Ionic polymerization

Ionic polymerization systems have been developed because some monomers which contain double bonds cannot be polymerized using free-radical initiators. Also ionic polymerization generally takes place at low temperatures and can offer better control of stereoregularity and relative molecular mass distribution. The chemical factors which control whether or not a particular monomer can be polymerized using free radical, cationic or anionic initiators are beyond the scope of this book. However, Table 2.8 gives an indication of what initiators can be used for different types of monomer. It can be seen that in general free radicals are less selective and can polymerize most types of monomers. The ionic initiators are only able to polymerize certain monomers which have specific types of substitutent groups. 

This type of polymerization takes place by the addition of monomer molecules to a positively charged growing chain, known as a carbonium ion. The reaction is initiated usually by the donation of a proton (H ) to a monomer molecule by a strong acid. For example an ordinary mineral acid such as perchloric acid will initiate the polymerization of styrene in ethylene dichloride solution 

The carbonium ion and the perchlorate ion remain closely associated as an ion pair. The most important types of initiators for cationic polymerization are Lewis acids or Friedel-Crafts catalysts such as BF3, AICI3 or SnCU. For Friedel-Crafts catalysts, proton transfer cannot take place without a source of ions. It is found BF3 will catalyse the polymerization of styrene in carbon tetrachloride only when traces of water are present. 

TABLE 2.8 Ability of various monomers to polymerize by addition polymerization using different types of initiator. 

Ethylene Styrene Vinyl halides Dienes Vinyl ethers Dialkyl olefins 

Cationic polymerization of comb LCPs was first reported by Percec who used the Lewis acid boron trifluoride etherate as initiator. This approach, illustrated in Fig. 7.22, gave useful solution preparations of polyvinylethers (R = H) and polypropenylethers (R = CH3). Recently Sagane and Lenz demonstrated that the use of the cationic initiator system HI/U brought about living polymerizations of similar monomers and greatly improved the molecular mass distribution of the products. 

Percec has extended the use of cationic procedures to the synthesis of vinyl ether copolymers based on constitutional isomers, analogous to the copolysiloxanes shown in Fig. 7.16 (Section 7.4.2). Other work, by Percec and Hahn, will prove useful in the controlled synthesis of siloxane backbones (Fig. 7.23). Cationic, ring-opening polymerization of cyclic tetrameric siloxanes, in the presence of an end capper (hexamethyldisiloxane) and initiated using triflic acid, can be used to prepare homo- (Y = n = 0) or copolysiloxanes whose DP and compositions are determined by the ratio of end-capper to monomer in the feedstock. These polymers are starting materials for siloxane comb LCPs (Section 7.5.5). 

Anionic poiymerization. Anionic polymerization is an addition polymerization in which the growing chain end bears a negative charge. The monomers suitable for anionic polymerization are those that have substituent groups capable of stabilizing a carbanion through resonance or induction. TVpical monomers that can be polymerized by ionic mechanisms include styrene, acrylonitrile, and methyl methacrylate (Table 14.20). 

Initiation. Initiation of anionic polymerization is brought about by species that undergo nucleophilic addition to a monomer. The most typically used anionic initiators can be classified into two basic types  

TABLE 14.23 Celing Temperatures of Some Common Polymers [21 ] 

Electron transfer initiators such as free alkali metals (e.g., Na, Li) or complexes of alkali metals and unsaturated or aromatic compounds (e.g., sodium naphthalene). These bring about initiation as shown in the following scheme  

During the initiation process, the addition of the initiator anion to a monomer (e.g., st n ene) produces a carbanion at the head end in association with a positively-charged metal counterion. 

As discussed earlier, ionic polymerization ean be eategorized according to the nature of the growing polymer centers, whieh yields the classifications cationic polymerization and anionic polymerization 

The growth center in this class of ionic polymerizations is eationic in nature. The polymer cation adds on the monomer moleeules to it sequentially, just as the polymer radical adds on the monomer in radical polymerization. The initiation of the polymerization is accomplished by catalysts that are proton donors (e.g., protonic acids such as H2SO4). The monomer molecules act like electron donors and react with the catalyst, giving rise to polymer ions. The successive addition of the monomer to the polymer ion is the propagation reaction. These two elementary reactions are expressed schematically as follows  

The presence of the gegen ion in the vicinity of the growing center differentiates cationic from radical polymerization. Other common reactions in cationic polymerization include the following. 

Transfer reactions. The positive charge of polymer ions is transferred to other molecules in the reaction mass. These could be impurity molecules or monomer molecules themselves. Because of these reactions, the resulting polymer has a lower molecular weight. As in the case of radical polymerization, the transfer reactions do not affect the overall reaction rate. 

