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Initiation

Initiation is the first step of the polymerization process in which the active center is created. There are two parts of initiation. In the first part, one or two radicals are created from the initiating molecules. There are several choices of initiators.

Part 1: Types of Initiation and Initiators

1. 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.
2. Photolysis: Radiation cleaves a bond homolytically, producing two radicals. This method is used most often with metal iodides, metal alkyls, and azo compounds.
3. Redox reactions: Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron.
4. Persulfates: The dissociation of a persulfate in the aqueous phase. This method is useful in emulsion polymerizations in which the radical diffuses into a hydrophobic monomer-containing droplet.
5. Ionizing radiation: α-, β-, γ-, or s-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical.
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. 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.

Part 2: Attack on a Monomer

The second part of initiation is when the radical initiator, produced in part one of initiation, attacks a monomer. This begins the polymer chain.

Ternary Initiating Systems

A ternary initiator is the combination of several types of initiators into one molecule. The types of initiators are chosen based on the properties they are known to have. 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.

Metallocenes, such as indenylzicronium dichloride, in combination with initiators, such as benzoyl peroxide, accelerate polymerization of poly(methyl methacrylate) and produce a polymer with a narrower molecular weight distribution. Also, initiating systems containing heteroaromatic diketo carboxylic acids, such as 3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole, are known to catalyze the decomposition of benzoyl peroxide and have effects on the microstructure of the polymer. An initiator containing all three components was shown to accelerate the polymerization and produce polymers with enhanced heat resistance and regular microstructure.

Initiator Efficiency

Due to side reactions and inefficient synthesis of the radical species, chain initiation is not 100%. The efficiency factor, f, is used to describe the effective radical concentration. The maximum value of f is 1.0, but values typically range from 0.3-0.8. The following is a list of reactions that decrease the efficiency of the initiator.

• Primary recombination: Two radicals re-combine 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 re-combine 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

Once a chain has been initiation, the chain propagates until there is no more monomer (living polymerization) or until termination occurs. The mechanism of chain propagation is as follows:

Termination

Chain termination will occur unless the reaction is completely free of contaminants. In this case, the polymerization is considered to be a living polymerization because propagation can continue if more monomer is added to the reaction. Living polymerizations are most common in ionic polymerization, however, 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.

1. Combination of two active chain ends: one or both of the following processes may occur.
   1. 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.
   2. 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.
2. Reaction of an active chain end with an initiator radical.
3. 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 (DPPH) are a few other inhibitors. The latter is an especially effective inhibitor because of the resonance stabilization of the radical.
4. 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 disporportionation, all chain transfer mechanisms also involve the abstraction of a hydrogen atom. There are several types of chain transfer mechanisms.
   1. 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 (more solvent leads to greater probability of transfer), the strength of the bond involved in the abstraction step (weaker bond leads to greater probability of transfer), and the stability of the solvent radical that is formed (greater stability leads to greater probability of transfer). Halogens, except fluorine, are easily transferred.
   2. 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.
   3. 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.
   4. To polymer: the radical of a polymer chain abstracts a hydrogen atom from somewhere on another polymer chain. This terminates one of the polymer chains, but allows the other to branch. When this occurs, the average molar mass remains relatively unaffected.
   5. Effects of chain transfer: The most obvious effect of chain transfer is a decrease in the polymer chain length. If the rate of termination is much larger than the rate of propagation, then very small polymers are formed with chain lengths of 2-5 repeating units (telomerization). The Mayo equation estimates the influence of chain transfer on chain length (X_n): . 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.

Methods of Radical Polymerization

There are four industrial methods of radical polymerization:

1. ''Bulk polymerization: reaction mixture contains only initiator and monomer, no solvent.
2. ''Solution polymerization: reaction mixture contains solvent, initiator, and monomer.
3. ''Suspension polymerization: reaction mixture contains an aqueous phase, water-insoluble monomer, and initiator soluble in the monomer droplets.
4. ''Emulsion polymerization: similar to suspension polymerization except that the initiator is soluble in the aqueous phase rather than in the monomer droplets. An emulsifying agent is also needed.

Controlled Radical Polymerization (CRP)

Also known as living radical polymerization, this method relies on completely pure reactions so that no termination caused by impurities occurs. Because these polymerizations stop only when there is no more monomer and not when termination occurs, the polymerization can continue upon the addition of more monomer.3 Block copolymers can be made this way. There are two main types of CRPs:

• Atom Transfer Radical Polymerization (ATRP): based on the formation of a carbon-carbon bond by atom transfer radical addition. This method requires reversible activation of a dormant species (such as an alkyl halide) and a transition metal halide catalyst (to activate dormant species).
• Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT): requires a compound that can act as a reversible chain transfer agent, such as dithio compounds.

Another, relatively new, form of CRP is Stable Free Radical Polymerization (SFRP). This process is used to synthesize linear or branched polymers with narrow molecular weight distributions and reactive end groups on each polymer chain. The process has also been used to create block co-polymers with unique properties. Conversion rates are about 100% using this process but require temperatures of about 135 °C. This process is most commonly used with acrylates, styrenes, and dienes. The following reaction scheme illustrates the SFRP process. Because the chain end is functionalized with the TEMPO molecule, premature termination by coupling is reduced. As with all living polymerizations, the polymer chain grows until all of the monomer is consumed.