Initiator of polymerization

Among other possible reactions, these free radicals can initiate ordinary free-radical polymerization. The Ziegler-Natta systems are thus seen to encompass several mechanisms for the initiation of polymerization. Neither ionic nor free-radical mechanisms account for stereoregularity, however, so we must look further for the mechanism whereby the Ziegler-Natta systems produce this interesting effect. 

Miscellaneous Reactions. Epoxy compounds yield chlorosubstituted carbonates (45). The reaction of chloroformates with hydrogen peroxide or metal peroxides results in the formation of peroxydicarbonates that are used as free-radical initiators of polymerization of vinyl chloride, ethylene, and other unsaturated monomers (46,47). 

This result reveals that exciplex formation plays a principal role in the initiation of polymerization. Since the absorption band is broadened toward longer wavelengths as the result of formation of CTC between AN and aniline, a certain concentration of aniline can be chosen so that 365-nm light is absorbed only by the CTC but not by the aniline molecule. Therefore, in this case the photopolymerization may be ascribed to the CTC excitation selected. For example, a 5 x 10 mol/L aniline solution in AN could absorb light of 365 nm, while solutions in DMF or cyclohexane with the same concentration will show no absorption. Obviously, in this case the polymerization of AN is caused by CTC excitation. The rates of polymerization for different amines were found to be in the following order (Table 12) ... 

Thus, deprotonation of the aminium radical from a secondary or primary amine will at last form an amino radical instead of an aminoalkyl radical and a CH2CH2CN radical. This amino radical will then serve as one of the active species for the initiation of polymerization. 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

Another mode of initiation of polymerization being vigorously studied in numerous laboratories involves the use of ionizing radiation. In the primary process a molecule (not necessarily the monomer) is ionized and thus a positive ion and an electron are formed. The latter is eventually captured by another molecule yielding a negative ion. On recombination of these ions, enough... 

Ideal localized adsorption, 5, 10 Infrared method, 373, 374 Initiators of polymerization, 148 Intramolecular London energies, 74, 78 Iodine molecule intramolecular London energy, 78... 

Largely for these reasons, radicals are most often characterized indirectly by examining the products of their reaction. Many of the methods used to study radical reactions have been applied to study initiation of polymerization. Some of these techniques are detailed in Section 3.5. 

Unsymmetrical azo-compounds find application as initiators of polymerization in special circumstances, for example, as initiators of living radical polymerization [e.g. triphenylmethylazobenzene (30) (see 9.3.4)], as hydroxy radical sources [e.g. a-hydroperoxydiazene (31) (see 3.3.3,1)1, for enhanced solubility in organic solvents [e.g. f-butylazocyclohexanecarbonitrile (32)J, or as high temperature initiators [e.g. t-butylazoformamide (33)]. They have also been used as radical precursors in model studies of cross-termination in copolymerization (Section... 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

The general chemistry of the peroxides has been covered in many books and reviews.2 46"32 1-4 131 Readers are referred in particular to Swem s Trilogy127120for an excellent background and a comprehensive coverage of the literature through 1970. The chemistry associated with their use as initiators of polymerization was reviewed by Moad and Solomon.2... 

Only a few diacvl peroxides see widespread use as initiators of polymerization. The reactions of the diaroyl peroxides (36, R=aryl) will be discussed in terms of the chemistry of BPO (Scheme 3.25). The rate of p-scission of thermally generated benzoyloxy radicals is slow relative to cage escape, consequently, both benzoyloxy and phenyl radicals are important as initiating species. In solution, the only significant cage process is reformation of BPO (ca 4% at 80 °C in isooctane) II"l only minute amounts of phenyl benzoate or biphenyl are formed within the cage. Therefore, in the presence of a reactive substrate (e.g. monomer), tire production of radicals can be almost quantitative (see 3.3.2.1.3). 

The chemistry of peroxydicarbonates (37) and their use as initiators of polymerization has been reviewed by Yamada e.t al., Hiatt1 3 and Strong.1 ... 

Aryl peroxyesters are generally unsuitable as initiators of polymerization owing to the generation of phenoxy radicals that can inhibit or retard polymerization... 

The common initiators of this class are f-alkyl derivatives, for example, t-butyl hydroperoxide (59), Aamyl hydroperoxide (60), cumene hydroperoxide (61), and a range of peroxyketals (62). Hydroperoxides formed by hydrocarbon autoxidation have also been used as initiators of polymerization. 

Benzoin and a wide variety of related compounds (e.g. 12, 70-74) have been extensively studied both as initiators of polymerization and in terms of their general photochemistry.271 2 3 The acetophenone chromophore absorbs in the near UV (300-400 nm). In the absence of hydrogen atom donors the mechanism of... 

Oxygen-centered radicals are arguably the most common of initiator-derived species generated during initiation of polymerization and many studies have dealt with these species. The class includes alkoxy, hydroxy and aeyloxy radicals and tire sulfate radical anion (formed as primary radicals by homolysis of peroxides or hyponitrites) and alkylperoxy radicals (produced by the interaction of carbon-centered radicals with molecular oxygen or by the induced decomposition of hydroperoxides). 

The first use of sterically hindered hexasubstituted ethanes [e.g. 33] as initiators of polymerization was reported by Bledzki et al.77,78 The use of related initiators based on silylatcd pinacols [e.g. 34, 35] has been reported by Crivcllo et a/.,7l>82 jjan( os et ai i anOther initiators of this class include 36 fi X/ and 37.The rates of decomposition of hexasubstituted ethanes and the derived macroinitiators are known to vary according to the degree of steric... 

The initiation of polymerization of styrene and isoprene in benzene by t-butyl lithium reveals some complexities129) (e.g. zero order kinetics in monomer) not observed in the reaction proceeding in cyclohexane. Further studies of that system are needed. 

In batch reactors, heat transfer will also limit the rate of heat-up to the required temperatures for initiation of polymerization. Use of a multiple catalyst system to provide lower temperature initiation has been proposed to minimize the time and energy required in heating. 

Ugelstadt, J., El-Aasser, M.S. and Vanderhoff, J.W. (1973) Emulsion polymerization initiation of polymerization in monomer droplets. Journal of Polymer Science Polymer Chemistry Edition, 11, 503-513. 

Sadurm, N., Solans, C., Azemar, N. and Garci a-Celma, M.J. (2005) Studies on the formation of O/W nano-emulsions, by low-energy emulsification methods, suitable for pharmaceutical aplications. Emulsion polymerization initiation of polymerization in monomer droplets., 26, 438-445. 

Polymerizations. The polymerizations were carried out in an argon atmosphere in capped glass bottles fitted with a neoprene rubber gasket inner liner. In charging the polymerizations, the order of addition of materials was solvent first, then metal alkyls, next the barium salt, and finally the monomer(s). The amount of metal alkyl charged was sufficient to titrate the acidic impurities present in the solvent and polymerization bottle, plus the calculated amount for initiation of polymerizations. The mole ratio of barium to metal alkyl(s) was based on the moles of total alkalinity of barium to the moles of carbon-metal assayed. Unless otherwise stated,... 

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