Polymerizations, anionic

The polymerization of monomers with strong electronegative groups — acrylonitrile, vinyl chloride, styrene, and methyl methacrylate — can be initiated by either mechanism (2) or (3) of Section 4.1. 

In (2), an ionic or ionogenic molecule is required that is capable of adding the anion to the vinyl double bond and so creating a carbanion. 

The gegen ion may be inorganic or organic, and typical initiators include KNHj, n-butyl lithium, and Grignard reagents (alkyl magnesium bromides). 

If the monomer has a strong electron-withdrawing group, then only a weakly positive initiator (Grignard) will be required for polymerization, but when the side group is phenyl or the electronegativity is low, a highly electropositive metal initiator, such as a hthium compound, is needed. 

Mechanism (3) is the direct transfer of an electron from a donor to the monomer to form a radical anion. This can be accomplished by means of an alkali metal, and Na or K can initiate the polymerization of butadiene and methacrylonitrile the latter reaction is carried out in liquid ammonia at 198 K. 

Anionic polymerization, particularly of vinyl monomers, has achieved a position of special importance because the possibility of obtaining a system free of any termination step can often be realised. The attraction of this is two-fold (i) the cleanness of such systems facilitates kinetic study and (ii) the reactive chain ends open valuable synthetic routes to telechelic polymers and block copolymers, possibly stereoregular in structure. 

Considerable control over the mode of addition to dienes can be effected by manipulation of conditions - in particular solvent and counter-ion. Although this has been known for a long time, it is only during the past few years that systematic studies have begun to make real progress in establishing the mechanistic details. 

The degree of subdivision of metal has been found to be of importance in the case of barium. With a-methyl styrene, barium powder gave mostly the dicarbanionic dimer but with a dispersion of metal the principal product was the monocarbanionic oligomer. Dicarbanionic oligostyryl barium forms a cyclic associate in THF and in tetrahydropyran. The adsorption maximum is sensitive to the ring size, ranging from 368 nm for the dimer to 359 nm when the 

When barium tetraphenylborate is added to the barium salt of mono-carbanionic polystyrene, the mixed salt formed has a very small dissociation constant and markedly reduces the rate of polymerization.  

The presence of cross-associates needs to be considered in the interpretation of copolymerization kinetics. It has been found that the reaction of poly-butadienyl-lithium with /Mlivinylbenzene in benzene solution proceeds at a rate which increases very markedly with time. Such a result would seem to imply that the polybutadienyl-lithium dimer is very much less reactive than is the mixed dimer formed between polybutadienyl-lithium and the vinylbenzyl-lithium generated by its reaction with p-divinylbenzene. Interestingly, no accelerations 

Anionic polymerization, due to the charged nature of the chain end, is much more prone for exerting control than any radical polymerization since typical termination reactions like radical-radical combination and disproportionation 

TABLE 3.1 Differences and Common Features of Living Anionic and Controlled Radical Polymerization Processes (Kinetic Constants According to Fig. 3.4) 

Growing species is active throughout the full polymerization 

Limited freedom in monomer structure due to high sensitivity to protonic impurities 

Very stringent polymerization conditions and often low temperatures are necessary 

Anionic polymerization is initiated by compounds that release anions in the reaction mass. Cationic and anionic polymerization are very similar in nature, except in their termination reactions. Termination reactions can occur easily in cationic polymerization, whereas they are almost absent in anionic polymerization. In both cases, there is a gegen ion adjacent to the growing center. Therefore, their initiation and propagation rates have similar characteristics. 

Anionic polymerization normally consists of only two elementary reactions initiation and propagation. In the absence of impiuities, transfer and termination reactions do not occur therefore, in this treatment, we do not discuss these reactions. 

The following are commonly nsed initiator systems for anionic polymerization 1. Alkali metals and alkali metal complexes (e.g., Na, K, Li, and their 

Lewis bases (e.g., ammonia, triphenyl methane, xanthene, aniline) High-energy radiation 

High-energy radiation will not be discussed here because it has little commercial importance. The first system of initiation, method 1, differs Ifom methods (2) and (3) in the process of producing growth centers. Alkali metals and alkali metal complexes initiate polymerization by transfer of an electron to the double bond of the monomer. For example, a sodium atom can attack the monomer directly to transfer an electron as follows  

Naturally, anionic polymerization is not compatible with H-bonding moieties, thus requiring efficient modification after the polymerization reaction (Ilhan et al., 2001 Schadler et al., 1998). An excellent example (Karatzas et al, 2006) of an efficient postmodification method was presented starting from living PS-PI anions, which was quenched by only one unit of oxirane, furnishing the hydroxy-telechelic PS-PI BCP (15). This in turn was reacted with the isocyanate (16), attaching the ureidopyrimidone moiety to the final polymer (17). 

