Addition polymerization links vinyl monomers by activating their double bonds, following a chain addition process with initiation, propagation, and termination steps.
The initiation phase can be triggered by either free radical or ionic systems. Here, we’ll focus on a free radical system. A substance that decomposes into free radicals through warming, the presence of a promoter, or exposure to ultraviolet light is added to the monomer, resulting in radical formation. Examples include benzoyl peroxide and azodi-isobutyronitrile, both decomposing as illustrated in Figure 1.1.
Such free radical formation maybe generally indicated as
The radical formation rate hinges on factors such as initiator concentration, temperature, and the presence of other agents. As the subsequent polymer growth stages are nearly instantaneous, the comparatively slow nature of this phase extends overall conversion times in most polymerizations to at least 30 minutes, and occasionally much longer.
The radicals generated can subsequently engage in addition reactions with a monomer molecule, creating another radical.
This radical proceeds to interact with an additional monomer molecule, producing another free radical with similar reactivity.
The reaction can repeat numerous times, joining several thousand monomer units in approximately 1 second, resulting in the formation of a long-chain free radical. This stage is known as propagation or growth.
Termination can occur through various methods, including:
(1) Mutual combination of two growing radicals
(2) Disproportionation between growing radicals
(3) Reaction with an initiator radical
(4) Chain transfer with a modifier
This response concludes the chain growth without a net reduction in radical concentration, thus having no impact on the reaction’s velocity.
(5) Chain transfer with monomer
(6) Interaction with a molecule to create a stable free radical, for instance, hydroquinone.
Termination mechanisms (1) and (2) are common, while (4) and (6) are technologically significant. Although increasing the polymerization temperature can reduce molecular weight to some extent, there are limits, and higher temperatures may lead to undesirable side reactions. Alternatively, incorporating small amounts of modifiers, known as chain transfer agents and regulators, allows for controlled chain growth without affecting the reaction rate. Examples include chlorinated materials like carbon tetrachloride, trichloroethylene, and mercaptans such as dodecyl mercaptan.
In mechanism (6), inhibitors like quinone, hydroquinone, and tertiary butylcatechol can prevent chain growth by preferentially reacting with free radicals, forming a stable product. These inhibitors are valuable for preventing the premature polymerization of monomers during storage or manufacturing. Polymerizing two monomers together can result in a binary copolymer, while copolymerizing more than two monomers produces a ternary copolymer or terpolymer. Homopolymers are made from a single monomer, and other copolymer forms include alternating copolymers, block copolymers, and graft polymers.
Figure 1.2 shows various ways to combine monomers A and B in one chain. Polymerization can occur in bulk, in a solvent, or in suspension or emulsion. While detailed considerations for specific polymers are in later chapters, some general points can be noted. Bulk polymerization is theoretically straightforward, yielding products with clarity and electrical insulation characteristics expected for a given material. However, due to the exothermic nature of polymerization reactions and the low thermal conductivity of polymers, there are significant risks of overheating and loss of control in bulk polymerization.
Commercially, bulk reactions are employed, demanding precise temperature control. Polymerization in a suitable solvent dilutes the reacting material, minimizing exothermic issues through convective movement or stirring. However, solvent removal poses challenges, including solvent recovery issues and potential fire and toxicity hazards.
An alternative to address the exotherm issue is suspension polymerization. Here, monomers are vigorously stirred in water to create small droplets. To prevent cohering at the sticky stage, suspension or dispersion agents like talc, poly (vinyl alcohol), or gelatine are added to coat each droplet protectively. Polymerization happens within each droplet, producing small beads relatively free from contaminants, provided a monomer-soluble initiator is used.
The reaction undergoes significant modification with the emulsion polymerization technique. In this method, the reaction mixture comprises approximately 5% soap and a water-soluble initiator system. The monomer, water, initiator, soap, and other ingredients are stirred in the reaction vessel. Monomer droplets are emulsified by some soap molecules, and excess soap forms micelles of about 100 molecules. In these micelles, the polar ends of the soap molecules face outward toward the water, while the non-polar hydrocarbon ends face inward (see Figure 1.3).
Monomer molecules, with limited solubility in water, diffuse through the water, swelling soap micelles. The initiator decomposes into free radicals that activate polymerization within the micelle. Chain growth continues until a second radical enters, initiating another chain. Kinetic analysis reveals that two growing radicals in the same micelle survive only briefly before mutual termination. The micelles then remain inactive until a third radical enters, starting the growth of another chain, and this pattern continues. Statistically, the micelle is active half the time, with half of them containing growing chains at any given moment.
As the reaction progresses, swollen micelles expel polymer particles stabilized by soap molecules, serving as sites for additional polymerization and absorbing monomer molecules. The resulting polymerized product consists of particles much smaller (50-500 nm) than those produced in suspension polymerization. While emulsion polymerization enables the rapid production of high molecular weight polymers, the inevitable inclusion of substantial soap amounts adversely impacts electrical insulation properties and polymer clarity.
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