Iniferter thermal

The results of the radical polymerization of MM A in bulk at 60-100 °C with 6 as the A-B type thermal iniferter are shown in Fig. 1 [16,81]. In this polymerization, the molecular weight of the polymers produced increased with the reaction time. 

Because the polymerization with the thermal iniferters previously described was performed at a high temperature, some side reactions might be unavoidable, e.g., ordinary bimolecular termination between polymer radicals, disproportionation between a polymer radical and a small radical leading to deactivation of the iniferter site, initiation by the radical generated from the iniferter sites, rearrangements of the structure of the iniferter sites, and spontaneous initiation of polymerization. 

To develop thermal iniferters which act at a lower temperature, Otsu and Tazaki proposed a redox iniferter system [153,154]. For example, reduced nickel (Ni°) reacts with organic halides (R-X) such as benzyl chloride to form a radical, which can initiate polymerization (Eqs. 33-35) ... 

In polymerization with the compounds having a photodissociable DC group as photoiniferters, the polymerization can be performed at low temperature, such as room temperature, in contrast with thermal iniferters. Moreover, we can readily prepare many kinds of DC derivatives with various structures, indicating that the functionalization and molecular structure design are easy [156]. 

Thermal gravimetric analysis (tga), 18 851 Thermal imaging, of plastics, 19 589 Thermal imidization, 20 269-270, 282 Thermal indicators, virtual two-way SMA devices in, 22 348-349 Thermal iniferters, 14 297 Thermal insulation... 

Qin SH, Qiu KY, Swift G et al. (1999) Living radical polymerization of methyl methacrylate with diethyl 2,3-dicyano-2,3-diphenylsuccinate as a thermal iniferter. J Polym Sd Part A Polym Chem 37 4610-4615... 

Reghunadhan Nair CP, Chaumont P, Clouet G (1994) Application of thermal iniferters in free radical polymerization a new trend in macromolecular design. In Mishra MK (ed), Macromolecular Design Concepts and Practice. Polymer Frontiers International, Hope-well Jet, 11 433... 

Recently, the use of thermal iniferters in radical polymerization have also been discussed in a review by G.Clouet and C.P.R. Nair.28... 

Iniferter polymerizations were also combined with anionic polymerization. The representative example involves the synthesis of PCL-l7-(PMMA-co-PSt)-l7-PCL. ° A polymeric thermal iniferter, PCL-substituted tetraphenylethane, was prepared by anionic polymerization of CL in the presence of aluminum triisopropoxide and benzopinacol. The benzopinacolate groups incorporated into the polymer chain initiated the polymerization of St and MMA via a controlled radical mechanism at 95 °C to yield the desired block copolymers (Scheme 47)... 

Qin, D.-Q., Qin, S.-H., and Qiu, K.-Y. (2000). A reverse ATRP process with a hexasubsti-tuted cethane thermal iniferter diethyl 2,3-dicyano-2,3-di(p-tolyl)succinate as the initiator. Macromolecules, 53(19) 6987-6992. 

Thiuram disulfides, described above as photoiniferters, can also act as thermal iniferters. The mechanism of the polymerization is the same and polymer chains are invariably end-capped at both ends with iniferter segments. The use of thiurams as thermal or photoiniferters for the preparation of block copolymers greatly depends on the quantum yield of dissociation. The synthesis of dithiocarbamate functional polymers by direct photolysis of the iniferters is limited due to the low quantum yield of dissociation, especially in the case of the thiuram disulfide (e.g., the quantum yield of dissociation (rf) of TD is 0.0025 in cyclohexane [81]). This very low value makes the photochemical dissociation much less attractive than the thermal one. It was suggested that better dithiocarbamate functionalization can be achieved by either thermal initiation with TD at 95°C or polymerization in the presence of AIBN as a thermal initiator and TD as a chain transfer agent. In the latter case, monofunctional or bifunctional TD-PSt were formed, depending on the mole ratio AIBN/TD. Interestingly, the quantum yield of BDC was found to be 0.06, which is 24 times higher than that of TD. Thus, BDC can be used both as a thermal initiator and as a photoiniferter [81]. 

Apart from thiuramdisulfides, some other compounds were shown to be useful thermal iniferters, Otsu and Tazaki [89] showed that compounds of the phenylazotriphenylmethane type can act as a thermal iniferter, where the phenyl radical initiates the polymerization, and the triphenylmethyl (tri-tyl) radical is a stable radical which couples with the growing chain, exhibiting a kind of living radical nature ... 

Such polymerizations were also reported with other thermal iniferters such as highly substituted ethanes and silyl pinacole ethers. It was shown [91] that sterically hindered carbon—carbon single bonds such as those found in hexaphenylethane were easily thermally cleaved to produce trityl radicals. Subsequently, it was found that the tetraarylethane and benzpinacole could also decompose in the same way and could be used [92,93] as initiators for... 

Depending on the stimulus which activates the polymerization, thermal iniferters or photoiniferters can be used. The chemistry of thermal iniferters is mainly based on the thermolysis of triphenylmethane derivatives (Chart 3.1a), whereas photodissociation of carbon-carbon or carbon-sulfur bonds is the main process in photoiniferters (Chart 3.1b). 

Most of the thermal iniferters containing carbon-carbon bonds are symmetrical disubstituted tetraphenylethane derivatives, such as tetraphenylsuccino-dinitrile, tetra(p-methoxy phenyl) succinodinitrile, pentaphenylethane,... 

Table 3.1 Structures of typical thermal iniferters and their polymerization conditions.

Scheme 3.3 C/LRP of methyl methacrylate using phenylazotriphenyhnethane as thermal iniferter.

Thermal iniferters have been mainly used for the polymerization of methyl methacrylate. It was reported that polymerization of styrene with some thermal iniferters such as l,l,2,2-tetraphenyl-l,2-bis(phenoxy)ethane and tetraarylsc-cinonitriles proceeds via a dead-end polymerization mechanism, i.e., it does not follow living radical polymerization conditions. On the other hand, Jongh et al and Chernikova et al reported that some specially designed hexa-substituted-ethane derivatives can act as iniferters for styrene polymerization. 

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