11) repeated reaction with fmoc-Ala4-OH 2) reaction with diacetylen 

The stability and reactivity of anionic species can be deduced from values for the equilibria depicted in Equation 7.1 [2, 3]. The more acidic conjugate acids (lower pfifa values) are associated with a correspondingly more stable anionic species. 

In general, these anions are associated with a counterion, typically an alkali metal cation. The exact nature of the anion can be quite varied depending on the structure of the anion, counterion, solvent, and temperature [3-5]. The range of possible propagating species in anionic polymerization is depicted in terms of a Winstein spectrum of structures as shown in Equation 7.2 for a carbanionic chain end (R ) [3, 6]. In addition to the aggregated (associated) (I) and unaggregated (unassociated) (2) species, it is necessary to consider the intervention of free ions (5), contact 

One unique aspect of anionic polymerization is that the reactive propagating species are not transient intermediates. Carbanions and organometallic species can be prepared and investigated independently of the polymerization process. These species can also be characterized and monitored during the polymerization. 

One of the most important advances in the science and technology of anionic polymerization was the report in 1956 by Michael Szwarc and coworkers delineating the characteristics of living anionic polymerizations, that is, that they proceed in the absence of the kinetic steps of chain transfer 

Handbook of Polymer Synthesis, Characterization, and Processing, First Edition. Edited by Enrique Saldivar-Guerra and Eduardo Vivaldo-Lima. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

By means of anionic polymerization, it is possible to produce in the laboratory linear polymers that are nearly monodisperse and have many types of branching such as multi-armed stars and combs and H-shaped molecules. For example, there have been reports of studies of anionically polymerized polystyrene, polybutadiene, and polyisoprene. An example of the anionic polymerization of a branched polymer is the technique of Roovers and Toporowski [22] for making comb polystyrenes. The varieties of model branched polymer that can be produced today by means of block polymerization and coupling chemistries include stars, H-shaped molecules, super-H molecules (multi-armed stars at both ends of a backbone segment), and combs of various types [23]. So-called pom-pom polymers are of special interest, because their rheological behavior has been modeled by McLeish and Larson [24]. These molecules have several arms at each end of a central crossbar, and polybutadienes having this structure have been synthesized [25,26]. 

Substituent groups on the double bond must stabilize the negative charge that develops in the transition state for the monomer addition step. They must also be stable to reactive anionic chain ends [13, p. 94]. Monomers that can be polymerized anionically include vinyl, diene and some carbonyl-type and cyclic monomers. We note that because of its lack of any substituent group, polyethylene cannot be polymerized anionically. We describe in a later section how to make a living polymer that is very similar to polyethylene. 

Styrene polymerizes spontaneously on heating, but samples for use in research are made by anionic polymerization. Of particular interest are structures having well-defined branching structures [28-31]. Several rheological studies of branched polystyrenes are discussed in later chapters. 

Conceptually, anionic polymerizations are related to free radical polymerizations in that they are chain processes with a reactive end group on the growing chain, in this case an anion rather than a radical. However, the reaction conditions and ultimate polymer properties are much different for the two types of polymerization. 

The prototype monomer is a monosubstituted olefin with a substituent that can stabilize a negative charge. Typical monomers include butadiene, styrene, acrylonitrile, and methyl methacrylate. Mixtures of styrene and butadiene are used in running shoes, and described 

A common material made by anionic polymerization is the random copolymer composed of a roughly 1 3 mixture of polystyrene and polybutadiene known as SBR (for styrene butadiene rubber). Both of these monomers are compatible with anionic polymerization, and indeed SBR is made this way as a popular substitute for natural rubber, often used in the automotive industry. Styrene and butadiene are also both compatible with radical polymerization, and SBR can be made under emulsion polymerization conditions using radical initiators. 

A monomer such as butadiene opens up another stereochemical option—that is, whether the remaining double bond is cis or trans. Not surprisingly, the differing stereochemistries have different properties, with the trans tending to have higher transition temperatures. Conditions have been found that allow some, but not complete, control over the cis / trans content of the polymer. 

Another feature of anionic polymerizations is that they are often living. Under appropriate conditions carbanions can be much more stable (actually, persistent is a better term) than radicals, and so it is not unreasonable that once all the monomer is exhausted, the polymer chains can retain a reactive, anionic group at their termini. As noted above, having a living polymerization opens up the possibility of preparing block copolymers. 

