Conventional radical initiators

Alternating copolymer can be synthesized using most of the ylides as radical initiators even in the absence of Lewis acids otherwise essential for other conventional radical initiators. 

Conventional radical initiators, various redox systems, /3-diketonates of transition metals alone and in conjunc-... 

Dithiols and dienes may react spontaneously to afford dithiols or dienes depending on the monomer dithiol ratio.221 However, the precise mechanism of radical formation is not known. More commonly, pholoinilialion or conventional radical initiators are employed. The initiation process requires formation of a radical to abstract from thiol or add to the diene then propagation can occur according to the steps shown in Scheme 7.17 until termination occurs by radical-radical reaction. Termination is usually written as involving the monomer-derived radicals. The process is remarkably tolerant of oxygen and impurities. The kinetics of the tbiol-ene photopolymerizalion have been studied by Bowman and... 

So-called reverse ATRP has been described where a conventional radical initiator (e.g. AIBN) and a transition metal complex in its Higher oxidation state are used. 85"288 One of the first systems explored was ( uBr- 133 AIBN VI VIA. It is important that the initiator is completely consumed early in the polymerization. The use of peroxide initiators in reverse ATRP can be problematical depending on the catalyst used and the reaction temperature.286 289 The system CuBr2/133/BPO/MMA at 60°C was found to provide no control,286 In ATRP at lower temperatures (40 °C), the system CuCl/133/BPO/MMA was successful though dispersities obtained were relatively broadf89 Radicals are produced from the redox reaction between the catalyst in its reduced form and BPO. 

The product from this system can be cured by using a conventional radical initiator such as benzoyl peroxide (BPO). To a sample (100 g) of this product, 1.5 g of BPO in a small amount of styrene was added. The mixture was poured into a mold and heated at 50 °C overnight and then at 75 °C and 100 °C for 2h each, and finally at 135 °C for 3 h in an air oven. The material hardened to a transparent solid with a very good surface. 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

Wang J-S, Matyjaszewski K (1995) Living Vcontrolled radical polymerization. Transition-metal-catalyzed atom transfer radical polymerization in the presence of a conventional radical initiator. Macromolecules 28 7572... 

Another route to the metal-catalyzed living or controlled radical polymerization is through initiation by a conventional radical initiator (A—A) such as AIBN in conjunction with a metal complex [M"1 X,i L, ] at a higher oxidation state, for example, CuC12/L-1 (Scheme 4). This system is sometimes... 

Figure 10. Conventional radical initiators employed for metal-catalyzed polymerizations.

Rh138 was almost the same (almost atactic, slightly syndiotactic) as the tacticity of those obtained with conventional radical initiators such as AIBN under similar conditions. The triad ratio of rr.mr.mm as determined by 13C NMR is usually 58 38 4 and does not change even with the use of chiral and/or bulky ligands.103116 These results may exclude a coordination mechanism and suggest a radical nature. However, the stereochemical structure alone is not strong evidence for the radical polymerization because, for example, group-transfer polymerization, basically via an anionic mechanism, results in a stereo structure of PMMA similar to those for free radical processes.263... 

The kinetic parameter for the radical dissociation of a carbon—halogen terminal was obtained with the use of an isolated polystyrene with a terminal C—Br bond in the presence of a copper catalyst and a conventional radical initiator with a long half-life.282-283 The result was compared with that of low molecular weight compounds of similar carbon-halogen bonds.163 The second-order rate constant of the model compound 1-13 (X = Br), an effective initiator for styrene, is comparable to that of the polymer terminal. Alternatively, rate constants can be obtained by using a combination of nitroxide-exchange reactions and HPLC analysis.242... 

Note that MW of the samples synthesized using closo complexes, are noticeably lower as compared to the polymers obtained in the presence of either the exo-nido complexes or conventional radical initiators (such as AIBN and peroxides) where the molecular weights of polyacrylates range from several hundred thousands to several millions. Since mthenacarboranes with the closo stmcture are more promising for the synthesis of narrow-dispersed polymers, we studied the kinetics of MMA polymerization in the presence of the above mentioned catalysts and analyzed the molecular-weight characteristics of the polymers in more detail, using complex 4 as an example. 

