Reactivity in cationic polymerization

Competing side reactions in cationic polymerization of carbonyl monomers include cyclotrimerization and acetal interchange. Cyclotrimerization is minimized by low-polarity solvents, low temperatures, and initiators of low acidity. Acetal interchange reactions among different polymer chains do not occur except at higher temperatures. Acetaldehyde and higher aldehydes are reasonably reactive in cationic polymerization compared to formaldehyde. Haloaldehydes are lower in reactivity compared to their nonhalogen counterparts. 

Whether all chains bear a terminal vinylic double bond has not been clearly established, and it would be somewhat astonishing if vinylic double bonds did not undergo side reactions since their reactivity in cationic polymerization is quite high. However, the occurrence of terminal p-vinyl benzyl groups is confirmed by the fact that the formed macromonomer readily copolymerizes with butyl acrylate. 

Use of stable organic cations to initiate cationic polymerization allows characterization of free ions and ion pairs as intermediates and in some cases facilitates measurement of their respective absolute reactivities. Further work is in progress to extend the range of catalysts and monomers which may react in this way and therefore to extend our knowledge of the absolute reactivity in cationic polymerization. 

Gorbachev, S.G. 1978. Synthesis of 9-alkenylcarbazoles and some aspects of their reactivity in cationic polymerization. PhD dissertation, Tomsk. 

The conversion of aromatic monomers relative to C-5—C-6 linear diolefins and olefins in cationic polymerizations may not be proportional to the feedblend composition, resulting in higher resin aromaticity as determined by nmr and ir measurements (43). This can be attributed to the differing reactivity ratios of aromatic and aHphatic monomers under specific Lewis acid catalysis. Intentional blocking of hydrocarbon resins into aromatic and aHphatic regions may be accomplished by sequential cationic polymerization employing multiple reactors and standard polymerization conditions (45). 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

It is generally accepted that there is little effect of counterion on reactivity of ion pairs since the ion pairs in cationic polymerization are loose ion pairs. However, there is essentially no experimental data to unequivocally prove this point. There is no study where polymerizations of a monomer using different counterions have been performed under reaction conditions in which the identities and concentrations of propagating species are well established. (Contrary to the situation in cationic polymerization, such experiments have been performed in anionic polymerization and an effect of counterion on propagation is observed see Sec. 5-3e-2.)... 

Ledwith, A. and D. C. Sherrington, Reactivity and Mechanism in Cationic Polymerization, Chap. 9 in Reactivity, Mechanism and Structure in Polymer Chemistry, A. D. Jenkins and A. Ledwith, eds., Wiley-Interscience, New York, 1974. 

Medium-size members of homologous polymeric series such as dimers, trimers, etc. are called oligomers. They can be linear or cyclic and are often found as byproducts of polymer syntheses, e.g., in cationic polymerizations of trioxane or in polycondensations of e-aminocaproic acid (see Example 4-9). For the preparation of linear oligomers with two generally reactive end groups, the so-called telechelics, special methods, i.e., oligomerizations, were developed. 

The addition of the anion takes place at the unsubstituted carbon atom, which, in this case, carries a partial positive charge. Since the growing chain end is a genuine anion, chain termination can occur by addition of a reactive cation. As in cationic polymerization, combination of two growing ends is not possible. Chain transfer with electrophiles can also occur. 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

Since carbocations are involved in cationic polymerization, a possible side reaction is their isomerization through hydride (alkyde) migration to more stable (less reactive) carbocations. This can lead to a polymer of broad molecular weight distribution or, if the isomerization is irreversible, to termination. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then react with excess monomer to start propagation. The kinetics and mechanisms of cationic polymerization and polycondensation have been studied extensively.925-928 Kennedy and Marechal926 have pointed out that only cations of moderate reactivity are useful initiators, since stable ions such as arenium ions were found to be unreactive for olefin polymerization. On the other hand, energetic alkyl cations such as CH3CH2+ were too reactive and gave side products. 

Most of the reported polyfvinyl ether) macromonomers have been prepared with a methacrylate end group which can be radically polymerized and which is non-reactive under cationic polymerization conditions [71-73]. Generally, the synthesis was based on the use of the functional initiator 30, which contains a methacrylate ester group and a function able to initiate the cationic polymerization of vinyl ethers. Such initiator can be obtained by the reaction of HI and the corresponding vinyl ether. With initiator 30 the polymerization of ethyl vinyl ether (EVE) was performed using I2 as an activator in toluene at -40 °C. The MW increased in direct proportion with conversion, and narrow MWD (Mw/Mn= 1.05-1.15) was obtained. The chain length could be controlled by the monomer to initiator feed ratio. Three poly(EVE) macromonomers of different length were prepared by this method Mn=1200,5400, and 9700 g mol-1. After complete... 

The alkenes most reactive to cationic polymerization contain electron-donating functional groups that can stabilize the carbocation intermediate. The reactivity order of substituents in cationic polymerization is similar to the reactivity order of substituted benzenes in electrophilic aromatic substitution reactions. 

The relative rates of oxirane ethanolysis with acid and basic catalysts are summarized in Table 9. It can be seen that the relative reactivity of the monomer in cationic polymerization is controlled by the basicity of the cyclic ether. 

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]... 

Contact and solvent-separated ion pairs can be distinguished in anionic systems the interionic distance of the former is usually 1-3 A, which increases to 4 or even 7 A in solvent-separated ion pairs [21]. There is apparently no further minimum in the potential energy diagram. The reactivity of solvent-separated ion pairs and free ions in anionic systems are similar, being a few orders of magnitude more reactive than contact ion pairs. In contrast, contact ion pairs in cationic systems are separated by 4-6 A, and therefore resemble the solvent-separated species of anionic systems in terms of structure, as well as their relative reactivity and ability to dissociate. The existence of solvent-separated ion pairs in cationic polymerization is questionable and has not yet been proven spectroscopically. 

The most reactive monomers in cationic polymerizations, Af-vinyl carba-zole, vinyl ethers, and p-methoxy-a-methylstyrene provide the most stable carbenium ions. Carbenium ions generated from the first two monomers are stabilized by a-heteroatoms, whereas the latter monomer generates a tertiary carbenium ion with a strongly electron donating p-substituent. Carbenium ions with two alkoxy groups, such as 1,3-dioxo-lane-2-ylium cations and their acyclic analogs (Chapter 6) are stable at room temperature in the absence of moisture. 

In general, styrene and its substituted derivatives are less reactive than vinyl ethers in cationic polymerization, although the reactivity depends considerably on the nature of the substituents. This in turn requires some care in synthesizing block copolymers of styrene derivatives by sequential living cationic polymerization. For example, styrene-methyl... 

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