Living radical polymerization functionalization

The majority of polymers formed by living radical polymerization (NMP, ATRP, RAFT) will possess labile functionality at chain ends. Recent studies have examined the thermal stability of polystyrene produced by NMP with TEMPO (Scheme 8.3),2021 ATRP and RAFT (Scheme 8.4).22 In each case, the end groups... 

Oganova et a/. observed that certain cobalt (II) porphyrin complexes reversibly inhibit BA polymerization presumably with formation of a cobalt (111) intermediate as shown in Scheme 9.27. Thus, it seemed reasonable to propose these species may function as initiators in living radical polymerization.250 259... 

Most reviews on living radical polymerization mention the application of these methods in the synthesis of end-lunctional polymers. In that ideally all chain ends are retained, and no new chains are formed (Section 9.1.2), living polymerization processes are particularly suited to the synthesis of end-functional polymers. Living radical processes are no exception in this regard. We distinguish two main processes for the synthesis of end-functional polymers. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

Behl, M., Hattemer, E., Brehmer, M. and Zentel, R. (2002) Tailored semiconducting polymers living radical polymerization and NLO-functionalization of triphenylamines. Macromol. Chem. Phys., 203, 503-510. 

The silica microspheres provide some diversity but not enough for many complex discrimination tasks. To introduce more sensor variety, hollow polymeric microspheres have been fabricated8. The preparation of these hollow microspheres involves coating silica microspheres by living radical polymerization, using the surface as the initiation site. Once the polymer layer forms on the silica microbead surface, the silica core is removed by chemical etching. These hollow spheres can be derivatized with the dye of interest. The main advantage of these polymer microspheres is the variety of monomers that can be employed in their fabrication to produce sensors with many different surface functionalities and polymer compositions. 

It was confirmed that the resulting polymers obtained from the St polymerization with 13 induced further photopolymerization of MMA to produce a block copolymer, and the yield and molecular weight increased as a function of the polymerization time, similar to the results for the polymerization of MMA with 13, indicating that this block copolymerization also proceeds via a living radical polymerization mechanism [64]. Similar results were also obtained for the photoblock copolymerization of VAc. Thus, various kinds of two- or three-component block copolymers were prepared [157,158]. 

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. 

Pendant-functionalized polymers are obtained by polymerization of a monomer containing the desired functional group. Conventional and living radical polymerization are both useful. ATRP, NMP, and RAFT have been studied, but RAFT much less than NMP, and NMP less than ATRP at this time [Coessens et al., 2001 Harth et al., 2001 Patil et al., 1998]. 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Since the benzyl esters are found to be a suitable functional group for connecting two polymer skeletons, the next problem is to connect two equivalent polymer skeletons with benzyl esters. This can be achieved by applying recently developed living radical polymerization [21-23]. 

A dense polymer brush is obtained using the grafting from techniques. Surface-initiated polymerization in conjunction with a living polymerization technique is one of the most useful synthetic routes for the precise design and functionalization of the surfaces of various solid materials with well-defined polymers and copolymers. Above all, surface-initiated living radical polymerization (LRP) is particularly promising due to its simplicity and versatility and it has been applied for the synthesis of Au NPs. 

To prepare block copolymers by ATRP, the initiation site for living radical polymerization can be introduced at the end of a polymer chain. In this context, terminally functionalized POs are useful for the synthesis of block copolymers. 

The rapid progress and proliferation of metal-catalyzed living radical polymerization has allowed a variety of vinyl monomers to be polymerized into well-defined polymers of controlled molecular weights and narrow MWDs. Most of them are conjugated monomers such as methacrylates, acrylates, styrenes, acrylonitrile, acrylamides, etc., except dienes, which possess not only alkyl substituents but also aprotic and protic functional groups. This fact attests to the versatility and flexibility of metal catalysis for precision polymerization. 

Polymerization of ethylene is one of the most important unsolved problems in metal-catalyzed living radical polymerization. This is due to the difficulty of the activation of the primary carbon-halogen bond. The unpolymerizable nature of ethylene can be utilized for the end-functionalization of PMMA with a terminal CH2CH2Br group (section III.B.2).226... 

In homogeneous free radical polymerization, water is often employed as solvent for water-soluble monomers and polymers with more polar functional substituents such as hydroxyl, amino, oxyethylene, ammonium, and carboxylate groups, along with emulsion, suspension, and dispersion processes. This is also the case for metal-catalyzed living radical polymerization. 

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