide groups, derived from grafted maleic anhydride groupings (229). 

Wei and co-workers [38] grafted P(3HB) onto cellulose by the radical initiation of dicumyl peroxide with in situ reactive extrusion. The findings revealed that the reaction time and concentration of dicumyl peroxide significantly influenced the grafting efficiency. SEM confirmed the presence of the grafting reaction between the surface of the cellulose and PHB. DSC analysis showed a reduction of the crystallinity of the graft polymer. TGA analysis further showed that the graft polymer is more thermally stable than P(3HB). 

Steinbuchel and T. Lutke-Eversloh, Biochemical Engineering Journal, 2003,16, 2, 81. 

Ramachandran, S. Kannusamy, K-H. Huong, R. Mathava and A-A. Amirul in Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites, Eds., L Roy and RM. Visakh, The Royal Society of Chemistry, London, UK, 

Bruzaud and Y. Grohens, Macromolecular Materials and Engineering, 2013,298,11,1176. 

Bates and G.H. Fredrickson, Annual Review of Physical Chemistry, 1990, 41, 525. 

Reaction conditions necessary to carry out the modifications described earlier in this section usually result in some decomposition of amylose and amylopectin, even when simple substitution, addition, or crosslinking are involved. As a rule, graft copolymerization produces derivatives of significantly increased molecular weight. Starch grafting usually entails etherification, acetalation, or esterification of starch with vinyl monomers to introduce a reaction site for the further formation of a copolymeric chain. Such a chain would typically consist of either identical or different vinyl monomers (block polymers), or it may be grafted onto another polymer altogether. 

Aluminoxanes suppressed side reactions involving hydrogen transfer. They also formed cyclic structures with starch, giving copolymers that were coated with crystalline polyethylene. A catalyst composed of dicyclopentadienylzirconium dichloride and trimethylaluminium permitted polymerization of ethylene on starch in a toluene suspension at 60 °C for 2h.2806 Graft copolymerization of methyl methacrylate onto starch was also performed with an acetylacetone-copper(II) complex in trichloroacetic acid.2807 The grafting yield and efficiency were proportional to the initiator concentration up to 7.0 x 10-3 mole/L. 

Grafting of methyl methacrylate on starch occurs in an aqueous, saturated starch solution at 85 °C in the presence of carbon tetrachloride or copper(II) ions,2808,2809 and the grafting efficiency could be as high as 72%. At a low concentrations of the vinyl compound, the reaction rate was proportional to its concentration. The reaction rate became independent of concentration at higher concentrations of the vinyl monomer. Grafting in the presence of 0.1M nitric acid and ammonium selenate was also reported.2810 

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Koutsos V, van der Vegte E W, Pelletier E, Stamouli A and Hadziioannou G 1997 Structure of chemically end-grafted polymer chains studied by scanning force microscopy in bad-solvent conditions Macromolecules 30 4719-26... 

Koutsos V, van der Vegte E W and Hadziioannou G 1999 Direct view of structural regimes of end-grafted polymer monolayers a scanning force microscopy study Macromolecules 32 1233-6... 

In many colloidal systems, both in practice and in model studies, soluble polymers are used to control the particle interactions and the suspension stability. Here we distinguish tliree scenarios interactions between particles bearing a grafted polymer layer, forces due to the presence of non-adsorbing polymers in solution, and finally the interactions due to adsorbing polymer chains. Although these cases are discussed separately here, in practice more than one mechanism may be in operation for a given sample. 

W. J. Budant and A. S. Hoffman, Block and Graft Polymers, Reinhold Puhhshing Corp., New York, 1960. 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

Gross-Linking. A variety of PE resins, after their synthesis, can be modified by cross-linking with peroxides, hydrolysis of silane-grafted polymers, ionic bonding of chain carboxyl groups (ionomers), chlorination, graft copolymerization, hydrolysis of vinyl acetate copolymers, and other reactions. 

In addition to the primary appHcation of PTMEG ia polyurethanes, polyureas, and polyesters, a considerable number of reports of other block and graft polymers highlighting PTME units have appeared. Methods have been developed that allow the conversion of a cationicaHy polymerizing system to an anionic one or vice versa (6,182). 

Chemical Grafting. Polymer chains which are soluble in the suspending Hquid may be grafted to the particle surface to provide steric stabilization. The most common technique is the reaction of an organic silyl chloride or an organic titanate with surface hydroxyl groups in a nonaqueous solvent. For typical interparticle potentials and a particle diameter of 10 p.m, steric stabilization can be provided by a soluble polymer layer having a thickness of - 10 nm. This can be provided by a polymer tail with a molar mass of 10 kg/mol (25) (see Dispersants). 

