Mechanochemical grafting

While mechanochemical grafting can be effective, it strongly depends on processing conditions and is often inhibited by the levels of antioxidant/stabi-lizer used in many commercial formulations. Antioxidant interference can be overcome by the addition of a very small amount of peroxide. Table 6.3 illustrates these effects. 

Grafting of MA on both isotactic and atactic polypropylene has been studied under a variety of conditions.Maleic anhydride, maleic acid, and monomethyl maleate have also been grafted to polypropylene by a thermal process at 325°C. Mechanochemical modification of both atactic and isotactic polypropylene may be accomplished by blending polymer-anhydride mixtures in laboratory mills with peroxide initiators. Mechanochemical grafting can also be brought about by using peroxidized polypropylene. Extraction studies clearly showed grafting was achieved.An additional reaction of MA with isotactic or atactic polypro-... 

Mechanochemical and thermal processes have been used to graft MA to poly (isobutylene) and poly(vinyl chloride). Mechanochemical graft-... 

Mastication of polychloroprene with MA and MA-styrene or MA-vinyl acetate mixtures gives carboxylated chloroprene rubbers/ The extent of the mechanochemical grafting of the base polymer with anhydride is increased 3-4 times in the presence of the vinyl monomers. The results are akin to the case where styrene improves grafting of MA on polybutadiene. Ivan et have recently reported techniques for grafting MA on cyclopenta-dienylated polychloroprene. 

For performance study, the mechanochemically grafted Si02 nanoparticles were melt mixed with PP to produce bulk composites. Besides, two other types of nanocomposites were also manufactured. One consisted of PP and nanosilica as-received (i.e. Si02/PP), and the other was prepared by melt mixing PP, nano-silica and butyl acrylate without the pre-treatment in the ball mill. 

Note 1 Examples of reactive blending are (a) blending accompanied by the formation of a polymer-polymer complex, (b) the formation of block or graft copolymers by a combination of radicals formed by the mechanochemical scission of polymers during blending. 

Osada et al. [52] have also grafted PMAA to a porous substrate in order to achieve an environmentally sensitive membrane. This sensitivity was termed a chemical valve function because mechanochemical forces caused the pores to enlarge and contract. PMAA was grafted onto poly(vinyl alcohol) (PVA) films which had a mean pore radius of 4 pm. The water permeation of the membrane was strongly affected by the conformational state of the PMAA grafts. At low pH the chains were contracted and the water permeability was... 

The mechanical synthesis of block and graft copolymers by vibromilling a polymer-monomer blend has been performed by many researchers. Natural polymers (14,17) vinyl polymers (18—27), and heterochain polymers (18, 28-34) have been formed during polymer mechanochemical degradation. Importantly, Simionescu, Vasiliu-Oprea and Neguleanu studied the possibility of carrying out mechanically-induced polycondensations starting from polyesters and diamines (33,35-37). 

Polyamides. As already described, polyamide degradation takes place in the dry state at the C—C bonds, whereas in the wet state an activated mechanochemical hydrolysis predominates. However, only the macroradicals obtained from —C—C— bonds are able to initiate graft reactions. 

Baramboim and coworkers (29,30) studied the possibility of grafting solid acrylic add on polyamide, AK 60/40 by vibromilling at low temperature. Polyamide was dissolved in acrylic add, then the 25 % solution was frozen at dry ice temperature and ground for three minutes at 50 cycles/second. The reaction was completed in one minute and the mechanochemical degradation of the grafted polymer subsequently occurred. The structure of the polymers was studied by... 

Temperature Mechanochemical degradation of polyamide by vibromilling exhibited a negative temperature coefficient. The grafting yield should thus increase on decreasing temperature. In fact, the reaction is temperature insensitive. From 10 to 40° C, only a modest decrease of grafting yield was observed. Homopolymerization is almost unaffected by temperature (see Fig. 6). 

Experiments were also performed with the aim of polymerizing a mixture of two monomers (69). The reaction rate and the composition of the graft copolymer conform to the rules of mechanochemical synthesis and radical copolymerization. If the two monomers have almost equal reactivities, the composition of the copolymer is approximately that of the initial monomer mixture (Fig. 20). The... 

The fundamental task, in our opinion, is to correlate the principles and methods of the proposed synthesis with those of mechanochemical synthesis. Thus, besides the destruction processes and mechanochemical synthesis discussed in the literature, other lands of transformations sometimes occur as side reactions, or even as major processes. These include chemical fixation of small molecules (methyl chloride or butyl alcohol) on mechanically activated macromolecular backbones grafting of inorganic surfaces (quartz, metals, metallic oxides, inorganic salts, etc.) dispersed by vibratory milling on polymerized fragments synthesized from monomers present in the reaction medium, and activated by centers on the inorganic surface (14) and the possibility of some reactions (such as nitration), achieved so far on macromolecular supports and only as side reactions. 

Mechanochemical preparation of block copolymers is of historic interest. In a polymer blend, mechanical stress causes degradation, producing macroradicals which in turn lead to the formation of a large number of block and graft copolymers. Berlin assumes [269] that the radicals predominantly attack stress-activated backbone C—C bonds. Radical combination is not regarded as important as radical concentration in the system is negligible. 

Osada Y, Honda K, Ohta M. Control and water permeability by mechanochemical contraction of poly(methacryhc acid)-grafted membranes. J Membr Sci 1986 27 327-335. 

Mechanochemically Initiated Graft Copolymerization. According to the ESR study, it is clear that mechanoradicals were generated in cellulose either by means of mechanical cutting or ball milling. These mechanoradicals may be utilized as reaction sites for the initiation of vinyl polymerization which would result in graft copolymerization of cellulose. Based on this principle, the ability of cellulose mechanoradicals to initiate copol3nnerization was pursued. 

Mechanochemical initiation can be of some importance in the industrial production of block and graft copolymers. The remainder of the these methods of radical generation are not much used in technical applications. 

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