17 Controlled/Living Radical Polymerization Krzysztof Matyjaszewski and James Spanswick Center for Macromolecular Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania I. CHEMISTRY OF CONTROLLED/LIVING RADICAL POLYMERIZATION (CRP) Conventional free radical polymerization (RP) has many advantages (see Chapters 2–4 discussing specific polymers prepared by radical polymerization). The procedure can be used for the (co)polymerization of a very large range of vinyl monomers under undemanding conditions; requiring the absence of oxygen, but tolerant to water, and can be conducted over a large temperatur e range (À80 to 250  C) [1]. This is why nearly 50% of all commercial synthetic polymers are prepared using radical chemistry providing a spectrum of materials for a range of markets. Many additional vinyl monomers can be copolymerized via a radical route leading to an infinite number of copolymers with properties dependent on the proportion of incorporated comono- mers. The major limitation of RP is poor control over some of the key structural elements that allow the preparation of well defined macromolecular architectures such as molecular weight (MW), polydispersity, end functionality, chain architecture and composition. Living polymerization was first defined by Szwarc [2] as a chain growth process without chain breaking reactions (transfer and termination). Such a polymerization provides end-group control and enables the synthesis of block copolymers by sequential monomer addition. However, it does not necessarily provide polymers with MW control and narrow molecular weight distribution (MWD). Additional prerequisites to achieve these goals include that the initiator should be consumed at the early stages of polymerization and that the rate of initiation and the rate of exchange between species of various reactivity should be at least as fast as propagation [3–5]. It has been suggested to use the term controlled polymerization if these additional criteria are met [6]. This term was proposed for systems, which provide control of MW and MWD but in which chain breaking reactions continue to occur, as in RP. However, the term controlled does not specify which features are controlled and which are not controlled. Another option would be to use the term ‘living’ polymerization (with quotation marks) or apparently living which could indicate a process of preparing well-defined polymers under conditions in which chain breaking reactions undoubtedly occur, as in radical or carbocationic polymerization [7]. The term controlled/living could also describe the essence of these Copyright 2005 by Marcel Dekker. All Rights Reserved. systems [6] and will be used in this chapter as we discuss in detail the polymerization procedures that have been developed for control over radical copolymerization of vinyl monomers. Well-defined polymers with precisely controlled structural parameters are accessible through living ionic polymerization processes, however, ionic living polymerization requires stringent process conditions and the procedures are limited to a relatively small num ber of monomers [8–10]. Therefore, it remained desirable to prepare, by free radical means which are more practical for industrial manufacturing procedures, new well-defined block and graft copolymers, materials with star, comb and network topology, end-functional polymers and many other materials prepared under mil d conditions, from a larger range of monomers, than available for ionic living polymerizations [11]. The concept of living radical polymerization was first discussed by Otsu [12] but did not come to the forefront of scientific scrutiny until after the publishing of the influential work of Georges [13] in 1993 who had built upon the earlier work of Rizzardo [14]. Georges pointed out to the scientific community that controlled radical polymerization was feasible. This is one reason why since 1995 we ha ve witnessed a real explosion of academic and indust rial research on controlled/living radical polymerizations (CRP) with over five thousand papers and hundreds of patents devoted to disclosing, and improving the various types of CRP discussed in this chapter, and to developing an understanding of the implications of molecular structure on material properties. In all of the CRP processes developed to date there is a low occurrence of side reactions (e.g., terminat ion or chain transfer) due to creation of a dynamic equilibrium between a dormant species present in large excess and a low concentration of active radical sites. By reducing the instantaneous concentration of active radicals, and hence the number