The catalysts for anionic polymerization are alkali metals, alkali metal amides, alkoxides, and cyanides. The cocatalysts are organic solvents, such as heptane. An example of anionic polymerization is the synthesis of polystyrene  

The chain growth in anionic polymerization does not necessarily have to go in one direction, as shown in the above example. It can go through two, three, four, or more directions, depending on the catalysts  

1 Controlled/Living Anionic Polymerization of Vinyl Monomers 

Kinetic studies on anionic polymerization in a continuous flow mode have also been reported by Szwarc and coworkers [141, 142], Schulz and coworkers [141, 142], and Muller and coworkers [143, 144]. However, preparative anionic polymerizations in continuous flow mode have not been studied until recently. 

Anionic polymerization of styrenes is a highly useful technique for the synthesis of polystyrenes with precisely adjustable molecular weights and molecular weight distributions and is applied for the synthesis of structurally well-defined polymers such as end-functionalized polymers and block copolymers. 

Controlled/Living Anionic Polymerization of Styrenes in Polar Solvent Using Flow Microreactor Systems [145] 

In a conventional anionic polymerization of styrenes in polar solvents in a batch macroreactor, major drawbacks include the requirement of low temperature such as —78°C. In contrast, Nagaki et al. reported that controlled anionic polymerization of styrene can be conducted under easily accessible conditions such as 0°C in a polar solvent using a flow microreactor to obtain the polystyrene with narrower molecular weight distribution (M = 1,200-20,000, MJM = 1.09-1.13) (Fig. 9) [146]. Moreover, the molecular weight can be easily controlled by changing the flow rates of monomer and initiator solutions. Furthermore, these methods can be 

The first results of anionic polymerization (the polymerization of 1,3-butadiene and isoprene induced by sodium and potassium) appeared in the literature in the early twentieth century.168,169 It was not until the pioneering work of Ziegler170 and Szwarc,171 however, that the real nature of the reaction was understood. Styrene derivatives and conjugated dienes are the most suitable unsaturated hydrocarbons for anionic polymerization. They are sufficiently electrophilic toward carbanionic centers and able to form stable carbanions on initiation. Simple alkenes (ethylene, propylene) do not undergo anionic polymerization and form only oligomers. Initiation is achieved by nucleophilic addition of organometallic compounds or via electron transfer reactions. Hydrocarbons (cylohexane, benzene) and ethers (diethyl ether, THF) are usually applied as the solvent in anionic polymerizations. 

Organolithum compounds (lithium alkyls) are the most valuable initiators in anionic polymerization.120168 169172-175 Since living anionic polymerization requires the fastest possible initiation, sec- or ferf-butyllithium is usually used. Lithium alkyls add readily to the double bond of styrene [Eq. (13.32)] or conjugated dienes and form free ions or an ion pair depending on the solvent  

Since lithium alkyls are more stable in hydrocarbons than in ethers, hydrocarbon solvents are usually applied. 

Electron transfer from polycyclic aromatic radical anions in polar solvents can also initiate propagation.120 168 169173 One of the early and best understood systems is naphthalene-sodium, a green solution of stable, solvated naphthalene radical anion.176 177 The electron transfer from the radical anion to the monomer yields a new radical anion [Eq. (13.33)]. The dominant reaction of the latter is its head-to-head dimerization to the stabile dimeric dicarbanion [Eq. (13.34)], which is the driving force for the electron transfer even when electron affinity of the monomer is less than that of the polycyclic molecule. Propagation proceeds at both ends of the chain  

A similar but direct electron transfer from the metal to the monomer is operative when alkali and alkaline-earth metals (e.g., sodium) are used as the initiator.169,173,177 In this case, however, initiation is slow relative to propagation because of the low metal surface area available, and this method is used only for special purposes. 1,1-Dipehylethylene, for example, forms a dianion that, for steric reasons, is not capable for further head-to-tail addition of the monomer, but it can be used to initiate the polymerization of other monomers.178 

Write a mechanism for the anionic polymerization of 2-vinylpyridine initiated by butyllithium. Why is a regioregular polymer formed  

In each addition step, the regiochemistry is determined by the ability of the pyridine ring to stabilize the anion formed  

Addition polymerization may also be initiated by anions. Anionic polymerization has achieved tremendous commercial importance in the past few decades because of its ability to control molecular structure during polymerization, allowing the synthesis of materials that were previously difficult or impossible to obtain. A variety of anionic initiators has been investigated but the organic alkah-metal salts are perhaps the most common, as illustrated below for the polymerization of styrene with K-butyllithium  

The anionic (—) chain end then propagates the chain by adding another monomer molecule. Again, the gegen ion can sterically influence the reaction. 

