Free-radical-initiated chain polymerization polyvinyl chloride

Random copolymers are often formed by chain polymerizations when two or more monomers are polymerized together. Many commercial polymers belong to this group, e.g. styrene/acrylonitrile (SAN), polyvinyl chloride/ polyvinylidene dichloride (Saran film), polyvinylidene difluoride/polyhexa-fuoropropene (Viton) which are all produced using free radical initiators (section 1.8.1). Ethylene/propylene elastomers are random copolymers (section 1.15.1.4) and they are obtained with Ziegler catalysts. 

In addition polymerization, monomers react to form a polymer chain without net loss of atoms. The most common type of addition polymerization involves the free-radical chain reaction of molecules that have C = C bonds. As in the chain reactions considered in Section 18.4, the overall process consists of three steps initiation, propagation (repeated many times to build up a long chain), and termination. As an example, consider the polymerization of vinyl chloride (chloro-ethene, CH2 = CHC1) to polyvinyl chloride (Fig. 23.1). This process can be initiated by a small concentration of molecules that have bonds weak enough to be broken by the action of light or heat, giving radicals. An example of such an initiator is a peroxide, which can be represented as R—O—O—R, where R and R represent alkyl groups. The weak 0—0 bonds break... 

Addition polymerization takes place for unsaturated monomers. In the presence of a catalyst, such as a free radical, a pi bond in the monomer is disturbed, and the resulting molecule is. itself, a chemically active free radical. This first step of the process is called initiation. The process may then continue, with the new molecule bonding with additional monomers in the same manner, thus forming a chain. Following this propagation step, free radicals may combine, thus forming a more stable polymer chain. This final step is called termination. Peroxides, such as benzoyl peroxide, are common agents that, when heat is applied, form free radicals that can initiate the polymerization process. An example of addition polymerization is shown below for the monomer vinyl chloride, which forms polyvinyl chloride. 

Chain-reaction polymerizations (also referred to as addition polymerizations) require an initiator for polymerization to occur. Initiation can occur by a free radical, an anionic, or a cationic species. These initiators open the double bond of a vinyl monomer, and the reaction proceeds as shown above in Fig. 1.1. Chain-reaction polymers typically contain only carbon in their backbone and include such polymers as polystyrene and polyvinyl chloride. 

The chain addition polymerizations require monomers with double bonds. They require free radical or ionic initiators to open the double bond and form the polymerization path in the manufacture of polymers such as polyethylene, polypropylene, polystyrene, and polyvinyl chloride which together constitute the majority of polymers, about 70 % of all polymers produced. A wide range of copolymers or terpolymers are produced by chain addition polymerization of two or three different monomers with double bonds. 

Polyvinyl chloride (PVC) is a homopolymer of vinyl chloride. Eighty percent of commercial PVC in packaging is produced by chain-reaction polymerization using a suspension method. Other methods are emulsion and solution polymerization. Chain-reaction polymerization requires initiators to produce free radicals, then the reaction proceeds until the chain is terminated. The predominant configuration of the monomer in the polymer chain follows a head-to-tail alignment to yield a syndiotactic polymer. 

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