Polymerization iniferter initiator transfer agent

In each of the sections below, we will consider the initiation process separately. For each system, various initiation methods have been applied. In some cases the initiator is a low molecular weight analog of the propagating species, in other cases it is a method oT generating such a species. The initiators first used in this form of living radical polymerization were called iniferters (initiator - transfer agent - chain terminator) or initers (initiator - chain terminator). 

MIPs can be synthesized in the pores and on the surface of pre-made porous particles. Porous silica particles have been applied for this purpose. To ensure that the imprinted polymer is attached firmly to the particle, the particles are often chemically modified by coupling of polymerizable groups or initiator molecules to the particle surface prior to the MIP polymerization [100-102]. The use of immobilized initiators is often referred to as the iniferter (initiator-transfer agent-terminator) approach [103]. The method has been applied to the imprinting of a range of templates [104—107]. 

Indeed, it is now possible to extend the range of monomers incorporated into a block copolymer by conducting sequential RAFT and ATRP polymerizations employing a dual functional bromoxanthate iniferter (initiator-transfer agent-terminator). Poly(vinyl acetate)-1 -PS, poly(vinyl acetate)- 7-poly (methyl acrylate), and poly(vinyl acetate)- -PMMA block copolymers with low polydispersity 1.25) were prepared... 

One more option for increasing time scale tj of the formation of macromolecules in the processes of radical polymerization is the employment of nontraditional initiators, such, as iniferters. The term "iniferter," introduced by Otsu and Yoshida (1982), is a result of the fusion of three words initiator, transfer agent, and terminator. A special feature of an iniferter is its participation in each of the three reactions mentioned. 

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

Otsu, T. Masatoshi,Y. Role of initiator-transfer agent-terminator (iniferter) in radical polymerizations pol5nner design by organic disulfides as iniferters. Makromol. Chem. Rapid. Commun. 1982, 3, 127-132. 

The resulting polymers always have the same functional group X at both chain ends. Therefore, telechelic polymers can be readily synthesized by the two-component iniferter system. An example is the polymerization of several monomers with 4,4J-azobiscyanovaleric acid (16) and dithiodiglycolic acid (17) as the initiator and the chain transfer agent, respectively, to synthesize the polymers having carboxyl groups at both chain ends [69]. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

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]. 

Recently, Lambrinos et al. [101] observed some deviation from the proposed polymerization mechanism in iniferter systems. These authors pointed out the bimolecular termination leading to the deactivation of the iniferter site in the polymerization of n-butyl acrylate initiated by / -xylene bis(iV,iS -die-thyl-dithiocarbamate and claimed that the polymerization was not strictly living. Doi et al. [102] proposed a new two-cx)mponent iniferter system to prevent the deactivation of the iniferter site. In this system, BDC and TD act as an iniferter (or an initiator) and a chain transfer agent and/or a primary radical terminator, respectively. In the polymerization of methylacrylate (MA) with BDC bimolecular termination leading to the deactivation of the iniferter site occurred in preference to chain transfer to BDC and the dithio-carbamayl radical that produce the iniferter site, resulting in a deviation from... 

The Inifer technique enables us to fulfil some requirements of polymer architecture even in some radical processes. An amplified form may be applied, the Iniferter variant, where the radical initiator simultaneously acts as a transfer and terminating agent. Otsu et al. used sulphides and disulphides (tetraethylthiuram disulphide, PhSSPh, Ph2S, PhCH2SSCH2Ph) [96] and carbamates (benzyl-A,A-diethyldithiocarbamate, p-xylylene-A,7V-diethyl-dithiocarbamate) [97] in the photopolymerization of methyl methacrylate and styrene, and phenylazotriphenylmethane in the polymerization of methyl methacrylate [98]. Living radical polymerizations yield polymers with defined end groups or the required block copolymers. 

Otsu used the term initers (from mftiator-terminator) for compounds such as 5 or, because some of them were also able to undergo chain transfer to initiator, iniferters (from inftiator-transyer agent-terminator). Nevertheless, the essential features of a true living polymerization such as accurately controlled molecular weights and low polydispersities were absent with 5 (and other iniferters too) since the trityl radical 7 can also to some extent initiate polymerization. 

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