Cationic chain polymerization isomerization

The competing reactions are isomerization of the cationic chain end, transfer reactions to monomer, counterion and solvent, and also termination reactions. The actual process of propagation depends on the concrete interactions between the reactants present in the polymerizing system. A synopsis of interactions expected is given in Table 7. For the most important of them quantum chemical model calculations were carried out. 

Cationic polymerization was considered for many years to be the less appropriate polymerization method for the synthesis of polymers with controlled molecular weights and narrow molecular weight distributions. This behavior was attributed to the inherent instability of the carbocations, which are susceptible to chain transfer, isomerization, and termination reactions [48— 52], The most frequent procedure is the elimination of the cation s /1-proton, which is acidic due to the vicinal positive charge. However, during the last twenty years novel initiation systems have been developed to promote the living cationic polymerization of a wide variety of monomers. 

In some cationic polymerizations, the monomers may rearrange in the process of placements into the chains. The isomerizations are to eneigetically preferred configurations. The result is that the units in the final polymers are structurally different from the original monomers. Such rearrange-... 

Unwanted branching of many polymers probably occurs through such isomerizations. PP, formed using cationic polymerization, has methyl, ethyl, w-propyl, w-butyl, isopropyl, gem-dimethyl, isobutyl, and t-butyl groups connected to the main chain. 

Possibilities for chain transfer in cationic polymerization are abundant.119,138 Proton transfer to the monomer is a rather general transfer reaction leading to two isomeric unsaturated end structures ... 

P-Pinene which is a main component of natural turpentine can be polymerized by living cationic isomerization polymerization [82] (Scheme 10) using TiCl3(OfPr) as a Lewis acid in conjunction with rc-Bu4NCl in CH2C12 at -40 °C. When initiator 31 was used, polymerization led to a poly(P-pinene) macromonomer with a methacrylate function at the a end and a chlorine atom at the co chain end [83]. Three macromonomers were prepared with DPn=8,15, and 25 respectively they had narrow MWD (Mw/Mn= 1.13-1.22) and the reported functionality was close to 1 (Fn=0.90-0.96). 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

Because the chain transfer to polymer is fast as compared with reformation of active species of propagation [Eq. (128)] and there is a reaction pathway, which due to the formation of isomerized products is irreversible [reaction (129)], continuous degradation of the already formed polythiirane chains occurs if the reaction system is kept unterminated [159]. Also isolated polymers, treated with cationic initiators degrade to low molecular weight, predominantly cyclic oligomers. Consequently, cationic polymerization of thiiranes is very strongly affected by chain transfer to polymer processes. 

Decomposition of alkylbenzene Ph-Me, Ph-Et, Ph- Pr Friedel-Crafts acylation acetylation, benzoylation, and so on Isomerization of paraffin open-chain C1-C7, cyclic Cs-Cn Esterification -> AcOH + MeOH, EtOH, and so on CgOH + phthalic acid, and so on Cationic polymerization Me, Et, Bu vinyl ether Oligomerization 3-pinene, 1-octene, 1-decene Others -> aldol condensation, and so on... 

These results agree with those of various authors who have demonstrated the transformation of bicyclo[n. 1.0]alkane type structures under acid catalysis into compounds having a methyl group. However, it is not possible to visualize a prior isomerization into methylcycloalkenes followed by its polymerization. One must consider that polymerization occurs in a single step by transformation into the carbo cation. Even so, the presence of monomer units with the methyl group not in the side chain but carried by carbon atoms in the ring should not be excluded. If such units existed, they would be present in a very small proportion with respect to the main monomer unit. 

The cationic polymerization of propylene, 1-butene, and higher 1-alkenes yields only very low molecular weight polymers DP < 10 - 20) with highly complicated strucmres that arise due to various combinations of 1,2-hydride and 1,2-methide shifts, proton transfer, and elimination, besides chain transfer during polymerization. In the polymerization of ethylene, initiation involving protonation and ethylation is quickly followed by energetically favorable isomerization ... 

Sinn et al. (1961) pointed out that the effect of water in the stereoregular polymerization was like the cocatalytic effect of water in Friedel-Crafts polsmerizations. They concluded, therefore, that the ds polymerization of butadiene was a cationic coordinated mechanism. In this mechanism the monomer was first oriented by the cobalt, then released as the cationic end of the chain added. They suggested that the monomer did not isomerize because it was held in a caged structure. The orientation of the chain end was not considered. It should be noted that, if a relatively free carbonium ion existed, isomerization of the double bond would be expected according to Eq. (39). The cobalt must preserve, therefore, the stereochemistry of the chain end until addition of another monomer unit (Lehr, 1963). 

The chemistry of aromatic alkylation is more complicated than implied by equations (15, 16, 17). Polymerization, CP production, and the isomerization of heavier olefins also occur. Olson (32) has reported many details of the positional isomerization of 1-aIkenes in the Ce—Cm range. Using 1-dodecene as an example, the alkylated product is a mixture of 2- through 6-dodecyl benzenes. In the absence of isomerization, only 2-dodecyl benzene is produced. Attachment at the first carbon atom is not expected when propylene or heavier olefins are employed since primary cations would then be obtained. Secondary cations are however more stable (or preferred) and lead to the attachment at the second or higher carbon atom of the cation. Olson suggests that positional isomerization involves the formation of dodecyl acid sulfates or dodecyl fluorides when sulfuric acid or HF are used as catalysts reverse reactions then lead to the formation of olefins with double bonds in a new position on the chain. In one example reported, at least 80% of the dodecene isomerized before alkylation (by reactions similar to eqs. 16 and 17). Olson also found that some of the initial dodecylbenzene produced were isomerized. The 2-dodecyl benzene that was initially produced isomerized in the presence of AICI3 catalysts to give from 3-dodecyl to 6-dodecyl benzenes. 

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