Sodium and lithium metals were used to polymerize butadiene in Germany during World War 11. After the war, in the United States, it was discovered that under appropriate conditions, dispersions of lithium could lead to largely cis-1,4 addition of butadiene and isoprene (the latter being the synthetic counterpart of natural rubber). In these processes, a metal atom first reacts with the monomer to form an anion radical  

These anion radicals then react in either of two ways. One may react with another atom of lithium 

Either way, the result is a dianion that propagates a chain from each end. Other dianionic initiators have also been developed [5-8]. 

In contrast to free-radical polymerization, in anionic polymerization, the initiator (R ) is a earbanion (see Eq. 2.14). 

Aldehydes can also be polymerized because of the polarity of the C=0 bond. Some common initiators that are used in anionic pol5nnerization are alkyllithium reagents such as n-C4H9Li or organic radical anions such as sodium naphthalenide (Fig. 2.6). 

Because of the sensitivity of the initiators towards impurities, particularly water and CO2, rigorous precautions have to be taken to exclude moisture from the reaction medium. It is also logical that chlorinated solvents are not used for these reactions, as they themselves will react with initiators. 

Propagation involves the attack of the growing carbanion on successive monomeric units such that the growing polymer chain resembles a continuous insertion of the monomer molecules between the C M moiety (see Eq. 2.16). 

Presumably because the mechanism of polymerization involves the insertion of the monomer into the ion-pair, the addition of the monomers occurs in a stereoregular manner. 

Mechanisms depending on carbanionic propagating centers for these polymerizations are indicated by various pieces of evidence (1) the nature of the catalysts which are effective, (2) the intense colors that often develop during polymerization, (3) the prompt cessation of sodium-catalyzed polymerization upon the introduction of carbon dioxide and the failure of -butylcatechol to cause inhibition, (4) the conversion of triphenylmethane to triphenylmethylsodium in the zone of polymerization of isoprene under the influence of metallic sodium, (5) the structures of the diene polymers obtained (see Chap. VI), which differ. both from the radical and the cationic polymers, and (6) 

Initiation presumably involves metal alkyls as the primary source of carbanions. These are immediately available from the Grignard reagents, organosodium compounds, or sodium amide used as catalysts when the alkali metal itself or its solution in liquid ammonia is used, addition to the monomer may precede actual initiation.  

Ionic Copolymerization.—Ionic copolymerizations tend to be more discriminating than those propagated by free radicals. When, for example, an equimolar mixture of styrene and methyl methacrylate is polymerized by stannic chloride or by boron trifluoride etherate, the product obtained at low conversions is almost pure polystyrene metallic sodium yields a polymer consisting of over 99 percent methyl methacrylate units by free radical polymerization a copolymer is obtained whose composition approximates that of the feed. In a mixture of acrylonitrile with methyl methacrylate the former monomer is polymerized by sodium in liquid ammonia almost to the total exclusion of the latter neither monomer is subject to polymerization by a Friedel-Crafts catalyst, but copolymers are readily obtained using free radicals. Results such as these, even in the absence of other evidence, would demand postulation of different propagation mechanisms for the three polymerization types. 

Because of the reluctance of many pairs of monomers to copolymer- 

Alfrey, Jr., J. J. Bohrer, and H. Mark, Copolymerization (Interscience Publishers, New York, 1952). 

The polymerization of styrene in liquid ammonia, initiated by potassium amide, was one of the first anionic polymerizations to be studied in detail. In this polymerization chain transfer to ammonia terminates the growth of polymer chains and living polystyrene is not formed. The reaction is of minor importance nowadays but will be briefly considered since it provides a further example of kinetics analysis. 

Interest in anionic polymerization grew enormously following the work of Michael Szwarc in the mid-1950s. He demonstrated that under carefully controlled conditions carbanionic living polymers could be formed using electron transfer initiation. 