Chain termination. No mutual termination occius in cationic polymerization because of the repulsion between the like charges on the two polymer ions—a phenomenon absent in radical polymerization. However, the neutralization of the polymer ion can occiu by the abstraction of a proton from the polymer ion by the gegen ion, as follows  

This chapter will deal with ionic chain growth polymerization for monomers which are of some industrial importance. It will deal mainly with those polymer reaction engineering aspects which are relevant for designing processes and products. As it cannot cover the entire subject, the systems dealt with are chosen as examples of the most important features of ionic polymerization. 

There are several recent monographs [1-7] on anionic and cationic polymerization covering various mechanistic, kinetic and preparative aspects, so here just some fundamental issues which are relevant for reaction engineering will be discussed. 

Like free-radical polymerization ionic polymerization is also a chain polyaddition. After the formation of an active center (radical, anionic, or cationic species), monomer molecules are added to this active center to form long hain molecules. However, from a kinetic point of view there are two major differences between radical and ionic polymerization. 

The first difference is that free-radical polymerization is monomer-based, which means that the kinetics is (almost) exclusively determined by the monomer M. Once a radical R is formed and added to the monomer molecule to build the growing chain R---M, the reactivity of this growing chain in all reactions is determined by the nature of the monomer irrespective of the nature of the initiating radical, which just forms the tail of that growing chain. So, from a practical point of view, in radical polymerization it is sufficient to determine the kinetic scheme and parameters and their dependences on the system variables temperature and pressure for a monomer system with one kind of radical initiator. Furthermore, all active radical centers from one monomer are identical, and they are hardly in- 

1) The symbols used in this chapter are listed at the end of the text, under Notation . 

In this type of addition polymerization, the active species possess ionic charges. Depending on the kind of the ionic species, these polymerizations can be further divided into anionic and cationic polymerization teclmiques. 

AROP can be used in conjunction with other polymerization techniques to incorporate PEG or poly(ethylene oxide) (PEO) segments into the polymer backbone during the synthesis of various multi-block polymers to alter their water 

CROP has been extensively used for preparing poly(2-oxazoline)s, an important biomedical polymer characterized by its structural similarities with the naturally occurring polypeptides. Commonly used initiators for this CROP include aliphatic 

Cationic polymerization of vinyl ethers is extensively used for preparing various biodegradable polymers containing acid-labile acetal linkages with potential 

A major difference between radical polymerization and the various ionic methods is that, in the latter, the incoming monomer must fit between the growing chain end and an associated ion or complex. The growing radical chain, however, has no snch impediment at the growing end although the precise nature of the growing chain end in radical and living radical polymerization is not completely understood. 

There are further important distinctions between free-radical and ionic polymerizations. For example, many ionic polymerizations proceed at very much higher rates than is usual for free-radical polymerization, largely because the concentration of actively propagating chains is very much 

TABLE 2.9 Susceptibility of various types of monomers to free-radical, cationic and anionic polymerization  

Styrene, a-methyl styrene Vinyl halides Vinyl esters Vinyl ethers 

The electron-withdrawing nature of the constituent on a vinyl monomer (CH2=CHX) will affect the polymerization of that monomer. If the constituent is electron donating, then a cationic initiation mechanism is favored. If the constituent is electron withdrawing, then an anionic mechanism is favored. 

Isobutylene Alkyl vinyl ethers Derivatives of a-methylstyrene 

Vinyl esters Related cyano derivatives Nitroethylenes 

Acrylic and methacrylic esters Vinylidene esters Derivatives of acrylonitrile 

Schildknecht classified the type of chain polymerization suitable for the various monomers shown in Table 3.7 [13]. In the various ionic methods of polymerization, the monomer must fit between the growing chain end and an ion complex [13]. Cationic polymerizations proceed very rapidly, with the lifetimes of growing chains less than 1 s in the case of isobutylene. Stereoregularity is obtained as monomers are fit between chain and counter ions when polymerized. 

PoIy(vinyI alcohol) (water-soluble thickening agent) 

The two types of ionic polymerizations are anionic and cationic. The former involves carbanions C and the latter involves carbonium C ions. Catalysts and cocatalysts are needed in ionic polymerization. 

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The active centers that characterize addition polymerization are of two types free radicals and ions. Throughout most of this chapter we shall focus attention on the free-radical species, since these lend themselves most readily to generalization. Ionic polymerizations not only proceed through different kinds of intermediates but, as a consequence, yield quite different polymers. Depending on the charge of the intermediate, ionic polymerizations are classified as anionic or cationic. These two types of polymerization are discussed in Secs. 6.10 and 6.11, respectively. 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

Ionic polymerizations, whether anionic or cationic, should not be judged to be unimportant merely because our treatment of them is limited to two sections in this text. Although there are certain parallels between polymerizations which occur via free-radical and ionic intermediates, there are also numerous differences. An important difference lies in the more specific chemistry of the ionic mechanism. While the free-radical mechanism is readily discussed in general terms, this is much more difficult in the ionic case. This is one of the reasons why only relatively short sections have been allotted to anionic and cationic polymerizations. The body of available information regarding these topics is extensive enough to warrant a far more elaborate treatment, but space limitations and the more specific character of the material are the reasons for the curtailed treatment. 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

The reaction medium plays a very important role in all ionic polymerizations. Likewise, the nature of the ionic partner to the active center-called the counterion or gegenion-has a large effect also. This is true because the nature of the counterion, the polarity of the solvent, and the possibility of specific solvent-ion interactions determines the average distance of separation between the ions in solution. It is not difficult to visualize a whole spectrum of possibilities, from completely separated ions to an ion pair of partially solvated ions to an ion pair of unsolvated ions. The distance between the centers of the ions is different in... 

