Initiation, of radical polymerization

The active centres of polymerization are produced by the addition of the primary radical to the monomer, i. e. to a n electron system. Only rarely is this simple process, and almost all branches of theoretical chemistry and chemical physics have contributed to its elucidation. The addition is a bimolecular reaction interpreted kinetically as a second-order reaction [125]. Unfortunately, most studies have been concerned with reaction in the gaseous phase. In the condensed phase, the probability that the excess energy of the reaction product will be removed by collision with a third molecule is very much higher thus the results obtained in the gaseous phase need not be valid generally. 

The rate of hydrogen atom addition to alkenes grows in the order ethylene propene 1-butene 2-methylpropene. 

The rate of H addition to 1-pentene is roughly equal to the addition to 1-butene, of the H addition to cis and trans isomers of 2-pentene as to cis-and frans-butenes. Cycloalkenes add a H atom in a similar way to simple alkenes of comparable structure. H attacks either the terminal or the internal C atom of 1,3-butadiene the first way predominates, probably due to allylic or hyperconjugative stabilization of the generated radical. 

The addition rate of a methyl radical to ethylene obeys the equation [126] 

An attack of methyl radical on propene produces predominantly butyl (90%), but also the isobutyl radical [127]. In additions to higher alkenes, neither of the two C atoms of the double bond is preferred by the methyl radical, which lacks electrophilic character. The relative reactivity of methyl with respect to ethylene, propene, 1-butene, and 2-methylpropene is roughly equal [128], 

---

In more recent years, photoinitiation of polymerization proved to be of immense value in the understanding of the precise nature of polymerization. Several systems used for the initiation of radical polymerization were reviewed by Oster and Yang [3], Rabek [10], and Davidson [5,6]. 

The kinetics and mechanism of the thermal and photochemical decomposition of dialkyldiazenes (15) have been comprehensively reviewed by Engel. The use of these compounds as initiators of radical polymerization has been covered by Moad and Solomon2 and Sheppard.50 The general chemistry of azo-compounds has also been reviewed by Koga et cr/./11 Koenig,3 and Smith.3J... 

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ecntcrcd radicals yet react with carbon-ccntcrcd radicals by coupling at or near diffusion controlled rates.179,184 This capability was utilized by Rizzardo and Solomon181 to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, w hile an efficient... 

Table 3.3. Peroxo compounds for initiation of radical polymerization...

The initiation of radical polymerizations, various transfer, as well as termination reactions all lead to a variety of products and the makeup of the mixture can only be slightly influenced by varying the reaction conditions or the monomer concentration, the initiator or the solvent. Furthermore, radical block copolymerization leads inevitably to more or less homopolymer so that the products require careful separation before the block copolymer can be characterized. Nevertheless, the synthesis of block copolymers via a radical mechanism has several important advantages ... 

All the data discussed above are only an approximation or analogy of the real initiation of radical polymerization. The detailed investigation of the kinetics and mechanism of actual initiation processes is often our future task. Only rarely it is possible to obtain quantitative data on initiation, for example the value of the initiation rate constant, without simplifying assumptions. [133]. Some further information on this problem will be presented in Sect. 8.1 and in Chap. 5, Sect. 4.2 and in Chap. 8, Sect. 1.1. 

METHACRYLIC SYSTEM. Methyl methacrylate purchased from Merck and 1,1,1-trimethylol propane trimethacrylate (TRIM) supplied by Degussa, used as crosslinker, were dried over molecular sieves but not otherwise purified, so that they still contained 15 ppm and 100 ppm methylethylhydroquinone, respectively. The initiator of radical polymerization was azobisisobutyronitrile (AIBN). 

The photoinitiation of polymerization of pentaerythritol tetraacrylate using phenyl-(p-anisyl)-iodonium triflate or triphenylsulfonium hexafluorophosphate, sensitized with either l,6-diphenyl-l,3,5-hexatriene or 1,3-diphenyl-2-pyrazoline, was illustrated by Smith [111b] in 1981. Under his conditions, direct photolysis of the onium initiators failed to initiate polymerization. Baumann and co-workers, however, found conditions for initiation of radical polymerization on direct irradiation of onium salts [18,122], consistent with the hypothesized generation of radicals capable of cage escape in direct photolysis. 

Examples of indirect initiation will be encountered later in this chapter in the Rice-Herzfeld mechanisms and hydrocarbon oxidation (see next section). Also, initiation of radical polymerization usually is a two-step process (see Section 11.3). 

The influence of aromatic diketocarboxylic acids (DC) on the decomposition initiator of radical polymerization— azobisisobutyronitrile (AIBN) was studied by UV spectroscopy. The interaction occurs with the participation of carboxyl groups of DC with nitrile groups of the initiator. It is shown that polymer obtained in the presence of aromatic DC has mainly a syndiotactic stracture. 

Among the commonly used initiators of radical polymerization the AIBN is distinguished by the fact that it is inactive in interaction with other substances and induced decomposition. The initiation efficiency of AIBN is only slightly dependent on the nature of the monomer and temperature. Therefore, the increasing of AIBN initiating activity is important. 

Yarmukhamedova et al. in Chapter 7 of this volume investigate chemical physics properties of a class of aromatic compounds (diketocarboxylic acids) on the radical initiation properties of an initiator compound used in a polymerization reaction system. Thus as Yarmukhamedova et al. describe, the influence of aromatic diketocarboxylic acids on the decomposition initiator of radical polymerization - azobisisobutyronitrile was studied by U V spectroscopy. The interaction occurs with the participation of carboxyl groups of diketocarboxylic acids with nitrile groups of the initiator. It is shown that polymer obtained in the presence of aromatic diketocarboxylic acids has mainly a syndiotactic structure. And thus such work as that reported here by Yarmukhamedova et al. advances our imderstanding of the synthesis and properties of technologically important classes of radical polymerization polymers. 

Projea No. 2004-034-1-400 Critically evaluated propagation rate coefficients for free-radical polymerization of water-soluble monomers polymerized in the aqueous phase Projea No. 2009-050-1-400 Critically evaluated rate coefficients associated with initiation of radical polymerization... 

Uehara et al. [66] also reported the polymerization of methyl methacrylate initiated by bis(acetylacetonato) metal(II) and chloral, where the metal M is either Mn(II) or Co(II). It can be frequently seen that the activity of a metal complex as an initiator of radical polymerization increases in the coexistence of an organic halide. This effect was attributed to the redox reaction between the metal complex and the organic halide [67—69]. The mechanism may be presented as... 

---