Controlled radical polymerization systems

Figure 3.5 (a) The evolution of number-average MW versus conversion for controlled radical polymerization (CRP) and FRP systems, (b) In ([M q/[M ) versus time for an ideal controlled radical polymerization system. Deviations from linearity can result from slow initiation or loss of radicals by termination. 

For this reason, the focus of this chapter will be on the recent developments (since 2000) in p-star polymers synthesized by the above living anionic polymerization systems, with emphasis on the control of synthetic factors necessary to achieve well-defined structures of p-star polymers, that is, molecular weight, molecular-weight distribution, arm number, and composition. In the last 20 years, rapid progress in living/controlled radical polymerization systems as well as the application of click makes possible the synthesis of several new p-star polymers. Therefore, representative examples will also be described. The syntheses of p-star polymers before 2000 are beyond the scope of this chapter, although they will be briefly described in Section 4.2, since such subjects have been covered elsewhere by several excellent reviews (Hadjichristidis, 1999 Hadjichristidis et al, 2001). 

Although many examples have been developed based on GRIM polymerization in conjunction with the living/controlled radical polymerization system, tedious site-transformation reactions as well as the limited control nature of radical polymerization frequently cause broad molecular-weight distributions (> 1.3). Furthermore, a thorough purification process to remove the residual copper catalyst from the products is sometimes essential for optoelectronic application. 

This book will be of major interest to researchers in industry and in academic institutions as a reference source on the factors which control radical polymerization and as an aid in designing polymer syntheses. It is also intended to serve as a text for graduate students in the broad area of polymer chemistry. The book places an emphasis on reaction mechanisms and the organic chemistry of polymerization. It also ties in developments in polymerization kinetics and physical chemistry of the systems to provide a complete picture of this most important subject. 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. Fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [37]. Like those systems, success of this method requires a carefully controlled matching of the... 

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. 

Tab. 9.3 Specific surface modifications and SAM systems of particles or planar substrates for the surface-initiated controlled radical polymerization (CRSIP) of vinyl monomers. 

The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

The first workable capping agents for controlled radical polymerization were discovered by Rizzardo et al. [77, 78] who used nitroxides. The nitroxide reacts reversibly with radical chain ends but itself does not initiate the monomer. They called their new system Stable Free Radical Polymerization (SFRP). Scheme 32a depicts an example of SFRP using TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy). SFRP was developed independently by Georges at Xerox for the synthesis of styrene block polymer as dispersing agents [79]. 

Over the past 10 years the advent of controlled radical polymerization has resulted in an explosion of interest in the synthesis of block copolymer systems that were hitherto inaccessible [54]. The most commonly used methods of controlled radical... 

A review article by Qiu et al. [212] and references herein [217-226] covers NMCRP in miniemulsions up to 2001. Cunningham wrote a related review in 2002, also covering controlled radical polymerization in dispersed phase systems [227]. Here, the main results reported in the Qiu review will be summarized, and new developments in the field since then will be reviewed. 

Tsuyoshi Ando received his bachelor degree in 1995, master degree in 1997, and Ph.D. degree in 2000 from Kyoto University. His doctoral study was on the development of transition-metal-catalyzed living radical polymerization systems under the direction of Professor Mitsuo Sawamoto, where he received a Research Fellowship for Young Scientists of the Japan Society for the Promotion of Scientists (1998-2000). He joined the Kyoto University faculty, Department of Polymer Chemistry, Faculty of Engineering, as a research instructor in 2000. His research activity is focused on controlled reactions, including precision polymerization, catalyzed by metal compounds. 

The use of Cp or Cp -based ligands is also beneficial for the iron-based systems in controlling radical polymerization. For instance, FeCpI(CO)2 (Fe-3, X = I) induced a living radical polymerization of styrene in conjunction with an iodide initiator [(CH.s C-(C02C2H5)I] in the presence of Ti(Oi-Pr)4 to give very narrow MWDs (MJMn =1.1) and controlled molecular weights.72 The rate was increased with the use of the corresponding bromide, while the MWD was narrowed by replacement of Cp with Cp. 73 A faster and controlled polymerization was possible with dinuclear Fe(I) complexes (Fe-5 and Fe-6) in the absence of metal alkoxides. 

A series of a-halopropionates (1-21 and 1-22, X = Cl, Br), model compounds of the dormant polymer terminal of acrylates, are suitable for not only acrylates but also styrenes and acrylamides. Ethyl 2-chlo-ropropionate (1-21, X = Cl) was employed for the controlled radical polymerizations of MA and styrene catalyzed by CuCl/L-1 to afford relatively narrow MWDs (MwIMn 1.5).84 A better controlled polymerization of MA is achieved with the bromides 1-21 and 1-22 (X = Br) in conjunction with CuBr/L-1 to give narrower MWDs (MJMn 1.2).84 A similar result was obtained with the combination of 1-23 and CuBr/L-1 for the polymerization of styrene.166 A nickel-based system with Ni-2 and 1-21 (X = Br) gave another controlled polymerization of nBA.134 The iodide compound 1-21 (X = I) is specifically effective in conjunction with an iodide complex such as Re-1 to induce controlled polymerization of styrene.141... 

