Centres of cationic polymerizations

CENTRES OF CATIONIC POLYMERIZATION WITH ACTIVATED MONOMER... 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

The initiators of cationic polymerizations must produce sufficiently reactive cations which are able to yield active centres with the monomer. It is therefore useful to discuss each of the two main initiator types separately. 

Even carboxylate ions can serve as active centres of / -propiolactone polymerization [316]. Cationic polymerization is characterized by the formation of an oxonium transition salt generated by the reaction of an active centre with an exo- or endocyclic oxygen atom. The reaction mode depends on the kind of initiator and monomer [317]... 

All the above discussion has centred on cationic polymerizations. It should be realized that all the other types of homopolymerization mentioned for the monomers can occur in copolymerizations as well [14, 159]. Even cyclopolymerizations [160] and charge transfer reactions [161, 162] are known. But sorting out the exact reactions that are occurring and the efficiency with which they occur has a long way to go. 

Hydrocarbon Manmners.— Extermination of the nature of the propagating centres in cationic polymerizations of hydrocarbon monomers such as isobutylene and butadiene has been accomplished through model compound studies and detailed physico-chemical analyses of the polymerized products. 

The centre of experimental and theoretical investigation on cationic polymerization is the propagation reaction, Eq. (1), and the influence on it. 

Cationic transition metal amide complexes have been investigated in part because of their potential in catalysis p irticularly for olefin polymerization. Much of this work has concerned polydentate amido, linked cyclopentadienyl-amido or delocalized nitrogen centred bidentate ligands (see later). However, the structures of a small number of cationic complexes containing monodentate amido ligands have been determined. These include... 

An example of a grafting reaction via cationic active centres is the reaction of the allylic —Cl of polyvinylchloride (formed by partial loss of HC1 from the polymer) with A1R2C1, which leads to a carbocation along the polymer chain which, in the presence of a suitable monomer, can initiate a cationic polymerization 20). 

Non-stationary polymerization are complicated from the kinetic point of view. The changing concentrations of active centres, of monomer and possibly even of further components produce conditions unsuitable for an analysis of the process. Even technical and technological difficulties occur. Nevertheless, these have to be solved as most known coordination and cationic, and a considerable number of anionic, polymerizations are non-stationary. Information on the polymerization mechanisms of the more conventional monomers are summarized in Table 3. 

Busson and van Beylen [205] studied the role of the cation and of the carbanionic part of the active centre during anionic polymerization in non polar media. They were interested in the problem of complex formation between the cation and the monomer double bond [206] and they therefore measured the reaction of various 1,1-diphenylethylenes with Li+, K+ and Cs+ salts of living polystyrene in benzene and cyclohexane at 297 K. Diphenylethy-lene derivatives were selected for two reasons. 

Similar to their anionic counterpart (see Sect. 2.4), even with cationic polymerizations the structure and size of the molecule to which the active centre is bound plays an important role. The required macromolecules with one or two active ends are formed by living polymerizations. Modern macro-molecular syntheses use them as agents, especially for the preparation of... 

The pH dependence of the polymerization rate of acrylic acid [54] in the presence of various neutralizing agents does not exhibit a course. Between pH 2 and 6, the rate drops abruptly afterwards it grows, depending on the degree of neutralization and the kind of neutralizing agent. The rate increase is connected with the presence of a cation which restricts the repulsion between the anions of the active centre and the monomer... 

According to Olah et al. [109], the ethylium carbenium ion is more stable than the carbonium ion by 40 kJ mol-1. Thus the cationic polymerization centre of hydrocarbon monomers (in free ion form) has the trivial shape... 

The existence of centres with non-ionic character has already been suspected in studies of polymerizations which are supposed to proceed on carbocat-ions the theory of pseudo-cationic polymerization was proposed [137] (see Chap. 3, Sect. 3.1). The transformation of an ion pair to a covalent compound will evidently be easier for acid centres with heteroatoms, i.e. in heterocycle or vinyl ether polymerizations. Propagation on covalent bonds has actually been observed, first in the studies of oxazoline polymerization [138] and later even with THF [139, 140] and with other monomers (see, for example, refs. 131, 141 and 142). 

To date these have not been very important as active centres of polymerizations, mainly because of the limited number of conventional monomer types suitable for building such a centre. Nitrogen cations are an exception. Cyclic imines polymerize on centres [148, 149]... 

Centres of this kind might even gain technical importance in the future because N-substituted lactam derivatives can only be polymerized by a cationic process. 