We first examined thymol iodide (Tl) (Figure 1), a phenoxy iodide, as a catalyst (deactivator 1-A). Tl is commercially available and used as a medicine, and thymol (with an OH group instead of an 01 group) is a scent component of thyme. We examined the polymerization of styrene at 100 °C, using PE-I (Figure 1) as a low-mass alkyl iodide (dormant species), VRllO as a conventional radical initiator (an azo initiator), and Tl as a catalyst In this polymerization (Scheme lb). Polymer, which is originally supplied by VRl 10, is supposed to react with Tl (R-OI), in situ producing the activator radical, the thymoxy radical (R-0 ) (and Polymer-I). If R-0 effectively abstracts I from PE-... 

As a conventional radical initiator, the use of BPB, which decomposes faster than VRllO, led to a faster polymerization (Figure 2 (filled square) and Table 1 (entry 2)). In the examined case, the conversion reached 76 % for 7 h, keeping a good polydispersity control. 

Table 1. Bulk Polymerizations of Styrene (8 M) (100 equiv to [PE-I]o) with PE-I, Conventional Radical Initiator (I), and Catalyst (100 "C).

Since radicals arc neither formed nor destroyed during reversible chain transfer, RAFT polymerization must, like conventional radical polymerization, be initiated by a source of free radicals as shown in Scheme 9.38. RAFT polymerization is usually carried out with conventional radical initiators. Most often thermal initiators (e.g. AIBN, ACP, BPO, K2SnOs) are used. S polymerization may be initiated thermally between 100-130°C. Polymerizations initiated with UV irradiation, a gamma source " or a plasma tleld have been reported. In these polymerizations, radicals generated directly from the RAFT agent may be responsible for initiation. It was initially suggested by Pan and coworkers that the mechanism for molecular weight control in and y-... 

Various stable radicals such as nitroxide, triazolinyl, trityl, and dithiocarbamate have been used as the mediating or persistent radical (deactivator) for SFRP. Nitroxides are generally more efficient than the others. Cyclic nitroxide radicals such as 2,2,6,6-tetramethyl-l-piper-idinoxyl (TEMPO) have been extensively studied. SFRP with nitroxides is called nitroxide-mediated polymerization (NMP). Polymerization is carried out by two methods that parallel those used in ATRP [Bertin et al., 1998 Georges, 1993 Flawker, 1997 Flawker et al., 2001]. One method involves the thermal decomposition of an alkoxyamine such as 2,2,6,6-tetramethyl-l-(l-phenylethoxy)piperidine into a reactive radical and a stable radical (Eq. 3-236). The other method involves a mixture of a conventional radical initiator such as... 

NMP is based on the concept of a dynamic equilibration between dormant alkoxyamines and propagating radicals as shown in eqn [55].The choice of the persistent radical is cmcial for controlled polymerization. While styrene can be easily moderated by 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO), other monomers required the development of nitroxides that contain hydrogen atoms at the a-C. There are two different initiation methods for NMP. Conventional radical initiators (i.e., AIBN, BPO) in conjunction with a persistent radical were initially used to prepare polymers by NMP, but these systems were limited in the choice of monomer. Functionality could be incorporated via a functionalized initiator or a functionalized persistent radical. For example, Baumert and Mulhaupt prepared carboxylic acid-terminated polystyrene, poly(styrene-co-acrylonitrile), and polystyrene-b-poly (styrene-co-acrylonitrile) by the use of the functionalized initiator 4,4 -azobis(4-cyanopentanecarboxylic acid). The polymerization was controlled by the addition of 2,2,6,6-tetramethyl-l-piperidyloxyl radical, and polymers with... 

NMR spectra revealed that only one structure, C6Fi3(VPA) I, was obtained, indicating that the oligomers are synthesized from transfer reactions and that conventional radical initiation and termination did not occur (Scheme 4.14). 

We examined the polymerization of St at 80 °C, using 1-phenylethyl iodide (PE-I) as a low-mass alkyl halide initiator, GeLi as a deactivator, and benzoyl peroxide (BPO) as a conventional radical initiator. In this polymerization, P, which is originally supplied by BPO, is supposed to react with Gel4, in situ producing the activator Gelj (and P-I). If Gel effectively abstracts I from PE-I (or P-I) to produce PE (or P ), a useful sequence of activation and deactivation will be completed. 

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