of side reactions, polymerization is able to proceed in a controlled manner. This results in the formation of (co)polymers having predictable MW and controllable MWD with MW increasing as a function of time in a batch polymerization process, all the while maintaining a narrow MWD. In almost all of the references included in this chapter initiation efficiencies are high, and the experimental molecular weight is close to the theoretic molecular weight and MWD is less than 1.3. CRP is also able to produce materials with well-defined block length s, complex architecture, and functionalized chain ends. There are several requirements that have to be met for any process that claims to control radical polymerizations, including assuring quantitative initiation and suppressing the contribution of chain breaking reactions. All of the controlled/living radical based processes developed to meet these requirements, along with many other new living polymerization systems, such as carbocationic, ring-opening, group transfer, ligated anionic polymerization of acrylates, etc., depend upon the existence of a dynamic equilibration between an active and a dormant species. In CRP the equilibrium is between growing free radicals and some kind of dormant species [15]. The equilibrium is established via activation (k a ) and deactivation (k d ) steps. Currently three approaches generally appear to be successful at controlling radical polymerization and the major processes will be discussed in historical order. 1. Thermal homolytic cleavage of a weak bond in a covalent species which reversibly provides a growing radical and a less reactive radical (a persistent or stable free radical) (Scheme 1). There are several exampl es of persistent radicals but it seems that the most successful are nitroxides [13,14,16], triazolinyl radicals Copyright 2005 by Marcel Dekker. All Rights Reserved. [17,18], bulky organic radicals, e.g., trityl [19–21] or compounds with photolabile C–S bonds [22] and some organometallic species [23–26]. Scheme 1 A subset of this process is the transition metal catalyzed, reversible cleavage of the covalent bond in the dormant species via a redox process (Scheme 2). Since the key step in controlling the polymerization is transfer of an atom (or group) between a dormant chain and a transition metal catalyst in a lower oxidation state forming an active chain end and a transition metal deactivator in a higher oxidation state, this process was named atom transfer radical polymerization (ATRP) [27–32]. Scheme 2 2. The second approach to CRP is based on a thermodynamically neutral exchange process between a growing radical, present at very low concentrations, and dormant species, present at much higher concentrations (generally three to four orders of magnitude) (Scheme 3). This degenerative transfer process can employ alkyl iodides [33,34], unsaturated methacrylate esters [35,36], or thioesters [37,38]. The latter two processes operate via addition-fragmentation chemistry. Scheme 3 3. Finally, there is a third approach that has not yet been as extensively examined as the above systems. This process is the reversible formation of persistent radicals, by reaction of the growing radicals with a species containing an even num ber of electrons, which do not react with each other or with monomer (Scheme 4). Here, the role of a reversible radical trap may be played by Copyright 2005 by Marcel Dekker. All Rights Reserved. phosphites [39] or some reactive, but non-polymerizable alkene, such as tetrathiofulvalenes, stilbene or diphenylethylene [40,41]. Scheme 4 In the remaining pages of this chapter we will discuss the chemistry of these successful approaches to controlled/living radical polymerization and some examples of new materials prepared by these techniques will be discussed. Several reviews devoted to CRP have been already been published, and readers may refer to proceedings from ACS Meetings on CRP [42,43], general reviews on CRP [44–48], reviews on ATRP [30,49–54], on macromolecular engineering and materials prepared by ATRP [55], on nitroxide mediated polymerization (NMP) [56–58], on catalytic chain transfer [59,60], and on reversible addition fragmentation transfer polymerization, RAFT [61]. II. CONTROLLED/LIVING RADICAL POLYMERIZATION BASED ON REVERSIBLE THERMAL CLEAVAGE OF WEAK COVALENT BONDS The homolytic cleavage of weak covalent bonds results in the formation of an active radical capable of propagating the polymerization and a counter radical which, in principle, should only be involved in the reversible capping of the growing chains. The stable counter radicals should not