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... 

Metal-catalyzed living radical polymerizations of vinylpyridines were investigated with the copper-based systems. One of the difficulties in the polymerization is a decrease of catalytic activity imposed by the coordination of the monomers by the metal complex. Controlled radical polymerization of 4-vi-nylpyridine (M-33) was achieved by an initiating system consisting of a strong binding ligand such as L-32 and a chloride-based system [1-13 (X = Cl)/ CuCl] in 2-propanol at 40 °C.214 The Mn increased in direct proportion to monomer conversion, and the MWDs were narrow (MJMn = 1.1 —1.2). In contrast, 2-vinylpyridine (M-34) can be polymerized in a controlled way with chlorine-capped polystyrene as an initiator and the CuCl/L-1 pair in / -xylene at 140 °C.215 Block copolymers with narrow MWDs (Mw/Mn = 1.1 —1.2) were obtained therein. 

Metal-catalyzed living or controlled radical polymerizations can generally be achieved with initiating systems consisting of an organic halide as an initiator and a metal complex as a catalyst or an activator as described above. However, these polymerizations are slow in most cases due to low concentration of the radical species, as required by the general principle, the dormant-active species equilibria, for living radical polymerization (see the Introduction). 

For styrene-based random copolymers, functional groups can be introduced into the polymer chains via copolymerization with functional styrene derivatives, because the electronic effects of the substituents are small in the metal-catalyzed polymerizations in comparison to the ionic counterparts. Random copolymer R-6 is of this category, synthesized from styrene and />acetoxystyrene.372 It can be transformed into styrene// -vinylphenol copolymers by hydrolysis.380 The benzyl acetate and the benzyl ether groups randomly distributed in R-7 and R-8 were transformed into benzyl bromide, which can initiate the controlled radical polymerizations of styrene in the presence of copper catalysts to give graft copolymers.209 Epoxy groups can be introduced, as in R-9, by the copper-catalyzed copolymerizations without loss of epoxy functions, while the nitroxide-mediated systems suffer from side reactions due to the high-temperature reaction.317... 

Such a controlled radical polymerization can be performed even in the absence of free initiators, where larger amounts of Cu(II) species are added in the system.369 The polystyrene layer obtained from S-3 in the presence of 5 mol % Cu(II) relative to Cu-(I) increased up to 20 nm in thickness, in direct proportion to the Mn of the polymers prepared in the other experiments with ethyl 2-bromopropionate but without surface-confined initiator under similar conditions. For MA, the layer thickness increases up to 60 nm. Block copolymer layers were also prepared by block copolymerization of MA or tBA from the polystyrene. Modification of the hydrophilicity of a surface layer was achieved by the hydrolysis of the poly (styrene-A/oc7c-tB A) to poly (styrene- block-acry lie acid) and confirmed by a decrease in water contact angle from 86° to 18°. 

Sawamoto has described the controlled radical polymerization of MMA using [RuCl2(PPh3)3] as a catalyst [218]. In organic solvents, this ruthenium catalyst [197] necessitates the use of an aluminum-containing co-catalyst, usually A1(O IT), thus affording a very efficient control of the polymerization of methacrylic monomers. As the aluminum compound can not be used in neat water, polymerizations were initially carried out in a biphasic toluene water 1 1 mixture. Polymerizations in this aqueous system were found to be somewhat faster than in neat toluene. In view of these results, the authors have demonstrated that aqueous polymerizations do not necessitate the use of any aluminum co-cata-lyst. Thus, in the absence of an aluminum salt, suspension polymerization of MMA in water at 80°C was found to be efficient and controlled (83% conversion M =9.6xl0 g moT 1.42). 

Although more studies need to be performed to study the scope and generality of this system, the use of amine hydrochloride salts as initiators for controlled NCA polymerizations shows tremendous promise. The concept of fast, reversible deactivation of a reactive species to obtain controlled polymerization is a proven concept in polymer chemistry, and this system can be compared to the persistent radical effect employed in all controlled radical polymerization strategies [34]. Like those systems, the success of this method requires a carefully controlled matching of the polymer chain propagation rate constant, the amine/amine hydrochloride equilibriiun constant, and the forward and reverse exchange rate constants between amine and amine hydrochloride salt. This means it is likely that reaction conditions (e.g. temperature, halide counterion, solvent) will need to be optimized to obtain controlled polymerization for each different NCA monomer, as is the case for most vinyl monomers in controlled radical polymerizations. Within these constraints, it is possible that controlled NCA polymerizations utilizing simple amine hydrochloride initiators can be obtained. 

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