Radical polymerization controlled

Conventional free-radical polymerization reactions are used widely on an industrial scale to produce important commodity polymers and copolymers from vinyl-based monomers. As processes are relatively insensitive to impurities and can be used to polymerize a wide range of monomers, they are difficult to control because of the 

The significant difference between conventional radical polymerizations and all of the LRP techniques developed so far is the establishment of a rapid dynamic equilibrium between a very small amount of chain-growing free radicals and a large excess of the dormant species. The general scheme for this reversible activation process is represented schematically below  

Three principle processes have been developed that provide the activa-tion/growth/deactivation cycle, which can be repeated many times throughout the polymerization reactions. 

In each of these mechanisms, the reverse reaction dominates the equihbriimi and keeps the overall concentration of the propagating radical (P ) low, typically [Pn ]/[Pn— X] 10-5. If tijis reversible radical trapping process occurs frequently, it minimizes the irreversible termination reactions but also means that the polymer chains all have an equal chance to grow, resulting in polymers with a narrow molecular weight distribution. It also follows that, unlike conventional free-radical polymerizations, the polymer chain length will increase steadily with the reaction time, similar to living anionic polymerizations. 

In any CRP, the polymerization reaction is initiated by a species (P,— X), which in several cases may be a low-molecular-weight homologue of the polymer itself, produced in the early stages of the reaction. 

CRP (also referred to as living radical polymerization ) is a family of promising techniques for the synthesis of macromolecules with well-defined molecular weight, low polydispersities (often close to unity) and various architectures under mild conditions at 20-120°C, with minimal requirements for purification of monomers and solvents. A common feature of the variants is the existence of an equilibrium between active free radicals and dormant species. The exchange between active radicals and dormant species allows slow but simultaneous growth of all chains while keeping the concentration of radicals low enough to minimize termination. The ideal CRP is achieved if all chains are initiated 

With instantaneous initiation and no termination, [Ptot] remains constant such that a plot of ln([Af ]o/[Af ]) has a linear relationship with time for an isothermal batch reactor ([Af ]o concentration of monomer at time 0 [M ] concentration of monomer at time f), 

Since the active species are free radicals, it is impossible to entirely suppress bimolecular termination or other mechanisms such as chain transfer. Nonetheless, CRP chemistries allow imprecedented control of polymer microstructure not achievable by conventional FRP. 

Three variants of CRP have emerged as the most promising  

Three major systems are distinguished so far c) Atom trtmsfer radical polymerization (ATRP) 

ATRP is a very potent method for preparing block copolymers by sequential monomer addition as well as star polymers using multifunctional initators. Furthermore, it can be applied also in heterogenous polymerization systems, e.g., emulsion or dispersion polymerization. In Example 3-15 the ATRP of MMA in miniemulsion (see also Sect. 2.2A.2) is described. 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end or monomeric radical and the nitroxy radical. Monomer is inserted into this bond when it opens thermally. The free radical necessary to start the reaction can be created by adding a conventional radical initiator in combination with, e.g., TEMPO or by starting the reaction with a preformed adduct of the monomer with the nitroxy radical using so-called unimolecular initiators (Hawker adducts). 

The thermal lability of the R-C-O-N bond system controls the reversibility of the chain termination and limits also the use of NMP. SFRP of styrene at about 130 °C is studied intensively. In this case, high control and high-molar-mass products could be achieved. It was found that the thermal autopolymerization 

End-group functionalization in NMP can be achieved by using a functional radical initiator in combination with TEMPO. 

The first example of the use of a controlled radical polymerization technique for the construction of polymers with side chain peptide moieties was 

The same methodology was applied to construct side chain polymers containing the beta-sheet forming tetrapeptide Ala - Gly - Ala - Gly (AGAG) [42]. This side chain polymer was subsequently used as a macroinitiator for ATRP to prepare flanking blocks of amorphous poly methyl methacrylate. Infra-red spectroscopy clearly showed that the resulting well-defined triblock copolymer possessed a beta-sheet secondary structure. 