In ionic polymerizations termination by combination does not occur, since all of the polymer ions have the same charge. In addition, there are solvents such as dioxane and tetrahydrofuran in which chain transfer reactions are unimportant for anionic polymers. Therefore it is possible for these reactions to continue without transfer or termination until all monomer has reacted. Evidence for this comes from the fact that the polymerization can be reactivated if a second batch of monomer is added after the initial reaction has gone to completion. In this case the molecular weight of the polymer increases, since no new growth centers are initiated. Because of this absence of termination, such polymers are called living polymers. 

The molecular weight distribution for a polymer like that described above is remarkably narrow compared to free-radical polymerization or even to ionic polymerization in which transfer or termination occurs. The sharpness arises from the nearly simultaneous initiation of all chains and the fact that all active centers grow as long as monomer is present. The following steps outline a quantitative treatment of this effect ... 

Even though the catalyst may be only partially converted to H B", the concentration of these ions may be on the order of 10 times greater than the concentration of free radicals in the corresponding stationary state of the radical mechanism. Likewise, kp for ionic polymerization is on the order of 100 times larger than the sum of the constants for all termination and transfer steps. By contrast, kp/kj which is pertinent for the radical mechanism, is typically on the order of 10. These comparisons illustrate that ionic polymerizations occur very fast even at low temperatures. 

Ionic polymerizations are almost exclusively solution processes. To produce monodisperse polymers or block copolymers, they must be mn batchwise, so that all chains grow for the same length of time under identical conditions. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

The thermal (or photochemical) decomposition of the azo group gives rise to a radically initiated polymerization. The reactive site F, the transformation site, however, can, depending on its chemical nature, initiate a condensation or addition type reaction. It can also start radical or ionic polymerizations. F may also terminate a polymerization or even enable the azo initiator to act as a monomer in chain polymerizations. 

A new ionic polymeric polycarbamate was synthesized after steps of polyurethane chemistry using 3-iso-cyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate, 2,5-dimethyl-2,5-dihydroperoxyhexane, 1,6-butanediol, 2,4-tolylenediisocyanate, and N,N -bis(j3-Hydroxy-ethyOpiperazine [27]. Modification of the nitrogen of the piperazine ring into quaternary ammonium salt by treatment with methyliodide gave the MPI high electroconductivity. 

An important difference between free radical and ionic polymerization is that a counter ion only appears in the latter case. For example, the intermediate formed from the initiation of propene with BF3-H2O could be represented as... 

In ionic polymerizations, reaction rates are faster in solvents with high dielectric constants, which promote the separation of the ion pair. 

Addition of metallic oxides to isobutene polymerized by high energy radiation leads to a spectacular increase in the yield.313. It seems that some ions are stabilized by complexing with the surface of the oxide and such an interaction prevents their recombination with the gegen-ions. These observations confirm therefore the suggested cause of inefficient ionic polymerization in systems exposed to ionizing radiation. 

The nature of the gegen ion is probably of the greatest importance in determining the rate of propagation in ionic polymerization. However, it is not clear whether the presence of the gegen ion enhances or inhibits the propagation. One may argue that the... 

Ions and ion pairs interact strongly with the solvent, and hence an ionic polymerization is greatly influenced by the environment. Solvation tends to separate the ions and thus the system approaches a state which would be expected in a hypothetical solution deprived of gegen ions. At the same time formation of a solvation shell around the growing center probably slows down the addition. This effect is particularly notable in the termination step and will be discussed further in the next section of this paper. 

The distinction between coordination polymerization and ionic polymerization is not sharp. Let us consider for example a C—X bond, X being a halogen or a metal. Winstein54 and Evans14 have demonstrated that in a compound containing this type of bond an equilibrium may be established in a suitable solvent between... 

In an ionic polymerization the strong electrostatic field of the ion pairs should have a pronounced effect on the ratio of the probabilities of the two placements. Furthermore, solvation of an ion pair is much stronger than of a neutral radical, hence the influence of a solvent on stereospecificity of addition is expected to be much more pronounced in an ionic polymerization than in a radical polymerization. The nature of the gegen ion represents still another factor which is of extreme importance in determining the stereospecificity of the polymerization. 

Yoshida, H. and Hayashi, K. Initiation Process of Radiation-induced Ionic Polymerization as Studied by Electron Spin Resonance. Vol. 6, pp. 401—420. 

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