Preparation of Graft Copolymers with Ionic Chain-Growth and Step-Growth Polymerization Reactions
Both anionic and cationic mechanisms can be used to form graft copolymers. A typical example of graft copolymer formation by anionic mechanism is grafting polyacrylonitrile to polystyrene [364]:
Another example of ionic graft copolymerization in a reaction carried out on pendant olefinic groups using Ziegler-Natta catalysts in a coordinated anionic type polymerization [365]. The procedure consists of two steps. In the first one, diethyl aluminum hydride is added across the double bonds. The product is subsequently treated with a transition metal halide. This yields an active catalyst for polymerizations of a-olefins. By this method, polyethylene and polypropylene can be grafted to butadiene styrene copolymers. Propylene monomer polymerization results in formations of isotactic polymeric branches:
Another example is formation of graft copolymers of formaldehyde with starch, dextrin, and poly (vinyl alcohol) [366, 367]. This procedure is also carried out in two steps. Potassium naphthalene is first reacted with the backbone polymer in dimethyl sulfoxide. The formaldehyde is then introduced in gaseous form to the alkoxide solution. A similar reaction can be used to form graft copolymers of poly (ethylene oxide) on cellulose acetate [391]. Poly (ethylene oxide) can also be grafted to starch. For instance, a preformed polymer [392] terminated by chloroformate end groups can be used with potassium starch alkoxide:
The products are water-soluble. The efficiency of the coupling process, however, decreases with an increase in the DP of poly (ethylene oxide). Lithiated polystyrene reacts readily with halogen-bearing polymers like polychlorotrifluoroethylene [411]. This can be utilized in formation of graft copolymers. The reactions can be conducted in solutions as well as in preparations of surface grafts on films [411]. An example of a cationic grafting reaction is formation of graft copolymers of polyisobutylene on polystyrene backbones [393]. Polystyrene is chloromethylated and then reacted with aluminum bromide in carbon disulfide solution. This is followed by introduction of isobutylene:
The above, however, yields only 5–18% of a graft copolymer, even at 60C. It is possible that considerable amounts of cross-linking occur at the reaction conditions and may, perhaps, be the reason for the low yield [393]. Another example is grafting to cellulose. BF3 can be adsorbed to the surface of the polymer. It then reacts with hydroxy groups and yields reactive sites for cationic polymerization of a-methyl styrene and isobutylene [402]. These reactions are carried out at 80C. Cationic graft copolymerizations of trioxane can be carried out with the help of reactive C–O–C links in a number of polymers, like poly (vinyl acetate), poly (ethylene terephthalate), and poly (vinyl butyral) [403]. Many graft copolymers can also be formed by ring opening polymerizations [404]. The reactions with active hydrogens on the pendant groups, like hydroxyl, carboxyl, amine, amide, thiol, and others, can initiate some ring opening polymerizations. An example is preparation of graft copolymers of ethylene oxide with styrene [405]. Copolymers of styrene with 1–2% of hydrolyzed vinyl acetate (vinyl alcohol after hydrolysis) can initiate polymerizations of ethylene oxide and graft copolymers form. Recently, solutions of polysilanes were treated with controlled amounts of triflic acid (CF3SO2OH) in CH2Cl2 and afterwards with tetrahydrofuran. This yielded a graft copolymer of poly (tetramethylene oxide) on polysilane backbones [412]. An interesting series of papers were published by Kennedy and coworkers on use of alkylaluminum compounds as initiators of graft copolymerizations [366, 367]. Allylic chlorines form very active carbon cations in the presence of this initiator. This is also true of macromolecular carbon cation sources [402]. As a result, very high grafting efficiency is achieved in many different polymerizations using macromolecular cationogens and alkylaluminum compounds. In some instances, formation of graft copolymers is greater than 90%. The grafting reaction can be illustrated as follows [367].
The temperatures of the reactions and the nature of the aluminum compounds are the most important synthetic variables [367]. On the other hand, many other graft copolymerizations by cationic mechanism suffer from low grafting efficiencies. They are also often accompanied by large formations of homopolymers. Use, however, of living cationic processes appears to overcome this drawback. An illustration of this can be another preparation of a graft copolymer of polyisobutylene on a polystyrene backbone [413]:
