Solvation cationic polymerization

To illustrate Eq. (6), Table 5 contains the individual results which were added to each other to obtain the solvation energy. Dichloro methane, often used in cationic polymerizations, was applied in the calculations. 

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). 

It was possible to formulate a rule describing how the copolymerization parameters depend on the polarity of the solvent used. This rule is a result of contemplation about the connection between the copolymerization parameters and propagation rate constants during the cationic polymerization as well as about the changes of solvation of educts and activated complexes of the crossed propagation steps in solvents with varied polarity 14 U7). The rule is as follows ... 

The same type of addition—as shown by X-ray analysis—occurs in the cationic polymerization of alkenyl ethers R—CH=CH—OR and of 8-chlorovinyl ethers (395). However, NMR analysis showed the presence of some configurational disorder (396). The stereochemistry of acrylate polymerization, determined by the use of deuterated monomers, was found to be strongly dependent on the reaction environment and, in particular, on the solvation of the growing-chain-catalyst system at both the a and jS carbon atoms (390, 397-399). Non-solvated contact ion pairs such as those existing in the presence of lithium catalysts in toluene at low temperature, are responsible for the formation of threo isotactic sequences from cis monomers and, therefore, involve a trans addition in contrast, solvent separated ion pairs (fluorenyllithium in THF) give rise to a predominantly syndiotactic polymer. Finally, in mixed ether-hydrocarbon solvents where there are probably peripherally solvated ion pairs, a predominantly isotactic polymer with nonconstant stereochemistry in the jS position is obtained. It seems evident fiom this complexity of situations that the micro-tacticity of anionic poly(methyl methacrylate) cannot be interpreted by a simple Bernoulli distribution, as has already been discussed in Sect. III-A. 

All the enumerated examples indicate the insufficiency of reactions (59)-(64) to explain completely the initiation of cationic polymerizations. An inseparable aspect of initiation is the activation of the primary products produced by ionization or dissociation. Several kinds of ion pair of various reactivities are known to exist. The solvate envelope of free ions must affect the frequency of active ion-monomer collisions, i. e. the initiation rate. In the author s opinion, the key to our understanding of some co-initiation effects in cationic polymerization is a suitable interpretation of the Winstein dissociation scheme [247]... 

When an alkene molecule loses an electron, a cation radical is formed. The very reactive cation radical (CH3)2C—CHJ is generated from 2-methyl-propene in light in the presence of TiCl4. It can be detected by ESR in the frozen parent compound at 123 K [172], We assume that at higher temperatures these formations are dimerized to dications. The existence of a donor-acceptor complex is a necessary condition for the mechanism generating cation radicals (see Chap. 3, Sect. 5). a-Methylstyrene is cationically polymerized when illuminated in the presence of tetracyanobenzene in methylene chloride. From the two compounds, of which a-methylstyrene is the donor (D) and tetracyanobenzene the acceptor (A), the donor-acceptor complex is generated in the singlet and triplet states it dissociates to solvated ion radicals [173]... 

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. 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

Determination of propagation rate constants in cationic (and in anionic) systems is complicated by the simultaneous occurrence of different types of propagating sites. In olefin polymerizations, some portion of the active centers may exist as free ions and others as ion pairs of varying degrees of solvation. In the solvents in which cationic polymerizations are normally carried out, the polymerization is mainly due to free ions. In low dielectric constant media like benzene or hydrocarbon monomers, however, ion pairs will dominate the reaction. 

The existence of contact pair ions (as in Eq. 9-1) is neglected in this representation because the dielectric constants of the solvents usually used for cationic polymerizations are high enough to render concentrations of intimate ion pairs negligible compared to those of solvated ion pairs. 

In cationic polymerizations of aldehydes the growing cation can always be solvated by the acetalic oxygens in the polyoxymetiiylene chain, viz. 

Very little is known about the effect of this interaction and how important this equilibrium is for the cationic polymerization, especially in solid/liquid interface reactions. Triethyloxonium fluoroborate, an excellent initiator for formaldehyde polymerization, can be visualized as an ethylcarbonium ion solvated by one mole of diethylether. 

It would appear that this conclusion contradicts our earlier suggestion based on a study of the cationic polymerization of p-chloro-a-methylstyrene in a series of solvents of different polarities (methylene chloride, toluene and heptane)( ). In that case, the molecular weights of the polymers formed increased with increasing solvent polarity, but that effect may have been associated with the comparative reactivities of solvated ion pairs, not free ions. 

This high tendency of poly(ethylene oxide) to solvate cations and from the polymeric shell around the counterion leads to autoacceleration in polymerization (polymerization faster on solvated ion-pairs), and increase of conductivity with monomer conversion. Moreover, polymerization is not sensitive to the "external" solvating agents, e.g. crown ethers. 

The radlatlon-lnduced cationic polymerization of vinyl and unsaturated monomers In the liquid state has been studied for over 25 years, and the essential features of this type of polymerization appear to be well established (1, ). In contrast to cationic polymerization by catalysts where the propagating species Is usually described as a solvated Ion pair, the distinctive characteristic of cationic polymerization Induced by high energy radiation Is that propagation occurs by free Ions with very large rate constants, the range of kp values for observable polymerization being from 10 ... 

One can visualize a range of behavior from one extreme of a completely covalent species (I) to the other of completely free (and highly solvated) ions (V). The intermediate species include the tight or contact ion pair (II) and the solvent-separated or loose ion pair (III). The contact ion pair has a counterion (or gegenion) of opposite charge close to the propagating center (unseparated by solvent). The solvent-separated ion pair involves ions that are partially separated by solvent molecules. In cationic polymerization the chain end is cationic and has a negative counterion, while in anionic polymerization the chain end is anionic and has a positive counterion. 

Both the initiation and propagation processes are, moreover, influenced by equilibria between various degrees of association of the active center and its counterion. As a minimum, it is necessary to consider the existence of solvent-separated ion pairs, and free solvated ions. A simplified scheme [20] is shown in Fig. 8.7. The existence of contact (associated) ion pairs [as in Eq. (8.1)] is neglected in this scheme because the dielectric constants of the solvents usually used for cationic polymerizations are high enough (9-15) to render concentrations of intimate ion pairs negligible compared to those of solvated ion pairs. The observed kp in these simplified reactions will thus be composed of contributions from the ion pairs and free solvated ions ... 

Polymerization of epoxides occurs readily under the influence of strong bases in both protic and aprotic solvents, propagation involving stepwise growth of alkoxide ions. Dimethyl sulfoxide (DMSO) is the most useful of the dipolar aprotic solvents and shows a marked ability to solvate cations (especially K ) whilst leaving anions essentially unsolvated. As a consequence nucleophilic reactivity of anions is greater in solvents such as DMSO. 

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