Atom Transfer Radical Polymerizations

Polymer formation during the Kharasch reaction or ATRA can occur if trapping of the radical (123), by halocarbon or metal complex respectively, is sufficiently slow such that multiple monomer additions can occur. Efficient polymer synthesis additionally requires that the trapping reaction is reversible and that both the activation and deactivation steps are facile. 

The first purposeful use of ATRA in polymer synthesis was in the production of telomers, In this early work, comparatively poor control over the polymerization was achieved and little attempt was made to explore the wider utility of the process. Some analogies may also be drawn with the work of Baraford et al. and others on transition mctal/organic halide redox initiation (Sections 3.3.5.1 and 7.6.2).  

The first reports of ATRP (Atom Transfer Radical Polymerization), w hich clearly displayed the characteristics of living polymerization, appeared in 1995 

Kajimoto et The kinetics of ATRP arc considered in reviews by Fischer  

Ideally, the metal complex is a catalyst and, in principle, is only required in very small quantities. However, the kinetics of initiation for the systems described to date dictate that relatively large amounts are used and catalyst initiator ratios are typically in the range 1 1 to 1 10, The most commonly used catalysts are metal 

As in anionic addition polymerization, chain length can be reduced by using more initiators, but because these initiators are rather expensive, it is often preferable to use a chain-transfer agent instead. Also, at low monomer concentrations, termination can occur through cyclization of the chain end. 

In ATRP, there are reactive and dormant polymer species in equilibrium during the polymerizations, which alternate between halide-capped polymers (dormant) and growing (reactive) polymers with a free radical on the end. The choice of catalyst controls this equilibrium which in turn influences the polymerization rate and the distribution of chain lengths. The mechanism offers flexibihty to conduct reactions in bulk, solution, or emulsions/suspensions, just as fiee-radical polymerizations. Due to the capability to polymerize a large range of monomers with an inexpensive catalyst in a reactor, where purity is nearly as important as in anionic polymerizations, ATRP continues to grow in popularity. For further information, review articles written by the inventors are available [12,16]. 

FIGURE 10.3 Metal (often copper) hgand (M/L) structure to support ATRP reactions. Here, and Pm+c are polymers with length m and m+c, respectively, X is typically a hahde atom, such as Cl or Br, and the A s are kinetic rate constants for activation (ka), deactivation (kja), and termination (A ,)-Reprinted from [16] with permission from Elsevier. 

Acrylate monomers may also be polymerized by atom transfer radical polymerization (ATRP). The reader is referred to Section 9.1.3.3 for an overview of catalyst systems. 

The above-described ATRP mechanism is identical either if polymerization occurs in bulk or if started from a surface (i.e., surface-initiated). Many commercially available initiators (e.g., alkyl halides) can be used, as long as they present a weak halogen-heteroatom bond. The initiator will provide the polymer a surface with simple halogen as end groups, which is easily converted into useful functionalities. The transition metal complexes used (Ru, Cu, Fe, Ni, among others) are responsible for the conversion into useful functionalities, removing the halides from the polymer surface. The surface is then ready for polymerization [131]. 

Initiator (RX) + Met /2L (Alkyl halide initiator Metal/Ligand 

Xiao and coworkers investigated a new approach for chemically modifying the PDMS surface that has the potential of achieving a lasting hydrophilicity [145], The authors used ATRP to modify a UV-ozone oxidized PDMS surface. Further, polyacrylamide chains were used to increase the hydrophilicity of PDMS. It was found that after 30 min of exposure to UV-ozone the PDMS surface was damaged. To counteract this effect, surface ATRP would have to be performed before the occurrence of such bulk damage, ideally just following surface oxidation. Actually, unlike plasma 

In contrast, here a bifunctional initiator is employed and the polymerization order of the two blocks is inverted In a first step, the styrene block is synthesized by atom transfer radical polymerization (ATRP) followed by the addition of lactide via the recently developed organocatalytic ring-opening polymerization, as depicted in Fig. 3.1 [4, 5]. This synthesis route reduces the involved steps and enables a simplified and time-efficient preparation of copolymers with different block compositions. Importantly, both polymerization techniques offer precise and robust control over the copolymer composition, which is an essential requirement to reliably target the double-gyroid s narrow location in phase space [6]. 

In addition, diblock copolymers with different 4-X-styrene blocks, denoted as PXS, containing 4-chlorostyrene, 4-bromostyrene, or mixtures of 4-fluorostyrene and styrene, were synthesized. In contrast to poly(4-fluorostyrene), polymers of 4-bromostyrene and 4-chlorostyrene are cross-linkable by ultraviolet irradiation [7-9]. Cross-linked polymers typically show better chemical resistance and thermal stability, which are indispensable properties when used as template material for challenging templating methods. Furthermore, replacing the expensive 4-fluorostyrene 

Parts of this chapter were previously published [1]. 