react with themselves, with monomer to initiate the growth of new chains, or participate in other side reactions such as the abstraction of b-H atoms. These persistent radicals should be relatively stable, although some recent data indicate that their slow decomposi tion may help in maintaining appropriate polymeriza- tion rates [17]. There are several examples of persistent radicals used in controlled radical polymerization but perhaps the most extensively studied are nitroxides, specifically TEMPO [62,63] (Scheme 5). Hawker showed that the two radicals on the right-hand side of Scheme 5 were not closely associated with each other during the polymerization [64] allowing for a statistical replacement of the nitroxide with functional end groups [65]. Interesting results were also obtained with organometallic species, especially with paramagnetic high spin co balt (II) compounds [23]. However, often a particular trap acts efficiently only for one class of monomers. For example, Co(II) porphyrine derivatives are excellent for controlling the polymerization of acrylates [66] but poor for styrene, while for methacrylates they act as very efficient transfer reagents (catalytic chain transfer). The nitroxide TEMPO is efficient only for the CRP of styrene and its copolymers, however, some newly developed nitroxides have also been successful for acrylates [67]. Consequently, the range of monomers controllably polymerizable by this pro cedure is Copyright 2005 by Marcel Dekker. All Rights Reserved. slowly expanding [58]. Scheme 5 Nitroxides were originally described in the patent literature as agents in the polymerization of (meth)acrylates [14], but the resulting products were essentially either stable oligo-polymeric alkoxyamines or unsaturated species. It was only after the seminal paper by Georges using TEMPO in styrene polymerizations at elevated temperatures (>120  C), that real advances in controlled radical polymerization were made [13]. Initial results were most encouraging, since they employed very simple reaction conditions (bulk styrene, [BPO] o : [TEMPO] o ¼ 1.3 : 1 and simple heating) and obtained the desired outcome (DP n ¼ Á[Sty]/[TEMPO] o in the range of M n ¼ 1000 to 50,000 and with low polydispersities, M w /M n < 1.3). The reactions were slow with rates similar to the thermal polymerization of styrene. Under typical conditions, the majority of the chains are present in the form of alkoxyamines, which are the covalent bonded dormant species, while a very small fraction of radicals are continuously generated by thermal initiation and by the thermal cleavage of the alkoxyamines ([P*] % 10 À8 M) [68]. Chains continuously terminate by coupling/dispropor tionation and lead to an excess of TEMPO via the persi stent radical effect ([TEMPO] % 10 À5 M) [69–71]. The alkoxyamine functional group on the chain ends can also slowly decompose and generate unsaturated structures and a hydroxylamine (Scheme 6) [72] that can be reoxidized to TEMPO in the presence of traces of oxygen. Scheme 6 In the system described by Georges control was initially relatively good but decreased as the reaction progressed and molecular weights exceed M n ¼ 20,000, however, more recent work indicates that molecular weights over 150,000 can be obtained [58,67]. Typically, above 80% of chains are in the form of dormant, potentially active species but this number drops as chain length increases, the remaining $20% of chains are terminated and not capable of growth. Under appropriate conditions it is possible to conduct chain extensions and therefore prepare block copolymers. Several improvements to the original system have been made; these include the use of different initiators such as AIBN instead of BPO [73], using a simple pure thermal process [74,75], or preformed alkoxyamines, so-called unimolecular initiators [76]. Also di- and multi-functional initiators have been successfully used to make novel materials with chains growing in several directions, or from multiple sites on a backbone polymer [57,77]. The rate of polymerization can be increased over that of TEMPO mediated systems by using new nitroxides, which are sterically bulkier and dissociate easier, thereby providing a larger equilibrium constant. Examples include Copyright 2005 by Marcel Dekker. All Rights Reserved. phosphoric and phosphonic acid cyclic and acyclic nitroxide derivatives [78,79] includ- ing N,N-(2-methylpropyl-1)-(1-diethylphosphono-2,2-dimethyl-propyl-1-)-N-oxyl, (SG1) expanding the range of monomer s polymerizable by (NMP) (see Scheme 7) [67]. Scheme 7 Phosphorous containing nitroxides. Rates of propagation for