Peroxyesters polymerization initiation

Many types of peroxides (R-O-O-R) are known. Those in common use as initiators include diacyl peroxides (36), pcroxydicarbonatcs (37), peroxyesters (38), dialkyl peroxides (39), hydroperoxides (40), and inorganic peroxides [e.g. persulfate (41)1, Multifunctional and polymeric initiators with peroxide linkages are discussed in Sections 3.3.3 and 6.3.2.1. 

Thus, even if /-amyloxy radicals (101) show similar specificity for addition 1 5-abstraction to /-butoxy radicals, abstraction will be of lesser importance.42"42 The reason is that most /-amyloxy radicals do not react directly with monomer. They undergo [3-scission and initiation is mainly by ethyl radicals. Ethyl radicals are much more selective and give addition rather than abstraction. This behavior has led to /-amyl peroxides and peroxyesters being promoted as superior to the corresponding /-butyl derivatives as polymerization initiators.423... 

Peroxyesters Initiator for ABS polymerization Initiator for polystyrene polymerization Unsaturated polyester curing... 

Shown in Figure 10 is an Arrhenius plot of AX, for DTBP-induced ethene polymerizations at 2000 bar. According to the discussion in Reference 58, the initiator efficiency /dtbp of DTBP in ethene high-pressure polymerization is close to unity. The initiator efficienty/of an arbitrary peroxyester may be estimated from the AX value measured for ethene high-pressure polymerization initiated by this particular initiator. Using the known decomposition rate coefficients of this initiator, comparison of the measured AX, value with the reference AXr value of DTBP-induced ethene polymerization (for /= 1) obtained at a given temperature and otherwise identical reaction conditions directly yields the efficiency/for the initiator of interest. 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

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

The main industrial use of rert-alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. 

Initiators are used to initiate the curing reaction at elevated temperatures. Cross-linking or polymerization occurs by a free radical mechanism in which the double bond of the polyester chain reacts with the vinyl monomer that is usually styrene, and this reaction provides a three-dimensional network that converts the viscous resin to a hard thermoset solid. The initiators added decompose at elevated temperatures thus providing free radicals to initiate the cross-linking. Peroxyesters and peroxyketals are the most common classes of peroxides used as initiators. 

Peroxides are the most important initiators for free-radical polymerization. Among them, dialkyl or diaryl peroxides, peroxyesters, diacyl peroxides, and per-oxycarbonates are of particular relevance. For a large number of such peroxides, the temperature and pressure dependence of the decomposition rate coefficient. 

Tsubokawa, N. Fujiki, K. Sone, Y. (1988a). Graft Polymerization of Vinyl Monomers onto Carbon Black Surface Initiated by Peroxyester Groups. Polymer Journal, 20, 213-... 

Initiator efficiency within ethene high-pressure polymerization has been measured for a wide range of organic peroxyesters. Determination of / has been carried out via the quantity reduced peroxide-induced monomer conversion (AXJ presented in Reference 58. AXr has been determined under stationary conditions in a CSTR (Figure 4) from the concentration of initiator decomposed in the CSTR under polymerization conditions Ci,dec and monomer conversion AX. The quantity AXr is related to the rate coefficients of propagation kp and termination ky respectively, to CSTR residence time r and to initiator efficiency/via eqn [5] ... 

The efficiency values of the peroxyesters under investigation listed in Table 4 are in the range of 0.4tert-Butyl peroxy-2-ethylhexanoate (TBPEH) and tert-Butyl peroxypivalate (TBPP), no literature values for/have been reported so far. Luft and co-workers measured the initiator consumption of TBPEH and TBPP in ethene polymerization at 1700 bar and found that of TBPEH to... 

Table 4 Initiator efficiencies f of peroxyesters in high-pressure ethene polymerization at 2000 bar ...

Figure 11 Initiator efficiency f of fert-butyl peroxyesters in high-pressure ethene polymerization at 2000 bar. The upper, middle, and lower lines indicate the arithmetic mean values of ffor primary, secondary, and tertiary peroxyesters, respectively. For explanation of the abbreviations, see Table 3, From Buback, M, Fischer, B, Hinrichs, S, etal. Macromol. Chem. Phys. 2007, 208, 772, ...

Organic peroxides act through the splitting of the —0—0— bond into free radicals, thereby initiating the polymerization or crosslinking of monomers or polymers. Their exceptionally broad line includes diacyl peroxides, dialkyl peroxides, hydroperoxides, ketone peroxides, peroxyketals, peroxydicarbonates, and peroxyesters. The last two are particularly important in PVC resin manufacture as initiators in the polymerization of vinyl chloride monomer. 

Peroxyesters can be reduced by metal ions such as copper or iron to produce free radicals that will initiate chain growth polymerization. 

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