Initiation involves dissociation of potassium amide followed by addition of the amide ion to styrene. Termination occurs by proton abstraction from 

NH2 + CH2=CHPh H2NCH2—CHPh H2N-(-CH2—CHPh H2—CHPh + CH2=CHPh 

The second step of initiation usually is rate-determining and so the amide ion produced upon chain transfer to ammonia can initiate polymerization, but at a rate controlled by the rate constant, for initiation. Therefore, it is normal to consider this chain transfer reaction as a true kinetic-chain termination step so that application of the steady-state condition gives 

Nonempirical calculations of reaction complexes formed during anionic polymerization ofbutadiene, in the presence of stilbenes, have been made [87]. The mechanism of cis-trans isomerization in the terminal unit of the living polymer consisted in concerted rotation about the Cp bond, and the migration of Li between and Cy 

Anionic copolymers, trans-stilbenebutadiene copolymer, trans-stilbene-isoprene copolymer, and trans-stilbene-2,3-dimethylbutadiene copolymer, copolymerized using BuLi initiator, were studied in THE at 0°C and in benzene at 40°C [89]. It was shown that the rate of monomer consumption (excluding stilbene) decreased as follows butadiene isoprene 2,3-dimethylbutadiene. Anionic copolymerization 

A significant event in the period covered by the present survey was the ACS conference on anionic polymerization in 1980 - the first major conference on the subject for many years. The abstracts of the papers presented at that meeting are available and a book containing the full texts is in preparation. A review of anionic polymerization has been written by Richards. A review of organolithium catalysis of olefin and diene polymerization, which contains some previously unpublished results, has also appeared.  

There is a growing interest in the application of quantum mechanics to interpreting the mechanisms of anionic polymerizations. Papers have appeared on the interaction of butadiene with active anionic sites and on that of polyacrylonitrile anion with the lithium counterion. The use of a semi-empirical model for the interaction of the polystyryl anion with solvent and counterion has led to the prediction of rates of propagation in encouraging similarity to observation.  

Nuclear magnetic resonance spectroscopy is a powerful means of exploring the structure of the terminal carbanion. Bywater has written a short, but valuable, summary of some of the major findings to date. One of the most intriguing observations is the sensitivity of the conformation of the allylic ions derived from butadiene and isoprene to solvent and counterion. Broadly, the irons conformation is favoured in hydrocarbons and the cis by polar solvents, but there are some 

The polymerization of acrylate esters by organomagnesium reagents is best described as pseudo-anionic monomer is complexed to a magnesium atom covalently bonded to carbon. The anionic polymerization of methyl methacrylate in benzene yields the syndiotactic polymer when the initiator is an alkali metal complexed with 18-dicyclohexyl-6-crown. When a solution of methyl methacrylate in toluene is placed below a solution of butyl-lithium in the same solvent at —78 °C so as to avoid any mechanical mixing, the polymer obtained has a higher isotacticity than that observed if the solutions are stirred.  

Mathis and Francois have described the preparation of cumyl barium and strontium in THF by an adaptation of Ziegler s method. Very conveniently, they included details of the synthesis of the precursor, cumyl methyl ether. Nakh-manovich and Arest-Yakubovich have investigated the kinetics of the polymerization of styrene by organo-alkaline earth initiators in THF they report that the rate increases in the sequence Ba Sr Ca. A study of stability was made by Mathis et al., who found that the slow decomposition of e oHlicarbanionic polystyryl barium and strontium in THF is due to protonation by the solvent. An important, and unexpected, feature is that the termination is not random - partially deeomposed polymer is devoid of monocarbanionic content. 

1-diphenylethylene is required in order to decrease down the too high reactivity of the polybutadienyl anions toward the initiation of the purely anionic ROP of sCL. Furthermore, due to the inevitable inter- and intramolecular transesterification reactions in anionic ROP, especially at higher monomer conversion, the sCL polymerization time has been strictly controlled and the copolymerization was stopped by hydrolytic deactivation well before reaching quantitative sCL conversion. 

The latter of the three procedures above is particularly convenient and a brief description of the experimental procedure involved is given in Protocol 9. 

Caution Carry out all procedures in a well-ventilated fume-hood, wear appropriate disposable gloves, a lab-coat, and safety glasses. All vacuum-line work should be performed while standing behind a protective Perspex screen. Never use flat-bottomed flasks with rotary evaporators. 

Preparation. Dry all glassware in an electric oven set at 125°C for 24 h prior to reaction. Distil a suitable quantity ( 50 ml) of THF into a dry receiver flask fitted with a septum, distil styrene from calcium hydride under reduced pressure into a dry receiver flask fitted with a septum. Butyllithium is titrated with 1-pyreneacetic acid, to obtain an approximate value for its activity.  

While the apparatus is still hot, set up the two-necked flask with a septum cap and magnetic stirrer bar. Connect to the double manifold with a gas inlet adapter, evacuate the flask and then fill with nitrogen. Allow the flask to cool to room temperature.  