The SFRP or NMP has been studied mainly using the stable free radical TEMPO (2,2,6,6-tetramethyl-l-piperidinyloxy) or its adducts with, e.g., styrene derivatives. It is based on the formation of a labile bond between the growing radical chain end 

The thermal lability of the R-C-O-N bond system controls the reversibility of the chain termination and limits also the use of NMP. SFRP of styrene at about 130°C is studied intensively. In this case, high control and high-molar-mass products could be achieved. It was found that the thermal autopolymerization of the styrene monomer plays an important role in the mechanism of the reaction. Therefore, first experiments using different monomers in the presence of TEMPO and a radical initiator failed with regard of the control. However, new nitroxy adducts with a different R-O-N bond stability have been developed, e.g., by Hawker which work also for styrene derivatives as well as for acrylates. 

In the RAFT mechanism, the chain equilibrium process is based on a transfer reaction, no radicals are formed or destroyed, and, when the RAFT agents behave ideally, the kinetics can be compared to the one of a conventional free radical polymerization. The release of initiating radicals through chain transfer (b) at the 

There has been a revolution in free radical polymerization chemistry that began in the 1980s with the seminal patent of Solomon et al. (1986). These scientists found that it was possible to obtain controlled radical polymerization of monomers such a styrene and alkyl (meth) acrylates by effecting free radical polymerization in the presence of stable nitroxyl radicals as shown below. It has been found that these controlled polymerizations carried out 

The kinetics of the stable free radical polymerization are controlled by the persistent radical effect which has been clearly elucidated by Fischer (1997,1999). 

Careful and extensive investigations of these nitroxide-mediated polymerizations (also referred to as stable free radical polymerization) have established optimum conditions for controlled radical polymerization of a variety of vinyl monomers (Matyjaszewski, 1998,2000). Variables examined include the structure of the nitroxide and the presence of other additives to control spontaneous polymerization of monomers such as styrene. It is noteworthy that in place of alkoxyamine initiators, a mixture of a normal free radical initiator such as an azo compound or a peroxide can also be used. 

The application of these procedures to 1,3-dienes has presented problems. The rates of polymerization were observed to decrease and then stop due to a buildup of excess free nitroxide (Keoshkerian et al., 1998). An effective procedure for the controlled polymerization of isoprene at 145°C involved the addition of a reducing sugar such as glucose in the presence of sodium bicarbonate to react with the excess nitroxide (Keoshkerian et al., 1998). After 4 h, polyisoprene with M = 21,000 and My,/M = 1.33 was obtained in 25% yield. The reaction of TEMPO-terminated polystyrene with either butadiene or isoprene resulted in the formation of the corresponding diblock copolymers that were characterized by NMR and SEC (Georges et al., 1998). No evidence for either polystyrene or polydiene homopolymers was reported. 

Initial studies utilized a poly(tert-butyl acrylate)-SGl macroinitiator for the chain extension of isoprene, but no kinetic data was reported (Matsuoka et ah, 2009). More recently, Nicholas and coworkers reported the polymerization of isoprene with a variety of SGI-based alkoxyamine initiators, and investigated the effect of temperature, alkoxyamine concentration, and structure on the polymerization kinetics (Harrisson et ah, 2011). Optimal control over the polymerization was observed at 115°C where 40% conversion was attained after 16h,butonlymodestmolecular weights were observed (M 8350).Thebest control over the polymerization was observed with SGI-based alkoxyamines which contain secondary and tertiary nonacid groups. 

The newest form of living polymerization and potentially the most versatile is living radicai polymerization, more formally known as controlled radical polymerization. First discovered for the polymerization of low-molecular-weight methacrylates, it can be applied to styrenics, acrylates, methacrylates, and a variety of other vinyl monomers. A major advantage over anionic polymerization is the relative insensitivity to functional groups. Like all living polymerizations, it was important to establish rapid initiation with slower overall rate of polymerization and at the same time to inhibit any termination or transfer reactions. Normally, this is very complicated in a radical polymerization, so a radical trap is used in combination with high temperatures. The molecules used have much in common with the inhibitors used to stabilize monomers and prevent premature polymerization. 

