Free radical polymerization is a method of polymerization by which a polymer forms by the successive addition of free radical building blocks. Free radicals can be formed via a number of different mechanisms usually involving separate initiator molecules. Following its generation, the initiating free radical adds (nonradical) monomer units, thereby growing the polymer chain.

Free radical polymerization is a key synthesis route for obtaining a wide variety of different polymers and material composites. The relatively non-specific nature of free radical chemical interactions makes this one of the most versatile forms of polymerization available and allows facile reactions of polymeric free radical chain ends and other chemicals or substrates. In 2001, 40 billion of the 110 billion pounds of polymers produced in the United States were produced by free radical polymerization.

                                                                    Initiation

Initiation is the first step of the polymerization process. During initiation, an active center is created from which a polymer chain is generated. Not all monomers are susceptible to all types of initiators. Radical initiation works best on the carbon-carbon double bond of vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones. Initiation has two steps. In the first step, one or two radicals are created from the initiating molecules. In the second step, radicals are transferred from the initiator molecules to the monomer units present. Several choices are available for these initiators.

Types of initiation and the initiators

1. Thermal decomposition: The initiator is heated until a bond is homolytically cleaved, producing two radicals (Figure 1). This method is used most often with organic peroxides or azo compounds.
   

   Figure 1: Thermal decomposition of dicumyl peroxide.
2. Photolysis: Radiation cleaves a bond homolytically, producing two radicals (Figure 2). This method is used most often with metal iodides, metal alkyls, and azo compounds.
   

   Figure 2: Photolysis of azoisobutylnitrile (AIBN).
3. Redox reactions: Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron (Figure 3). Other reductants such as Cr²⁺, V²⁺, Ti³⁺, Co²⁺, and Cu⁺ can be employed in place of ferrous ion in many instances.
   

   Figure 3: Redox reaction of hydrogen peroxide and iron.
4. Persulfates: The dissociation of a persulfate in the aqueous phase (Figure 4). This method is useful in emulsion polymerizations in which the radical diffuses into a hydrophobic monomer-containing droplet.
   

   Figure 4: Thermal degradation of a persulfate.
5. Ionizing radiation: α-, β-, γ-, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical (Figure 5).
   

   Figure 5: The three steps involved in ionizing radiation: ejection, dissociation, and electron-capture.
6. Electrochemical: Electrolysis of a solution containing both monomer and electrolyte. A monomer molecule will receive an electron at the cathode to become a radical anion, and a monomer molecule will give up an electron at the anode to form a radical cation (Figure 6). The radical ions then initiate free radical (and/or ionic) polymerization. This type of initiation of especially useful for coating metal surfaces with polymer films.
   

   Figure 6: (Top) Formation of radical anion at the cathode; (bottom) formation of radical cation at the anode.
7. Plasma: A gaseous monomer is placed in an electric discharge at low pressures under conditions where a plasma (ionized gaseous molecules) is created. In some cases, the system is heated and/or placed in a radiofrequency field to assist in creating the plasma.
8. Sonication: High-intensity ultrasound at frequencies beyond the range of human hearing (16 kHz) can be applied to a monomer. Initiation results from the effects of cavitation (the formation and collapse of cavities in the liquid). The collapse of the cavities generates very high local temperatures and pressures. This results in the formation of excited electronic states which in turn lead to bond breakage and radical formation.
9. Ternary Initiators: A ternary initiator is the combination of several types of initiators into one initiating system. The types of initiators are chosen based on the properties they are known to induce in the polymers they produce. For example, poly(methyl methacrylate) has been synthesized by the ternary system benzoyl peroxide-3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole-di-η⁵-indenylzicronium dichloride (Figure 7).
   

   Figure 7: benzoyl peroxide-3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole-di-η⁵-indenylzicronium dichloride