nitroxide mediated systems follow a simple law (Eq. 1) and depend on the concentration of radicals, which are defined by the equilibrium constant (K eq ), and the concentration of dormant species [P-SFR] and SFR (Eq. 2), where [SFR] is the concentration of the persistent radical. R p ¼Àd½M=dt ¼ k p ½M½P à ¼k p ½MK eq ½P-SFR=½SFRð1Þ ½P à ¼K eq ½P-SFR=½SFRð2Þ However, when the equilibrium constants are very small the polymerizations are slow, as in the classic case of the TEMPO mediated polymerization of styrene, K eq % 10 À11 M at 130  C. In that case, the rate can be increased to an acceptable level by increasing the number of radicals either from thermal initiation by the monomer or by adding a second conventional radical initiator, which has an appropriate lifetime at the polymerization temperature, such as dicumyl peroxide [68,80,81]. In that case, the concentration of radicals is defined by the balance between rates of initiation and termination: ½P à ¼R i =R t ð3Þ A stationary concentration of SFR must therefore self adjust and be reduced to fulfill the equilibrium requirement and obey both equations (2) and (3). Another approach to increase rates is to reduce the concentration of the SFR, such as TEMPO, by other reactions. The lower thermal stability of 4-oxoTEMPO results in its continuous decomposition, thereby reducing its concentration and resulting in a shift of the equilibrium towards more growing radicals, and finally faster rates. The decomposition/ dissociation may also be catalyzed intra- or inter-molecularly by addition of acid derivatives and acetyl compounds (potentially acid generators) [82,83]. The principle of low thermal stability of persistent radicals was also employed in the use of triazolinyl radicals, which decompose at elevated temperatures and spontaneously reduce their concentration [17]. Research is presently being focused on the high throughput synthesis for the design of new alkoxyamine initiators for nitroxide mediated living free radical procedure [84] and Hawker has shown that the rates of polymerization can be significantly enhanced, even when compared to the second generation a-hydrido-based alkoxyamines recently developed. He has demonstrated that intramolecular H-bonding is a powerful tool for increasing the performance of alkoxyamine initiators for nitroxide mediated Copyright 2005 by Marcel Dekker. All Rights Reserved. living free radical polymerizations. Increases in the rate of polymerization (ca. 1000%) were observed for polar monomers such as acrylamides and especially acrylates [67], while only moderate improvements were obtained for non-polar monomers, such as styrene and isoprene. In each case, the degree of control during the polymerization was improved, leading to lower polydispersities and a better correlation between experimental and theoretical molecular weights. Nitroxide mediated polymerization has also been conducted in heterogeneous systems including emulsion [85,86], miniemulsion [87], and suspension [88,89], howeve r, as fully discussed below for biphasic ATRP reactions, an understanding of partition coefficients for all components of the system between all phases is critical for a controlled polymerization [90,91]. Probably the most important factor for the future of NMP will be the development of new compounds that allow polymerization and copolymerization of a broader range of mon omers under milder reaction conditions; we should however note that nitroxide mediated polymerization has already been applied to styrene [92], acrylates [93], acrylamides [94], acrylonitrile [67], dienes [95], and recently polymerization of ethylene has been claimed to be controlled [96,97]. NMP has also been extended to functional monomers such as sodium styrene sulfonate [98], 2-vinylpyridine [99,100], 3-vinyl pyridine [101,102], and 4-vinylpyridine [103]. However, since a nitroxide residue ends up at the end of each chain, these new compounds should be inexpensive, and introduce no adverse properties (color, poor thermal stability, etc.) to the final material. III. TRANSITION METAL CATALYZED PROCESSES — ATOM TRANSFER RADICAL POLYMERIZATION Atom transfer radical polymerization (ATRP) is based on the reversible transfer of halogen atoms, or pseudo-halogens, between a dormant species (P n –X) and a transition metal catalyst (M n t /L) by redox chemistry. The alkyl (pseudo)halides are reduced to active radicals and transition metals are oxidized via an inner sphere electron transfer process [28,50]. In the most studied system, the role of the activator is played by a copper(I) species complexed by two bipyridine ligands and the role of