Poly (vinylidine chloride)
Vinylidine chloride homopolymers form readily by free-radical polymerization, but lack sufficient thermal stability for commercial use. Copolymers, however, with small amounts of comonomers find many applications.
The monomer, vinylidine chloride, can be prepared by dehydrochlorination of 1,1,2-trichloroethylene:
It is a colorless liquid that boils at 32C. Also, it is rather hard to handle as it polymerizes on standing. This takes place upon exposure to air, water, or light. Storage under an inert atmosphere does not completely prevent polymer formation. Poly (vinylidine chloride) can be formed in bulk , solution, suspension, and emulsion polymerization processes. The products are highly crystalline with regular structures and a melting point of 220C. The structure can be illustrated as follows:
This regularity in structure is probably due to little chain transferring to the polymer backbone during polymerization. Such regularity of structure allows close packing of the chains and, as a result, there are no effective solvents for the polymer at room temperature. Copolymerization of vinylidine chloride with vinyl chloride reduces the regularity of the structure. It increases flexibility and allows processing the polymer at reasonable temperatures. Due to extensive crystallization, however, that is still present in 85:15 copolymers of vinylidine chloride with vinyl chloride, they melt at 170C. The copolymerization reactions proceed at slower rates than do homopolymerizations of either one of the monomers alone. Higher initiator levels and temperatures are, therefore, used. The molecular weights of the products range from 20,000 to 50,000. These materials are good barriers for gases and moisture. This makes them very useful in films for food packaging. Such films are formed by extrusion and biaxial orientation. The main application, however, is in filaments. These are prepared by extrusion and drawing. The tensile strength of the unoriented material is 10,000 lb/in.2 and the oriented one 30,000 lb/in.2. Vinylidine chloride is also copolymerized with acrylonitrile. This copolymer is used mainly as a barrier coating for paper, polyethylene, and cellophane. It has the advantage of being heat sealable.