Sokolskii et al. [233] noticed that ZN coordination centres spontaneously change to cationic centres during polymerization. In our laboratory we have attempted to obtain quantitative data on a similar reaction by means of models [232], Tsuchyia and Tsuruta [234] pointed out the possibility of choice between anionic and cationic polymerization of methyloxirane with diethylzinc —HzO (see Chap. 3, Sect. 2.1). 

When the active centre concentrations change during propagation, the whole polymerization is non-stationary. Kinetically the process becomes more complicated and often even experimental control of the process becomes more difficult. On the other hand, a non-stationary condition can be utilized in studies of the elementary polymerization steps. To this end, the non-stationary phases of radical polymerizations are suitable, where outside these phases the process is essentially stationary [23-25]. Hayes and Pepper [26] called attention to the existence and solution of a simple non-stationary case caused by slower decay of rapidly generated cationic centres. In more complicated cases, exact analysis of the causes of a non-stationary condition is often beyond present possibilities. Information from the process kinetics is often not conclusive. It should be mentioned that, even when the condition d[Ac]/dt = 0 is strictly valid, polymerizations may be non-stationary, particularly in those cases when during propagation the more active form of the centres is slowly transformed to the less active form or vice versa. 

Rakova and Korotkov compared the rates of homopolymerization and copolymerization of styrene and butadiene [226], Styrene polymerizes very rapidly and butadiene slowly. Their copolymerization is slow at first, with preferential consumption of butadiene. When most of the butadiene is consumed, the reaction gradually accelerates yielding a product with a high styrene content. In the authors opinion, this is caused by selective solvation of the active centres by butadiene only after butadiene has polymerized, does styrene gain access to the centres [227], A similar behaviour was observed by Medvedev and his co-workes [228] and by many others. In our laboratory we observed this kind of behaviour in the cationic polymerization of trioxane with dioxolane. Although trioxane is polymerized much more rapidly than dioxolane, their copolymerization starts slowly, and is accelerated with progressing depletion of dioxolane from the monomer mixture [229],... 

Cationic polymerizations are less well understood than their anionic counterparts, particularly concerning the participation of various ionic forms of active centres in propagation. The values k+, k +, and fc(+ )s have mostly not been safely determined (with the exception of some heterocycles, see below). The main reason is probably contamination of centres by solvating molecules, and the instability of various centre types caused by the simultaneous solvating and polymerizing ability of the monomers. 

Similarly, when the polymer produced by cationic polymerization is more basic than the monomer, the centres can lose their reactivity by interaction with the nucleophilic sites on the chains. This situation can be demonstrated by the observation of Penczek and Kubisa during polymerization of 3,3-bis-(chloromethy 1 )-oxetane [113]... 

When the monomer is attached to the active centre in a manner leading to the formation of a weakly reactive ion, growth is terminated. A typical example of this kind of termination is the formation of an unreactive ion during cationic polymerization of vinyl carbazole. Instead of propagation... 

Transfer to polymer was observed, for example, in the polymerization of p-isopropyl-a-methylstyrene initiated by sodium naphthalene. The macromolecule can eliminate a proton (in the presence of cationic initiators a hydride ion, in radical polymerizations of this monomer a hydrogen atom). An active centre is generated... 

Cationic polymerization of 2-methylpropene at temperatures about 170 K may be almost flash-like the transformation of tetrahydrofuran to an equilibrium polymer-monomer mixture may last tens to hundreds of hours at 260 K. Evidently the overall polymerization rate is a function of many factors which may be interconnected or may act separately. The aim of kinetic measurements is to describe the polymerization, and to find conditions under which it would proceed in the desired manner. This is usually only possible after the various factors and their consequences have been isolated and investigated. The rate of monomer consumption during polymerization mostly depends on the generation rate of active centres, and on their concentration and reactivity. 

The living character of some cationic polymerizations greatly reduces the number of overlapping effects. Even transitions between forms of growth centres of various activity become accessible to reliable measurements. The best studied reaction from this point of view is the polymerization of te-trahydrofuran by centres with stable counter-ions. 

In cationic polymerizations, initiation occurs by attachment of a proton or some other Lewis-acidic cation X" to the H2C=CR2 double bond of a vinyl monomer to form a new carbon-centred cation of the type XH2C-CR2, which then grows into a polymer chain by subsequent H2C=CR2 additions (Figure 2, bottom). This type of polymerization works well - and is used in practice - only for olefins such as isobutene, where 1,1-disubstitution stabilizes the formation of a cationic centre. Since side reactions, such as release of a proton from the cationic chain end, occur rather easily, cationic polymerization usually gives shorter chains than anionic polymerization. 

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