Addition (Chain-reaction) Polymerization

In addition polymerization, the three steps to focus on are: 1. Chain initiation. This step involves the generation of active species. With the energy provided by light or heat, a peroxide molecule breaks up into two reactive free radicals. Then the free radical initiator attacks and attaches to a monomer molecule. This forms the activated monomer. 2. Chain propagation. This step is the major component and lasts much longer than the other two steps of a chain reaction. The newly-formed activated monomer attaches to the double bond of another monomer molecule. This addition reaction proceeds as there are still monomers present, and creates a continuous polymer chain with repeating monomers connected in a head-to-tail manner. 3. Chain termination. Theoretically the propagation step would occur again and again until all monomers are consumed. However, the termination can take place via combination or disproportionation of two radicals, which annihilate their activities. The molecular weight of polymer chains increases rapidly at early stage and remains approx. the same throughout the polymerization. A bulk addition polymerization is a homogeneous system with an organic initiator. If the polymer is insoluble in monomer, then initiation, propagation, and termination might happen in the monomer phase. In solution polymerization, a diluent (either water or an organic solvent) is added to the initial reaction. The main advantage of solution polymerization over bulk polymerization is better heat control; the disadvantage of solution polymerization is the extra step of removing diluent from the polymer, which considerably increases the cost.. Suspension polymerization is a heterogeneous radical polymerization process that uses mechanical agitation to mix a monomer or mixture of monomers in a liquid phase, such as water, while the monomers polymerize, forming spheres of polymer. The advantages include better heat control of the reaction, and separation is much easier than in solution polymerization. The disadvantage is that only a few monomers are water soluble. The initial reaction component for an emulsion polymerization include: a water soluble initiator, a monomer that has limited miscibility in water, and a chemical emulsifier. The two differences between emulsion and suspension polymerization are: a suspension polymerization is a mechanical process, and must have a stabilizing agent until the droplets are far apart, while the emulsion polymerization is a chemical process which monomer requires a surfactant (emulsifier) to form micelles.

The key to obtain polymers with desired molecular weight and narrow polydispersity is the use of a properly selected polymerization method that affords optimal controllability and durability during the polymer synthesis. Controlled radical polymerization (CRP) offers such an opportunity. Compared to conventional polymerization methods, CRP minimizes the side reactions and allows synthesis of unprecedented functional polymers with narrow molecular weight distribution while retaining the tolerance of free radical polymerization (FRP) to various reaction conditions. The control over molecular weight is achieved by a dynamic equilibrium between active (growing radical) and dormant (deactivated) species so that the active species is present at low concentrations at any moment. We mainly utilize three approaches to synthesize free polymers: reversible addition-fragmentation chain-transfer (RAFT), atom transfer radical polymerization (ATRP), and nitroxide-mediated polymerization (NMP).

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