Functional iniferters

The photochemically and thermally induced iniferter properties of the tetraalkylthiuram disulfides during free radical polymerization were also exploited to end functionalize PMMA and PSt (Scheme 3.6). Table 3.4 summarizes the functional iniferters used for obtaining telechelic polymers. 

Controlled Living Radical Polymerization in the Presence of Iniferters Table 3.4 Functional iniferters. 

Polyfunctional iniferters have also been employed to prepare branched, network and star polymers (8-10, Chart 3.3). For example, Ishizu et showed that functionalized iniferters ((A, A -diethyldithiocarbamyl) methyl styrene or 2-(A, A -diethyldithiocarbamyl)ethyl methacrylate) can be utilized for the synthesis of hyperbranched polymers (UV irradiation), star polymers (copolymerization of vinyl head with crosslinking agent in dark condition), and rigid polymer brushes (vinyl homopolymerization of macroiniferters and subsequent treatment by internal domain locking with diamine compounds). 

These radical polymerizations may simply be considered as an insertion of monomer molecules into the R-R bond of the initiator leading to the polymer with two initiator fragments. Thus, the end groups of the polymer are controlled by the initiator used. Otsu proposed the name iniferter (im tiator-trans/er agent-terminator) for the initiators with such functions [64]. Many radical initiators, such as peroxides, azo compounds, tetraphenylethane derivatives, and organic sulfur compounds, may be expected to serve as an iniferter, if monomers and polymerization conditions are selected. Some peroxides show relatively high... 

Monofunctional Iniferter R-X > R-tMJj-X End-functional polymer, AB-type block copolymer... 

The A-B type iniferters are more useful than the B-B type for the more efficient synthesis of polymers with controlled structure The functionality of the iniferters can be controlled by changing the number of the A-B bond introduced into an iniferter molecule, for example, B-A-B as the bifunctional iniferter. Detailed classification and application of the iniferters having DC groups are summarized in Table 1. In Eqs. (9)—(11), 6 and 7 serve as the monofunctional iniferters, 9 and 10 as the monofunctional polymeric iniferters, and 8 and 11 as the bifunctional iniferters. Tetrafunctional and polyfunctional iniferters and gel-iniferters are used for the synthesis of star polymers, graft copolymers, and multiblock copolymers, respectively (see Sect. 5). When a polymer implying DC moieties in the main chain is used, a multifunctional polymeric iniferter can be prepared (Eqs. 15 and 16), which is further applied to the synthesis of multiblock copolymers. 

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]. 

When the end groups of the polymers obtained by radical polymerization using certain iniferters still have an iniferter function, such radical polymerization is expected to proceed via a living radical mechanism even in a homogeneous system, i.e.,both the yield and the molecular weight of the polymers produced increase with reaction time. The generalized model is shown in Eq. (18) [16] ... 

The tetraphenylethanes described above are symmetrical compounds used to generate the same two radicals by dissociation, while pentaphenylethane (28) is an unsymmetrical derivative, giving two different radicals, triphenylmethyl and diphenylmethyl radicals [138]. The former cannot initiate radical polymerization, but the latter is available as an initiating radical to produce the polymer 28, which can function as the polymeric iniferter [106]. 

Nair et al. studied the kinetics of the polymerization of MMA at 60-95 °C using N,1SP-diethyl-NjW-di(hydroxyethyl)thiuram disulfide (30a) as the thermal in-iferter [142]. The dependence of the iniferter concentration on the polymerization rate was examined. The chain transfer constant of the propagating radical of MMA to 30a was determined to be 0.23-0.46 at 60-95 °C, resulting in the activation energy of 37.6 kj/mol for the chain transfer. Other derivatives 30b-30d were also prepared and used to derive telechelic polymers with the terminal phosphorus, amino, and other functional aromatic groups [143-145]. Thermal polymerization was also investigated with the end-functional poly(St) and poly(MMA) which were prepared using the iniferter 13 [146]. 

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]. 

The former possibility previously described could be refuted by the spin-trap-ping experiments and the living radical polymerization of St with 46. Therefore, 13 was added to the polymerization system to conserve the active site of the inifer-ter. It was expected to reproduce the iniferter site due to the formation of DC radicals which can function as primary radical terminators and/or the effective chain transfer ability of 13. It was pointed out that the DC radical generated from 13 had high selectivity for monomers, i.e., 13 acted as an initiator for the polymerization of St, but did not as an initiator for the polymerization of MA, VAc, and AN [72,175,177]. 

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. 

Surface modification can be achieved by the surface derivatization of functional-group-bearing low-molecular-weight substances, the grafting of polymer to and from the surface and amphiphilic polymer coating. The main theme of this article is focused on initiator-transfer-terminator (iniferter)-based grafting-from-surface and derivatization-on-surface approaches aiming at precision surface architectures, which are primary determinants of the biocompatibility of medical devices. 

Scheme 6 Surface functional derivatization based on simultaneous photolysis of iniferters in solution and on a surface...

Examples of surface functional derivatization by iniferter-based crossrecombination are shown below. A dithiocarbamated benzyl polymer surface was used as a model surface. When n-propyl N,N-diethyldithiocarbamate was used, XPS measurements showed that the complete loss of both N and S atoms but a marked increase in the C content of the surface was noticed. 

Byme et al. [124] have shown the possibility of creating imprinted polymer ordered micropattems, of a variety of shapes and dimensions, on polymer and silicon substrates using iniferters and photopolymerization. They applied this approach to the recognition of D-glucose using copolymer networks containing poly(ethylene glycol) and functional monomers such as acrylic acid, 2-hydro-xyethyl methacrylate, and acrylamide. 

Focusing on telechelic polymers, the concept of iniferter is probably more interesting. like initer, iniferter compounds will be able to initiate and terminate the polymerization. They also function as a CTA [84], To get telechelic polymers by radical polymerization, it is necessary to use compounds with high transfer constants along with the radical initiator. Cho and Kim [85] suggested such a system, based on the use of two compounds bear-... 

If the end groups of the polymers formed still have an iniferter function, the radical polymerization wiU be expected to show features of a living radical mechanism, i.e., both yield and molecular weight of the polymers produced would increase with reaction time (conversion). The proposed iniferter model (Otsu et al., 1989) is shown in Fig. 6.22. 

Disulfide derivatives and hexasubstituted ethanes may also be used in this context to make end-functional polymers and block copolymers. The use of dilhiuram disulfides as thermal initiators was explored by Clouet, Nair and coworkers. Chain ends are formed by primary radical termination and by transfer to the dilhiuram disulfide. The chain ends formed are thermally stable under normal polymerization conditions. I he use of similar compounds as photo-iniferters, when some living characteristics may be achieved, is described in Section 9.3.2.1.1. 

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