Radical polymerization

In polymer chemistry, radical polymerization is a method of polymerization by which a polymer forms by the successive addition of a radical to building blocks. Radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating radical adds monomer units, thereby growing the polymer chain.
Radical polymerization is a key synthesis route for obtaining a wide variety of different polymers and materials composites. The relatively non-specific nature of radical chemical interactions makes this one of the most versatile forms of polymerization available and allows facile reactions of polymeric 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 radical polymerization.
Radical polymerization is a type of chain polymerization, along with anionic, cationic and coordination 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

;Thermal decomposition: The initiator is heated until a bond is homolytically cleaved, producing two radicals. This method is used most often with organic peroxides or azo compounds.
;Photolysis: Radiation cleaves a bond homolytically, producing two radicals. This method is used most often with metal iodides, metal alkyls, and azo compounds. Photoinitiation can also occur by bi-molecular H abstraction when the radical is in its lowest triplet excited state. An acceptable photoinitiator system should fulfill the following requirements:
;* High absorptivity in the 300–400 nm range.
;* Efficient generation of radicals capable of attacking the alkene double bond of vinyl monomers.
;* Adequate solubility in the binder system.
;* Should not impart yellowing or unpleasant odors to the cured material.
;* The photoinitiator and any byproducts resulting from its use should be non-toxic.
;Redox reactions: Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron. Other reductants such as Cr²⁺, V²⁺, Ti³⁺, Co²⁺, and Cu⁺ can be employed in place of ferrous ion in many instances.
;Persulfates: The dissociation of a persulfate in the aqueous phase. This method is useful in emulsion polymerizations, in which the radical propagates initially in the water phase before entry into polymer particles.
;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.
;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. The radical ions then initiate free radical polymerization. This type of initiation is especially useful for coating metal surfaces with polymer films.
;Plasma: A gaseous monomer is placed in an electric discharge at low pressures under conditions where a plasma is created. In some cases, the system is heated and/or placed in a radiofrequency field to assist in creating the plasma.
;Sonication: High-intensity ultrasound at frequencies beyond the range of human hearing can be applied to a monomer. Initiation results from the effects of cavitation. 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.
;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 has been synthesized by the ternary system benzoyl peroxide and 3,6-bis-N-isopropylcarbazole and di-η⁵-indenylzirconium dichloride.This type of initiating system contains a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid. Metallocenes in combination with initiators accelerate polymerization of poly and produce a polymer with a narrower molecular weight distribution. The example shown here consists of indenylzirconium and benzoyl peroxide. Also, initiating systems containing heteroaromatic diketo carboxylic acids, such as 3,6-bis-N-isopropylcarbazole in this example, are known to catalyze the decomposition of benzoyl peroxide. Initiating systems with this particular heteroaromatic diket carboxylic acid are also known to have effects on the microstructure of the polymer. The combination of all of these components—a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid—yields a ternary initiating system that was shown to accelerate the polymerization and produce polymers with enhanced heat resistance and regular microstructure.

Initiator efficiency

Due to side reactions, not all radicals formed by the dissociation of initiator molecules actually add monomers to form polymer chains. The efficiency factor f is defined as the fraction of the original initiator which contributes to the polymerization reaction. The maximal value of f is 1, but typical values range from 0.3 to 0.8.
The following types of reactions can decrease the efficiency of the initiator.
;Primary recombination: Two radicals recombine before initiating a chain. This occurs within the solvent cage, meaning that no solvent has yet come between the new radicals.
;Other recombination pathways: Two radical initiators recombine before initiating a chain, but not in the solvent cage.
;Side reactions: One radical is produced instead of the three radicals that could be produced.

Propagation

During polymerization, a polymer spends most of its time in increasing its chain length, or propagating. After the radical initiator is formed, it attacks a monomer. In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule into another radical. This begins the polymer chain. Figure 12 shows how the orbitals of an ethylene monomer interact with a radical initiator.
Once a chain has been initiated, the chain propagates until there are no more monomers or until termination occurs. There may be anywhere from a few to thousands of propagation steps depending on several factors such as radical and chain reactivity, the solvent, and temperature. The mechanism of chain propagation is as follows:

Termination

is inevitable in radical polymerization due to the high reactivity of radicals. Termination can occur by several different mechanisms. If longer chains are desired, the initiator concentration should be kept low; otherwise, many shorter chains will result.

Combination of two active chain ends: one or both of the following processes may occur.
* Combination: two chain ends simply couple together to form one long chain. One can determine if this mode of termination is occurring by monitoring the molecular weight of the propagating species: combination will result in doubling of molecular weight. Also, combination will result in a polymer that is C₂ symmetric about the point of the combination.
* Radical disproportionation: a hydrogen atom from one chain end is abstracted to another, producing a polymer with a terminal unsaturated group and a polymer with a terminal saturated group.
Combination of an active chain end with an initiator radical.
Interaction with impurities or inhibitors. Oxygen is the common inhibitor. The growing chain will react with molecular oxygen, producing an oxygen radical, which is much less reactive. This significantly slows down the rate of propagation. Nitrobenzene, butylated hydroxyl toluene, and diphenyl picryl hydrazyl are a few other inhibitors. The latter is an especially effective inhibitor because of the resonance stabilization of the radical.
Chain transfer

Contrary to the other modes of termination, chain transfer results in the destruction of only one radical, but also the creation of another radical. Often, however, this newly created radical is not capable of further propagation. Similar to disproportionation, all chain-transfer mechanisms also involve the abstraction of a hydrogen or other atom. There are several types of chain-transfer mechanisms.

To solvent: a hydrogen atom is abstracted from a solvent molecule, resulting in the formation of radical on the solvent molecules, which will not propagate further. The effectiveness of chain transfer involving solvent molecules depends on the amount of solvent present, the strength of the bond involved in the abstraction step, and the stability of the solvent radical that is formed. Halogens, except fluorine, are easily transferred.
To monomer: a hydrogen atom is abstracted from a monomer. While this does create a radical on the affected monomer, resonance stabilization of this radical discourages further propagation.
To initiator: a polymer chain reacts with an initiator, which terminates that polymer chain, but creates a new radical initiator. This initiator can then begin new polymer chains. Therefore, contrary to the other forms of chain transfer, chain transfer to the initiator does allow for further propagation. Peroxide initiators are especially sensitive to chain transfer. File:Termination - chain transfer - initiator2.png|thumb|500px|center|Figure 21: Chain transfer from polypropylene to di-t-butyl peroxide initiator.
To polymer: the radical of a polymer chain abstracts a hydrogen atom from somewhere on another polymer chain. This terminates the growth of one polymer chain, but allows the other to branch and resume growing. This reaction step changes neither the number of polymer chains nor the number of monomers which have been polymerized, so that the number-average degree of polymerization is unaffected.

Effects of chain transfer: The most obvious effect of chain transfer is a decrease in the polymer chain length. If the rate of transfer is much larger than the rate of propagation, then very small polymers are formed with chain lengths of 2-5 repeating units. The Mayo equation estimates the influence of chain transfer on chain length :. Where k_tr is the rate constant for chain transfer and k_p is the rate constant for propagation. The Mayo equation assumes that transfer to solvent is the major termination pathway.