More recently, new nitroxide compounds have been developed that permit a broader range of monomers to be used. By fine-tuning the stability of the radical and the 

FIGURE 4.12 (a) The radical polymerization of styrene in the presence of TEMPO stable free radical, (b) the polymerization of methyl methacrylate in the presence of Cu bipy, and (c) the polymerization of methyl methacrylate in the presence of a RAFT agent. All additives serve to stabilize the radical by forming a bond with the radical chain end. Polymerization only takes place in the short time the capping agent is off the chain. 

The method is sufficiently versatile that methacrylates, acrylonitriles, dienes, and styrenes all are copolymerized. The proposed mechanism for ATRP is shown in the scheme below. The figure shows three possible activities for the radical (1) dormant—most of the time the radical is capped and unreactive, and (2) polymer formation—the radical is active and the halogen is complexed with the copper enabling the insertion of monomer. This state is short lived enough that side reactions and termination are unlikely. Finally (3) unwanted reactions such as termination may occur. Again the chance of these latter reactions are minimized enough that the polymerization can be considered living, that is, without termination. 

Switchable RAFT agents have been developed and permit the combination in block copolymer form of LAM- and MAM-type monomers in a single polymer. 

In a well-controlled radical system, the monomer conversion is first order, molar mass increases linearly with monomer conversion, and the molar mass distribution MJM is below 1.5. In addition, chain end functionalization and subsequent monomer addition allow the preparation of well-controlled polymer architectures, for example, block copolymers and star polymers by a radical mechanism, which had been up to now reserved for ionic chain growth polymerization techniques. 

Three major systems, and various subsystems, are distinguished so far  

Stable free radical polymerization (SFRP) or nitroxide-mediated radical polymerization (NMRP or NMP) 

Reversible addition-fragmentation chain transfer (RAFT). 

FIGURE 3.17 General principles of controlled radical polymerization reducing the concentration of active radicals. 

The term control in CRP refers to the capability of producing a polymer with low polydispersity and with a prespecifled MW. On the other hand, the term livingness refers to the potential of a chain to be extended by the addition of extra monomer (of the same or different chemical nature) after a first batch of monomer has been exhausted. 

CRP is one of the most rapidly expanding areas of chemistry and polymer science due to the effectiveness already demonstrated by these techniques and their enormous potential for the synthesis of a broad variety of polymeric or polymer-related materials. The degree of control of the molecular architecture that can be achieved with these techniques is the driving force that has led many groups interested in the synthesis of materials with specific functionality, properties, or structures, to work in this field. The number of publications in this field (papers and patents) has seen an exponential increase in the last decade and it is estimated that in a single year (2005) more than a thousand publications were brought out [47]. 

The mechanism of all CRP techniques is based on a dynamic equilibrium between very small concentrations of propagating radicals and dormant species (Fig. 4.1), which can be reactivated by virtue of this equilibrium [48, 2]. A key factor for achieving good control (low polydispersities and good MW prediction) is a fast 

There are several techniques for performing CRP, but the most popular and successful ones so far are as follows stable free radical (SFR) or nitroxide-mediated radical polymerization (NMRP) [44, 45, 49], atom transfer radical polymerization (ATRP) [50, 51], and degenerative transfer techniques, including particularly reversible addition-fragmentation transfer (RAFT) polymerization [3]. These are examined in some detail in the following sections. 

In general, CRP techniques are based on either an SFR that exhibits the persistent radical effect (PRE) [52-55] (e.g., N in NMRP), an inactive species HalTp 

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Does not distinguish forms of controlled radical polymerization. Includes most papers on ATR.P, RAFT and NMP and would also include conventional (non-living) but controlled radical polymerizations. It would not include papers, which do not mention the terms living , controlled or mediated . 