Carefully add the butyllithium to the reaction flask by inserting the syringe needle through the septum attached to the side-arm. A dark red colouration due to the presence ofthe styryl anion will be produced. Stir the polymerization mixture for 4 h. 

However, in most cases, propagation takes place through carboxylate chain-ends, although coexistence with alkoxide chain ends in the beginning of ROP 

Both intra- and intermolecular transesterification reactions in anionic polymerization have been observed [82,86-87]. Intercalation of alkaline metals in 

The anionic polymerization of DXO using f-BuLi or CH3Li in THF or toluene solution was unsuccessful and only oligomers could be obtained [88]. 

Although there is an unshared electron at the end of a growing chain in free radical 

FIGURE 3-30 Schematic representation of the anionic ring opening polymerization of ethylene oxide. 

FIGURE 3-31 Schematic representation of initiation by sodium amide and subsequent propagation for 

FIGURE 3-33 Formation of a styryl anion radical and subsequent dimerization. 

A knife handle made of Kraton which is a block copolymer of styrene and butadiene that is made by living anionic polymerization (Source www.knifeoutlet.com). 

In the Gilch reaction, a strong base is added to the monomer yielding polymers with high molecular weight. MEH-PPV can be obtained in the pres- 

Because the pKa of p-methoxyphenol allows a full deprotonation by potassium ferf-butoxide, it is suggested that the propagation results from nucleophilic attack of the phenoxide on an intermediate quinodimethane formed by dehydrohalogenation of the monomer. Polymerization under these conditions is found to yield polymers with very low polydispersity values.  

In addition, dihlock copolymers, triblock copolymers, star homopolymers, and block copolymers can be obtained via anionic polymerization methods using difunctional and trifunctional initiators. Suitable initiators include sulfonyldiphenol bisphenol A, and phloroglucinol. 

Because neat PPV is hard to process, a variety of related or modified polymers, in particular those with certain side groups, have been described. These varieties are smnmarized in Table 3.2. 

Amino substituted PPV, namely, poly(2-(V,V-dimethylamino) phen-ylene vinylene) can be made by reacting a bis-cycloalkylene sulfonium salt with sodium hydroxide at about 0°C. A cycloalkylene sulfonium salt precursor polymer is formed, which is heated to get the amino substituted PPV.22 

Household goods, packaging, cable insulation, bottles and much else 

shoe soles, general elastomeric products such a belting, flooring 

Household appliances, business machines, general engineering plastic 

Electrical applications, bearings, non-stick kitchen items 

A similar calcnlation predicts M = 78,000 for the triblock, which is somewhat lower than the measnred value. As will be seen later (see Section 6.2), a ratio of much less than 2 is indicative of a narrow molecular weight distribution and is typical of living polymers. The steps are snmmarized in Fignre 4.9. 

Other kinds of polymerizations such as those that involve opening rings can be initiated by cationic mechanisms. A variety of main chain polyethm such as poly tetrahydro-furan are made this way. Photogenanted acids can also be used to open oxirane rings, for example, and this is a very valuable method of jiioto-aoss-linking epoxy groups. 

Although there are several mechanisms by which living polymers can be prepared, anionic polymerization to date represents the most successful commercial application. The particular usefulness of lithium alkyls has been with dienes to give cw-polyisoprene and c -polybutadiene, although they can yield isotactic or atactic polymers of styrene and MMA. It is a peculiarity of polymerizations with lithium alkyl that there is no termination step. The rate of polymerization depends on the amount of initiator and monomer present [19]. 

Since the polymer chains are alive, a second monomer can be grown on a seed made from a different monomer, much as described earlier for living cationic polymerization. The final step of the polymerization is decomposition of the lithium alkyl 

FIGURE 4.10 First-order rate plots for seeded butyl lithium polymerizations for monomers in benzene, hexane, or tetrahydrofuran. (Data from Morton, M., AIChE Symp. Polymer Kinetics Catalyst Systems, December 1961.) 

In the following, two different types of functionalized cyclosUoxanes are described, namely those with functional groups in aU sUoxane units [(RMeSiO) hereafter referred to as symmetrical ], and those partly functionalized [(RiR2SiO(Me2SiO) i hereafter referred to as asymmetrical ], where n = 3 or 4. 

---

In the case of anionic polymerization (with 2-isoprOpenylthiazole) there is a chain-monomer equilibrium. Furthermore, lowering the temperature of polymerization increases the conversion of monomer to polymer (314). 

Stereoregular polymerizations strongly resemble anionic polymerizations. We discuss these in greater detail in Chap. 7 because of their microstructure rather than the ionic intermediates involved in their formation. 