Scherer, Double-Gyroid-Stmctured Functional Materials, Springer Theses, DOl 10.1007/978-3-319-00354-2 3, 

Fast initiation and rapid reversible deactivation are the key to a successful ATRP, combing of a small number of terminated chains and an uniform chain growth mechanism. Thus, the polymerization rateRp plays an important role and can be defined by the kinetic parameters and reagent concentrations 

We have seen previously that polymerization initiated by free-radicals suffers from some disadvantages. Mainly, the chain-length cannot be controlled and branching occurs. Some of these disadvantages are overcome in newer methods of radical polymerization. An important new development in this regard is the atom transfer radical polymerization (ATRP) [29-30]. In this process all the chains are initiated essentially at the same time (at the point of catalyst injection) and all the chains grow at the same rate until the monomer is consumed. The important principles of the atom transfer polymerization process are illustrated by the following sequence of reac- 

The catalyst being a Cu(l) complex, abstracts a chlorine from the initiator in this process the metal gets oxidized to Cu(Il) generating a free radical R. The latter can initiate polymerization of a styrene monomer (see Eqs. 2.37-2.38). 

Chlorine transfer can occur to the growing polymer radical (see Eq. 2.39). 

The following are the key points of the mechanism of atom transfer radical polymerization. 

The bidentate ligand viz., 2,2 -bipyridyl is crucial in increasing the solubility of Cu(I)Cl and it also affects the abstraction of a chlorine from the initiator 1-phenyl ethyl chloride as well as the dormant species Pi-Cl. It is not necessary that this ligand alone be used. Other ligands that can perform in a similar manner are also useful. 

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The facile and reversible reaction of propagating species with transition metal halide complexes to form a polymeric halo-compound is one of the key steps in atom transfer radical polymerization (ATRP, see Section 9.4). 

The first reports of ATRP (Atom Transfer Radical Polymerization), which clearly displayed the characteristics of living polymerization, appeared in 1995 from the Laboratories of Sawamoto,2 Matyjaszewski266 and Percec.267 The literature on ATRP is now so vast that a comprehensive review cannot be... 

ORl OX w di-Miutyl peroxyoxalalc deactivation by reversible chain transfer and bioinolecular aclivaiion 456 atom transfer radical polymerization 7, 250, 456,457, 458,461.486-98 deactivation by reversible coupling and untmolecular activation 455-6, 457-86 carbon-centered radical-mediated poly nierizaiion 467-70 initiators, inferlers and iriiters 457-8 metal complex-mediated radical polymerization 484... 

Polystyrene-Woc -polysulfone-/ /oc -polystyrene and poly(butyl acrylate)-Woc -polysulfone-/ /oc -poly(butyl acrylate) triblock copolymers were prepared using a macroinitiator.214 The hydroxyl-terminated polysulfone was allowed to react with 2-bromopropionyl bromide, an atomic transfer radical polymerization (ATRP) initiator, in the presence of pyridine. The modified macroinitiator could initiate die styrene polymerization under controlled conditions. 

Brzezinska KR, Deming TJ (2004) Synthesis of AB diblock copolymers by atom-transfer radical polymerization (ATRP) and living polymerization of alpha-amino acid-N-carboxyan-hydrides. Macromol Biosci 4 566—569... 

In 2003, the van Hest group produced elastin-based side-chain polymers [123]. This research was motivated by the demonstration of the occurrence of an inverse temperature transition in a single repeat of VPGVG [124]. A methacrylate-functionalized VPGVG was synthesized and used as a monomer to perform atom transfer radical polymerization (ATRP) to produce homopolymers (Fig. 16b) or... 

Star polymers are a class of polymers with interesting rheological and physical properties. The tetra-functionalized adamantane cores (adamantyls) have been employed as initiators in the atom transfer radical polymerization (ATRP) method applied to styrene and various acrylate monomers (see Fig. 21). 

Figure 21. Atom transfer radical polymerization (ATRP) synthetic route to tetrafunctional initiators of a star polymer with adamantyl (adamantane core). Taken from Ref. [91] with permission.

Block copolymers were synthesized by a combination of fipase-catalyzed polymerization and atom transfer radical polymerization (ATRE). " " At first, the polymerization of 10-hydroxydecanoic acid was carried out by using lipase CA as catalyst. The terminal hydroxy group was modified by the reaction with a-bromopropionyl bromide, followed by ATRP of styrene using CuCE2,2 -bipyridine as catalyst system to give the polyester-polystyrene block copolymer. Trichloromethyl-terminated poly(e-CL), which was synthesized by lipase CA-catalyzed polymerization with 2,2,2-trichloroethanol initiator, was used as initiator for ATRP of styrene. 

Pyun, J., Kowalewski, T. and Matyjaszewski, K. (2003) Synthesis of polymer brushes using atom transfer radical polymerization. Macromol. Rapid Commun., 24, 1043-1059. 

Ejaz, M., Yamamoto, S., Ohno, K., Tsujii, Y. and Fukuda, T. (1998) Controlled graft polymerization of methyl methacrylate on silicon substrate by the combined use of the Langmuir-Blodgett and atom transfer radical polymerization techniques. Macromolecules, 31, 5934-5936. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Synthesis of Block Copolymers by Atom Transfer Radical Polymerization, ATRP... 

Atom transfer radical polymerization, ATRP, is a controlled radical process which affords polymers of narrow molecular weight distributions. Strictly this is not a coordinative polymerization, but its dependency upon suitable coordination complexes warrants a brief discussion here. 

Recently, Kong et al. [159] functionalized MWNT with polyacrylic acid (PAA) and poly(sodium 4-styrenesulfonate) (PSS) by surface-initiating ATRP (atom transfer radical polymerization) following the Schemes 1 and 2 ... 

Structural and Mechanistic Aspects of Copper Catalyzed Atom Transfer Radical Polymerization... 

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