deactivator by the corresponding copper(II) species. Scheme 8, shows such a system with the values of the rate constant for activation (k a ), deactivation (k d ), propagation (k p ) and termination (k t ) for a bulk styrene polymerization at 110  C [32]. The rate coefficients of termination decrease significantly with the progress of the polymerization reaction due to the increase in the chain length and increased viscos ity of the system. In fact, the progressive reduction of k t is one of the most important features of many controlled radical polymerizations [104]. Scheme 8 Copyright 2005 by Marcel Dekker. All Rights Reserved. The main difference between nitroxide mediated systems and ATRP is that the latter can be used for a much larger range of monomers, including metha crylates, is practical for a full range of copolymerizations, and it is generally much faster [105]. The rate of propagation for an ATRP (Eq. 4) can be adjusted conveniently, not only by the concentration of deactivator but also by the concentration of activator, since catalysis is at the very nature of ATRP [51]. The activity of the catalyst can be adjusted by selection of the ligand [106,107] and optionally addition of a solvent [108]. The ligand can also be selected for the reaction medium and can encompass hydrophilic or hydrophobic substituents, or in the case of polymerization conducted in supercritical carbon dioxide, fluroalkyl groups [109]. R p ¼Àd½M=dt ¼ k p ½M½P à ¼k p ½Mfk a ½P-X½CuðIÞg=fk d ½X-CuðIIÞg ð4Þ Polydispersities in ATRP, and in other controlled radical reactions, depend on relative rates of propagation and deactivation [5] (Eq. 5): M w =M n ¼ 1 þ½ðk p ½RX o Þ=ðk d ½X-CuðIIÞÞð2=p À 1Þð5Þ Thus, polydispersities decrease with conversion, p, with the rate constant of deactivation, k d , and with the concentration of deactivator, [X-Cu(II)], however, they increase with the propagation rate constant, k p , and the concentration of initiator, [RX] o . This means that more uniform polymers are obtained at higher conversions, when the concentration of deactivator in solution is high and the concentration of initiator is low. Also, more uniform pol ymers are formed when the deactivator is very reactive (e.g., copper(II) complexed by bipyridine or triamine) and monomer propagates slowly (e.g., styrene rather than acrylate). Chain breaking reactions do occur in these controlled radical systems [110], fortunately, at typical reaction temperatures, the contribution of transfer is relatively small. For example, in the polymerization of styrene, less than 10% of chains participate in transfer to monomer before reaching M n ¼ 100,000. However, since the contribu- tion of transfer progressively increases with chain length molecular weights should be limited by the appropriate ratio of monomer to initiator concentrations (for styrene Á[M]/[I] o < 1000). Termination does occur in radical systems and currently cannot be completely avoided. On the other hand, since termination is second order with respect to radical concentration and propagation is first order, the contribution of termination increases with radical concentration, and therefore also with the polymerization rate, consequently, most controlled radical polymerizations are designed to be slower than conven tional systems. It is possible to generate relatively fast controlled radical polymerizations, but only for the most reactive monomers, such as acrylates, and/or for relatively short chains. For short chains, the absolute concentration of terminated chains is still high but their percentile in the total number of chains is small enough so as not to affect end functionalities an d blocking efficiency. A typical proportion of terminated chains lies between 1 and 10%, with a large fraction of those being very short chains that may not markedly affect the properties of the synthesized polymers and copolymers. It is possible to measure the evolution of concentration of terminated chains by following the copper(II) species by EPR in a system starting from pure copper(I) catalyst. Commercially in a system using a higher cost low molecular weight initiator the addition of copper(II) to the Copyright 2005 by Marcel Dekker. All Rights Reserved. system will increase initiator efficiency by reducing termination reactions between low molecular weight radicals. The list of monomers polymerized successfully by ATRP is extensive and polymerizations have been investigated with a wide range of transition metals including copper [27], ruthenium [111], iron [112–116], rhodium [117], rhenium [118]. The main requirement for a transition metal catalyst to