The kinds of vinyl monomers which undergo anionic polymerization are those with electron-withdrawing substituents such as the nitrile, carboxyl, and phenyl groups. We represent the catalysts as AB in this discussion these are substances which break into a cation (A ) and an anion (B ) under the conditions of the reaction. In anionic polymerization it is the basic anion which adds across the double bond of the monomer to form the active center for polymerization ... 

Just as anionic polymerizations show certain parallels with the free-radical mechanism, so too can cationic polymerization be discussed in terms of the same broad outline. There are some differences from the anionic systems, however, so the fact that both proceed through ionic intermediates should not be overextended. 

The principal differences between cationic and anionic polymerizations center around the following points ... 

The electron-releasing R group helps stabilize this cation. As with anionic polymerization, the separation of the ions and hence the ease of monomer insertion depends on the reaction medium. The propagation reaction may be written as... 

Since the coordination almost certainly involves the transition metal atom, there is a resemblance here to anionic polymerization. The coordination is an important aspect of the present picture, since it is this feature which allows the catalyst to serve as a template for stereoregulation. 

Rate of polymerization. The rate of polymerization for homogeneous systems closely resembles anionic polymerization. For heterogeneous systems the concentration of alkylated transition metal sites on the surface appears in the rate law. The latter depends on the particle size of the solid catalyst and may be complicated by sites of various degrees of activity. There is sometimes an inverse relationship between the degree of stereoregularity produced by a catalyst and the rate at which polymerization occurs. 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

Initiation of these anionic polymerizations is considered to take place via a Michael reaction ... 

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

A brief review has appeared covering the use of metal-free initiators in living anionic polymerizations of acrylates and a comparison with Du Font s group-transfer polymerization method (149). Tetrabutylammonium thiolates mn room temperature polymerizations to quantitative conversions yielding polymers of narrow molecular weight distributions in dipolar aprotic solvents. Block copolymers are accessible through sequential monomer additions (149—151) and interfacial polymerizations (152,153). 

Although the anionic polymerization mechanism is the predominant one for the cyanoacryhc esters, the monomer will polymerize free-radically under prolonged exposure to heat or light. To extend the usable shelf life, free-radical stabilizers such as quinones or hindered phenols are a necessary part of the adhesive formulation. 

Cyanoacrylate adhesives (Super-Glues) are materials which rapidly polymerize at room temperature. The standard monomer for a cyanoacrylate adhesive is ethyl 2-cyanoacrylate [7085-85-0], which readily undergoes anionic polymerization. Very rapid cure of these materials has made them widely used in the electronics industry for speaker magnet mounting, as weU as for wire tacking and other apphcations requiring rapid assembly. Anionic polymerization of a cyanoacrylate adhesive is normally initiated by water. Therefore, atmospheric humidity or the surface moisture content must be at a certain level for polymerization to take place. These adhesives are not cross-linked as are the surface-activated acryhcs. Rather, the cyanoacrylate material is a thermoplastic, and thus, the adhesives typically have poor temperature resistance. 

Many perfluoroaUphatic ethers and tertiary amines have been prepared by electrochemical fluorination (1 6), direct fluorination using elemental fluorine (7—9), or, in a few cases, by fluorination using cobalt trifluoride (10). Examples of lower molecular weight materials are shown in Table 1. In addition to these, there are three commercial classes of perfluoropolyethers prepared by anionic polymerization of hexafluoropropene oxide [428-59-1] (11,12), photooxidation of hexafluoropropene [116-15-4] or tetrafluoroethene [116-14-3] (13,14), or by anionic ring-opening polymeriza tion of tetrafluorooxetane [765-63-9] followed by direct fluorination (15). 

Cychc carbonates are prepared in satisfactory quaUty for anionic polymerization by catalyzed transesterification of neopentyl glycol with diaryl carbonates, followed by tempering and depolymerization. Neopentyl carbonate (5,5-dimethyl-1,3-dioxan-2-one) (6) prepared in this manner has high purity (99.5%) and can be anionically polymerized to polycarbonates with mol wt of 35,000 (39). 

The use of alkaU metals for anionic polymerization of diene monomers is primarily of historical interest. A patent disclosure issued in 1911 (16) detailed the use of metallic sodium to polymerize isoprene and other dienes. Independentiy and simultaneously, the use of sodium metal to polymerize butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene was described (17). Interest in alkaU metal-initiated polymerization of 1,3-dienes culminated in the discovery (18) at Firestone Tire and Rubber Co. that polymerization of neat isoprene with lithium dispersion produced high i7j -l,4-polyisoprene, similar in stmcture and properties to Hevea natural mbber (see ELASTOLffiRS,SYNTHETic-POLYisoPRENE Rubber, natural). 

