The Catalyst Review October 2019 - 8

SPECIAL FEATURE
This article focuses on analysis of polymerization initiators, i.e., organic peroxides, azo-compounds and ionic (anionic and cationic)
initiators. Table 1 presents a summary of polymerization routes, main used initiators and process technologies related to the
polymers of interest for this article.
Table 1. Polymerization production technologies using initiators.

Source: Author

Initiator Technology Advances
Free radical polymerization consists of three fundamental steps: initiation,
propagation, and termination. Initiation involves the formation of free radicals
followed by the reaction with a vinyl or diene monomer. The free radicals can be
produced in several ways, including thermal or photochemical decomposition of
organic peroxides, hydroperoxides, azo or diazo compounds. Other methods of free
radical generation are high-energy radiation and oxidation-reduction (redox) reactions
(CROW 2019). Peroxides are characterized by a structure of the R-O-O-R type,
hydroxides by an R-O-O-H structure while the azo compounds have the functional
group diazenyl R−N=N−R′, in which R and R′ can be either aryl or alkyl. One of the
most important classes of initiators are diacyl peroxides, which have the general
structure R1C(O)OOC(O)R2, wherein R1 and R2 represent alkyl and/or aryl groups.
They are thermally unstable, decomposing at relatively low temperatures thereby
generating free radicals. By far the most important organic peroxide initiator is benzoyl
peroxide (BPO). It consists of two benzoyl groups bridged by a peroxide link. BPO readily
undergoes symmetrical fission (homolysis), forming two benzoyloxy radicals (see Figure 2).

Figure 2. Free radical polymerization initiation
step.

Source: PSLC 2019

The reactivity of BPO depends on the solvent and monomer. In the absence of accelerators, the decomposition rate of diacyl
peroxides is of first order; that is, it is directly proportional to the initiator concentration. As a rough rule, the order of increasing
rate of decomposition of the peroxide in various solvents is highly halogenated aliphatics < aromatics < most aliphatics < ethers
and alcohols < amines. The effect of substituents in either or both aromatic rings has been investigated extensively. The rate of
decomposition increases in presence of electron donating groups and decreases in presence of electron withdrawing groups as
one might expect, because an increase in electron density on the oxygen atoms increases repulsion between them and therefore
enhances the rate of spontaneous cleavage of the O-O bond. The decomposition of benzoyl peroxide is considerably accelerated
by the presence of amines, particularly tertiary aromatic amines such as dimethylaniline. Di-t-butyl peroxide (DTBP) is another
important free radical initiator mainly used for the initiation of LDPE polymerization and for cross-linking silicone, EPDM, and other
rubbers. It is special as an initiator because its thermal decomposition temperature is above 100°C and, thus, it is much higher than
that of most other initiators. Another important class of initiators are peresters, which have the general structure R1C(O)OOCR2,
wherein R1 usually represents an aryl group (Ar) and R2 an alkyl group. Peresters are thermally unstable, decomposing at relatively
low temperatures, thereby generating free radicals. The thermal decomposition involves the release of CO2 and free radicals as
shown in the following: ArCH2CO−O−O−CMe3→ ArCH2+ CO2+OCMe3. One of the most common peresters is t-butyl perbenzoate
(TBPB) which contains a phenyl group as R1 and a tert-butyl group as R2. It is often used as a radical initiator in polymerization
reactions, such as the production of LDPE from ethylene, and for crosslinking, such as for unsaturated polyester resins. Aliphatic
azonitriles and related compounds are widely used as initiators for vinyl monomer polymerization. The by far most important
azonitrile is azobisdisobutyronitrile or 2,2'-azodi(isobutyronitrile), abbreviated AIBN. When AIBN decomposes, it forms two
2-cyanopropyl radicals with elimination of one molecule of nitrogen (see Figure 2). Other important azonitrile compounds include
1,1'-azodi(hexahydrobenzonitrile), and 2,2'-azodi(2-methylbutyronitrile). AIBN and some of its derivatives are usually safer to use
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The Catalyst Review

October 2019

The Catalyst Review October 2019

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