be suitable for an ATRP is an ability to undergo a one electron redox reaction with an appropriate redox potential selected for the (co)monomers being polymerized. The initial range of monomers, which started with polymerization and copolymerization of styrene, acrylates, and methacrylates [28,119], have been extended to substituted styrenes [120], including 4-acetoxy styrene [121], benzyl ethers [122], and 4-trimethylsilyl derivatives [123]; substituted acrylates include methyl and n-butyl [28,124–127], ethyl [128], t-butyl [129–132], and isobornyl [133,134]; substituted methyl methacrylates [29,112,135–138], and various other alkyl methacrylates [131,134,139–144], including hydroxyethyl methacrylate [145,146], 2-(N-morpholino)ethyl methacrylate [147], 2-(dimethylamino)ethyl methacrylate [148,149], acrylamides [150,151], including methacrylamides [152–154], and substituted acrylamides, N-t- butylacrylamide homopolymer and N-(2-hydroxypropyl)methacrylamide [153], also vinylpyridine [100,155] and dimethylitaconate [156]. In addition, several other monomers have been successfully copolymerized using ATRP and include, for example, isobutylene and vinyl acetate [157]. A big advantage of any radical process, ATRP included, is its tolerance to many functional groups such as amido, amino, ester, ether, hydroxy, siloxy and others. All of them have been incorporated as substituents into (meth)acrylate monomers and successfully polymerized. One current exception is a ‘free’ carboxylic acid group which potentially complexes with the catalyst and disables ATRP, and therefore, presently, it has to be protected. Recent work has shown that monomers bearing ionic substituents such as sodium 4-vinylbenzoate, sodium 4-vinylbenzylsulfonate and 2-trimethylammonioethyl methacrylate methanesulfonate and triflate, and dimethylaminoethyl methacryate can be polymerized directly [148]. Another advantage of ATRP is a multitude of commercially available initiators. Nearly all compounds with halogen atoms activated by the presence of b-carbonyl, phenyl, vinyl or cyano groups have been used as efficient initiators. Also compounds with a weak halogen–heteroatom bond can be used, such as sulfonyl halides [31]. Small molecule initiators can carry additional functionalities, a few examples are shown in Scheme 9, the functionality is incorporated at the residual chain end. Scheme 9 Some low MW functional ATRP initiators. Many compounds with multiple active halogen atoms have been used to initiate bi- or multi-directional growth to form ABA block copolymers and star-like polymers and copolymers [158]. Active halogens can be incorporated at the chain end s of polymers Copyright 2005 by Marcel Dekker. All Rights Reserved. prepared by other techniques such as cationic, anionic, ring-opening metathesis and conventional radical processes to form macroinitiators. Such macroinitiators have been successfully chain extended via ATRP to form novel diblock, and triblock copolymers [159–162]. A useful tool that is available for the preparation of block copolymers when the second monomer to be polymerized is a methacrylate is the halogen switch technique [163], which allows one to match the rate of initiation with the rate of propagation. When the active halogen is incorporated along the backbone of a (co)polymer, graft copolymers are formed. Many commercial polymers including modified poly- butene, polyisobutylene, polyethylene, and polyvinyl chloride have been used as macroinitiators for the preparation of graft copolymers by the ‘grafting from’ procedure [164–166]. The halogen atoms, at the active chain ends, can be removed either by a reduction process or transformed to other useful functionalities [167], as shown for styrene and acrylate systems (Scheme 10) [168]. Scheme 10 ATRP has been successfully carried out in bulk, in solution [27,28], as well as in aqueous solution [157], emulsion [169], miniemulsion [170], and suspension [135, 171], and in other media (e.g., liquid or supercritical CO 2 [109] or ionic liquids) [172,173]. Typical temperature range for a polymerization is from sub-ambient temperature to þ130  C. Molecular weights for linear and graft copolymers range from 200 < M n < 500,000 (however the molecular weight of bottle-brush copolymers and particle tethered copoly- mers can reach well into the millions), and polydispersities are low, 1.05 < M w /M n < 1.3, depending on the catalyst used, and also on the relative and absolute catalyst and initiator concentrations. Copyright 2005 by Marcel Dekker. All Rights Reserved. [...]