The mechanism of the anionic polymerization of styrenes and 1,3-dienes initiated by alkaU metals has been described in detail (3,20) as shown in equations 3—5 where Mt represents an alkaU metal and M is a monomer molecule. Initiation is a heterogeneous process occurring on the metal surface. The... 

Anionic polymerization of vinyl monomers can be effected with a variety of organometaUic compounds alkyllithium compounds are the most useful class (1,33—35). A variety of simple alkyllithium compounds are available commercially. Most simple alkyllithium compounds are soluble in hydrocarbon solvents such as hexane and cyclohexane and they can be prepared by reaction of the corresponding alkyl chlorides with lithium metal. Methyllithium [917-54-4] and phenyllithium [591-51-5] are available in diethyl ether and cyclohexane—ether solutions, respectively, because they are not soluble in hydrocarbon solvents vinyllithium [917-57-7] and allyllithium [3052-45-7] are also insoluble in hydrocarbon solutions and can only be prepared in ether solutions (38,39). Hydrocarbon-soluble alkyllithium initiators are used directiy to initiate polymerization of styrene and diene monomers quantitatively one unique aspect of hthium-based initiators in hydrocarbon solution is that elastomeric polydienes with high 1,4-microstmcture are obtained (1,24,33—37). Certain alkyllithium compounds can be purified by recrystallization (ethyllithium), sublimation (ethyllithium, /-butyUithium [594-19-4] isopropyllithium [2417-93-8] or distillation (j -butyUithium) (40,41). Unfortunately, / -butyUithium is noncrystaUine and too high boiling to be purified by distiUation (38). Since methyllithium and phenyllithium are crystalline soUds which are insoluble in hydrocarbon solution, they can be precipitated into these solutions and then redissolved in appropriate polar solvents (42,43). OrganometaUic compounds of other alkaU metals are insoluble in hydrocarbon solution and possess negligible vapor pressures as expected for salt-like compounds. 

Polymerization ofiVIasked Disilenes. A novel approach, namely, the anionic polymerization of masked disilenes, has been used to synthesize a number of poly(dialkylsilanes) as well as the first dialkylamino substituted polysilanes (eq. 13) (111,112). The route is capable of providing monodisperse polymers with relatively high molecular weight M = lO" — 10 ), and holds promise of being a good method for the synthesis of alternating and block copolymers. 

Other Organolithium Compounds. Organoddithium compounds have utiHty in anionic polymerization of butadiene and styrene. The lithium chain ends can then be converted to useflil functional groups, eg, carboxyl, hydroxyl, etc (139). Lewis bases are requHed for solubdity in hydrocarbon solvents. 

The vast majority of commercial apphcations of methacryhc acid and its esters stem from their facile free-radical polymerizabiUty (see Initiators, FREE-RADICAl). Solution, suspension, emulsion, and bulk polymerizations have been used to advantage. Although of much less commercial importance, anionic polymerizations of methacrylates have also been extensively studied. Strictiy anhydrous reaction conditions at low temperatures are required to yield high molecular weight polymers in anionic polymerization. Side reactions of the propagating anion at the ester carbonyl are difficult to avoid and lead to polymer branching and inactivation (38—44). 

Unlike ftee-tadical polymerizations, copolymerizations between acrylates and methacrylates ate not observed in anionic polymerizations however, good copolymerizations within each class ate reported (99). 

The anionic polymerization of methacrylates using a silyl ketene acetal initiator has been termed group-transfer polymerization (GTP). First reported by Du Pont researchers in 1983 (100), group-transfer polymerization allows the control of methacrylate molecular stmcture typical of living polymers, but can be conveniendy mn at room temperature and above. The use of GTP to prepare block polymers, comb-graft polymers, loop polymers, star polymers, and functional polymers has been reported (100,101). 

In anionic polymerization the reaction is initiated by a strong base, eg, a metal hydride, alkah metal alkoxide, organometaHic compounds, or hydroxides, to form a lactamate ... 

Nylon-6 can also be produced from molten caprolactam using strong bases as catalysts (anionic polymerization) this is used as the basis of monomer casting and reaction injection mol ding (RIM). Anionic polymerization proceeds much faster than the hydrolytic route but products retain catalysts which may need to be extracted. 

Fig. 2. Reaction scheme for the anionic polymerization of propylene oxide.

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