Grafting, anionic Chain reactions

Ionic routes such as formation of the polymer anion by reaction with a strong base or the direct reaction of a polyamide with sodium are less likely to be used in reactive processing than is free-radical initiation. The process of self-graft polymerization by chain transfer to polymer, when it occurs in a single monomer/polymer system during polymerization, is an example of chain branching that is discussed in the next section. 

Metalation Methods. It is possible to metalate a hydrocarbon polymer chain and thereby introduce a well-controlled number of random graft sites by appropriate control of the metalating agent (Reaction 25). Graft polymers are then easily made (69) by introduction of styrene or other monomers that can be anionically polymerized (Reaction 26). 

For example, Matzner et aL (1973) succeeded in grafting side chains of nylon 6 onto a polystyrene backbone by means of anionic polymerization of 8-caprolactam. To facilitate grafting, polystyrenes containing up to 5% of ethyl acrylate or methyl methacrylate as comonomer were synthesized. The ester group served as the initiator for the lactam polymerization, thus permanently attaching the side chain to the backbone. The mechanism of the reaction is as follows ... 

The lithiated polyethylene copolymer was then suspended in hexane or THF solvent. The graft-from reactions were carried out in slurry solution by reacting the lithiated polyethylene copolymer with anionic polymerizable monomers, such as styrene and p-methylstyrene. After certain reaction time, 10 ml of isopropanol was added to terminate the graft-from reaction. The precipitated polymer was filtered and then subjected to fractionation. Good solvents for backbone and side chain polymers were used during the fractionization, using a Soxhlet apparatus under N2 for 24 hours. The soluble fractions were isolated by vacuum-removal of solvent. Usually, the total soluble fractions were less than 5 % of the product. The major insoluble fraction was PE graft copolymer, which was completely soluble in xylene or trichlorobenzene at elevated temperatures. 

Ionic polymerisation is a well-known technique for the preparation of graft copolymers but the fate of these reactions is determined by the reaction conditions. Since the discovery of living polymerisation , (anionic polymerisation) [67] it has become an excellent method for the synthesis of block and graft copolymers. In anionic polymerisation the graft copolymerisation is initiated by the anion generated by the reaction of bases with acidic protons in the polymer chain as shown in Scheme 2. 

---