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

It remains a common misconception that radical-radical termination is suppressed in processes such as NMP or ATRP. Another issue, in many people s minds, is whether processes that involve an irreversible termination step, even as a minor side reaction, should be called living. Living radical polymerization appears to be an oxymoron and the heading to this section a contradiction in terms (Section 9.1.1). In any processes that involve propagating radicals, there will be a finite rate of termination commensurate with the concentration of propagating radicals and the reaction conditions. The processes that fall under the heading of living or controlled radical polymerization (e.g. NMP, ATRP, RAFT) provide no exceptions. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Aliphatic disulfides are not thought to be effective as initiators in this context. However, Endo et a . K have described the use of the cyclic 1,2-disulfides 11 and 12 as initiators in a controlled radical polymerization. Polymerization of S at 120 °C gave a linear increase in molecular weight with conversion and the PS formed was used as a macroinitiator to form PS-6/oet-PMMA. The precise mechanism of the process has not been elucidated. 

The preparation of polymer brushes by controlled radical polymerization from appropriately functionalized polymer chains, surfaces or particles by a grafting from approach has recently attracted a lot of attention.742 743 The advantages of growing a polymer brush directly on a surface include well-defined grafts, when the polymerization kinetics exhibit living character, and stability due to covalent attachment of the polymer chains to the surface. Most work has used ATRP or NMP, though papers on the use of RAFT polymerization in this context also have begun to appear. 

Matyjaszewski, K., Kd. ACS Symposium Series, Controlled Radical Polymerization ... 

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... 

Stalmbach, U., deBoer, B Videlot, C van Hutten, P. F. and Hadziioannou, G. (2000) Semiconducting diblock copolymers synthesized by means of controlled radical polymerization techniques. J. Am. Chem. Soc., 122, 5464-5472. 

We have demonstrated a new class of effective, recoverable thermormorphic CCT catalysts capable of producing colorless methacrylate oligomers with narrow polydispersity and low molecular weight. For controlled radical polymerization of simple alkyl methacrylates, the use of multiple polyethylene tails of moderate molecular weight (700 Da) gave the best balance of color control and catalyst activity. Porphyrin-derived thermomorphic catalysts met the criteria of easy separation from product resin and low catalyst loss per batch, but were too expensive for commercial implementation. However, the polyethylene-supported cobalt phthalocyanine complex is more economically viable due to its greater ease of synthesis. 

The controlled radical polymerization techniques opened up a new era in polymer synthesis, and further growth and developments are certain. However, the control of the molecular characteristics and the variety of macro-molecular architectures reported by these methods cannot be compared with those obtained by other living polymerization techniques such as anionic polymerization. 

Matyjaszewski K (ed) (1998) Controlled radical polymerization. ACS Symposium Series, Chap 10-15... 

Like all controlled radical polymerization processes, ATRP relies on a rapid equilibration between a very small concentration of active radical sites and a much larger concentration of dormant species, in order to reduce the potential for bimolecular termination (Scheme 3). The radicals are generated via a reversible process catalyzed by a transition metal complex with a suitable redox manifold. An organic initiator (many initiators have been used but halides are the most common), homolytically transfers its halogen atom to the metal center, thereby raising its oxidation state. The radical species thus formed may then undergo addition to one or more vinyl monomer units before the halide is transferred back from the metal. The reader is directed to several comprehensive reviews of this field for more detailed information. 

The tendency of nitrones to react with radicals has been widely used in new synthetic routes to well-defined polymers with low polydispersity. The recent progress in controlled radical polymerization (CRP), mainly nitroxide-mediated polymerization (NMP) (695), is based on the direct transformation of nitrones to nitroxides and alkoxyamines in the polymerization medium (696, 697). In polymer chemistry, NMP has become popular as a method for preparing living polymers (698) under mild, chemoselective conditions with good control over both, the polydispersity and molecular weight. 

Turgman-Cohen, S. and J. Genzer, Computer simulation of controlled radical polymerization Effect of chain confinement due to initiator grafting density and solvent quality in "grafting from Method. Macromolecules, 2010. 43(22) p. 9567-9577. 

Surface-initiated Polymerization Using Controlled Radical Polymerization... 

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. 

Within the short period of time since its discovery, ATRP has developed remarkably fast to become the most employed controlled radical polymerization technique... 

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. 

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