Emulsion polymerizations of vinyl acetate in the presence of ethylene oxide- or propylene oxide-based surfactants and protective coUoids also are characterized by the formation of graft copolymers of vinyl acetate on these materials. This was also observed in mixed systems of hydroxyethyl cellulose and nonylphenol ethoxylates. The oxyethylene chain groups supply the specific site of transfer (111). The concentration of insoluble (grafted) polymer decreases with increase in surfactant ratio, and (max) is observed at an ethoxylation degree of 8 (112). 

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). 

A series of graft polymers on polychloroprene were made with isobutjiene, /-butyl vinyl ether, and a-methylstyrene by cationic polymerization in solution. The efficiency of the grafting reaction was improved by use of a proton trap, eg, 2,6-di-/-butylpyridine (68). 

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). 

C. R. Cuervo and A. J. Maldonado, Solution Adhesives Based on Graft Polymers of Neoprene and Methyl Methacrylate, Du Pont Informal Bulletin, Wilmington, Del., Oct. 1984 K. Itoyama, M. Dohi, and K. Ichikawa, Nippon Setchaku Kyokaishi 20, 268 (1984). 

Other copolymer forms are alternating copolymers, block copolymers and graft polymers. 

At one time butadiene-acrylonitrile copolymers (nitrile rubbers) were the most important impact modifiers. Today they have been largely replaced by acrylonitrile-butadiene-styrene (ABS) graft terpolymers, methacrylate-buta-diene-styrene (MBS) terpolymers, chlorinated polyethylene, EVA-PVC graft polymers and some poly acrylates. 

Today the common practice is first to dissolve the rubber in the styrene monomer and then to polymerise the styrene in the usual way. By this process the resultant blend will contain not only rubber and polystyrene but also a graft polymer where short styrene side chains have been attached to the rubber molecules. This gives a marked improvement in the impact strengths that can be obtained. 

To produce the Type 2 polymers, styrene and acrylonitrile are added to polybutadiene latex and the mixture warmed to about 50°C to allow absorption of the monomers. A water-soluble initiator such as potassium persulphate is then added to polymerise the styrene and acrylonitrile. The resultant materials will be a mixture of polybutadiene, polybutadiene grafted with acrylonitrile and styrene, and styrene-acrylonitrile copolymer. The presence of graft polymer is essential since straightforwsird mixtures of polybutadiene and styrene-acrylonitrile copolymers are weak. In addition to emulsion processes such as those described above, mass and mass/suspension processes are also of importance. 

Interesting graft polymers based on silicone polymers are finding use in the manufacture of polyurethane foams, particularly, of the polyether type (see Chapter 27), because of their value as cell structure modifiers. 

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. 

Both rubber-styrene and rubber-methyl methyacrylate graft polymers have been produced on a pilot plant scale. The side chains have unit weights of the order of 5000 compared with values of 70 000-270 000 for the main rubber chain. 

She et al. [128] used rolling contact to estimate the adhesion hysteresis at polymer/oxide interfaces. By plasma oxidation of the cylinders of crosslinked PDMS, silica-like surfaces were generated which could hydrogen bond to PDMS r olecules. In contrast to unmodified surfaces, the adhesion hysteresis was shown to be larger and proportional to the molecular weight of grafted polymer on the substrate. The observed hysteresis was interpreted in terms of the orientation and relaxation of polymer chains known as Lake-Thomas effect. 

Polychloroprene elastomer. Neoprene AC and AD are the most commonly used, mainly Neoprene AD because of its superior viscosity stability. For difficult-to-bond substrates, graft polymers Neoprene AD-G or AF) show better performance. For sprayable adhesives or high-viscosity mastics, the Neoprene AG offers excellent results. When specific properties (e.g. increase tack, improve wetting, increase peel strength) need to be met, blends of Neoprene AC or AD with Neoprene AG provide adequate performance. 

Shoe adhesives. CR adhesives are used for the permanent attachment of shoe soles. For difficult-to-bond sole materials (plasticized PVC, EVA foaming soles, thermoplastic rubber, SBR) graft polymer solutions of Neoprene AD-G combined with a polyisocyanate provide a good adhesion. Another major area for CR contact adhesives is the manufacture of leather goods, particularly leather shoe sole bonding and belt lamination. 

This principle is applied not only to the PVA-PVAc composites but to other polymer composites. The composite structure does not always need to be porous but may be powders and gels designed for the wettability by solvents and the extension of the surface area in soluble polymers. From this point-of-view, the present work sheds a new light on the research on composite materials related to graft polymers and copolymers. 

R. J. Ceresa, Block and Graft Polymers, Ch. 5, Butter- 110. worths, London (1962). 

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