... segmented copolymers, i.e., block and graft copolymers Block copolymers can be prepared in one of two manners: through the use of macroinitiators or by sequential addition of monomer Macroinitiators can be prepared by a number of polymerization techniques, including controlled/living radical polymerization In this case, a monomer is polymerized and the polymer is isolated then dissolved in a second monomer... free radical process of all the controlled/living polymerization processes developed to date A recent review of various methods of telomer synthesis [180] discusses the different types of transfer agents and monomers and the contribution of the techniques of telomerization to CRP (includes discussion of iodine transfer polymerization, RAFT, and macromolecular design through interchange of xanthates (MADIX))... approach to block copolymers a second monomer can be added at the end of the polymerization of the first monomer This sequential addition of monomer may result in a slight taper or gradient of the transition from block A Copyright 2005 by Marcel Dekker All Rights Reserved Table 1 Summary of CRP copolymerizations TEMPO derivatives St/nBMA; St/ClMS; St/MMA [214]; St/AN [215,216]; St/NVC [215, 217] ; St/VP [218];... addition of the persistent radical alone was also shown to be effective at providing controlled polymerization from surfaces [359] Two groups of workers initially examined functionalization of silica surfaces followed by polymerization of a range of vinyl monomers forming homopolymers and block copolymers [344,359] Monomers included styrene [360], MMA [140], and acrylamide Amphiphilic block copolymers... commercial uses, but one of when and how XI CONCLUSIONS Radical polymerizations are widely used in industrial processes, accounting for the synthesis of nearly 50% of all polymeric materials The widespread use of radical polymerization is due to its unique ability to easily and readily prepare high MW polymers Copyright 2005 by Marcel Dekker All Rights Reserved from a variety of monomers, under relatively... presence of ionic liquids with ferrous and cuprous anions Macromolecular Chemistry and Physics, 202 (17) : 3379–3391 174 Arehart, S V., and Matyjaszewski, K (1999) Atom transfer radical copolymerization of styrene and n-butyl acrylate Macromolecules, 32(7): 2221–2231 175 Moineau, G., et al (2000) Synthesis of fully acrylic thermoplastic elastomers by atom transfer radical polymerization (ATRP) 2 Effect of the... and stereoregularity of methyl methacrylate and butyl acrylate statistical copolymers synthesized by atom transfer radical polymerization Macromolecules, 34 (17) : 5833–5837 226 Kotani, Y., Kamigaito, M., and Sawamoto, M (1998) Living random copolymerization of styrene and methyl methacrylate with a Ru(II) complex and synthesis of ABC-type ‘blockrandom’ copolymers Macromolecules, 31 (17) : 5582–5587 227... by radical polymerization processes which are more tolerant of impurities, functional groups and are applicable to a wider range of monomers This increased level of control over radical polymerization will allow industry to tailor a material to the requirements of a specific application using the most robust polymerization process available, ensuring the polymers have the optimal balance of physical...Copolymerization is facile and many statistical, gradient and block copolymers have been prepared [143 ,174 ,175 ] The reactivity ratios are nearly identical to conventional radical processes [50 ,176 ] The key feature of ATRP is a transition metal compound, that is made available to participate in a redox cycle with the initiator or growing polymer chain, most often this is accomplished by complexation of. .. efficiency of the block copolymer synthesis, as well as the consumption of the initial transfer agent depends strongly on the structure of the alkyl precursor For example, cumyl derivatives have been excellent transfer agents in RAFT but, isobutyrate derivatives were unsuccessful in polymerization of MMA As described by Moad [61], the choice of CTA is critical in producing nearmonodisperse polymers via . enables the synthesis of new block copolymers. However, the efficiency of the block copolymer synthesis, as well as the consumption of the initial transfer agent depends strongly on the structure of the. copolymers by sequential polymerization of three different monomers [241]. In another approach to block copolymers a second monomer can be added at the end of the polymerization of the first monomer controlled/‘living’ radical polymerization. Such ‘mechanism transformation’ can be used to prepare a wide array of novel polymers; block copolymers of combinations of radically prepared polymers with those