Propagation cationic polymerization

Fig. 2.15 Mechanism of cationic polymerization propagation (counter-ions omitted). Adapted from Ref. [4]...

Chain transfer reactions are far more important for cationic polymerization. Propagating chains can be terminated by reactions of the cationic centers with cocatalyst or other protonic sources... 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

We shall consider these points below. The mechanism for cationic polymerization continues to include initiation, propagation, transfer, and termination steps, and the rate of polymerization and the kinetic chain length are the principal quantities of interest. 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

In view of the great structural similarity between the propagating sites in the cationic polymerization of P-PIN and isobutylene and their respective polymers (4), and our considerable experience accumulated with the LC Pzn of isobutylene [1-3], efforts have been made to adapt LC Pzn conditions found to yield living polyisobutylenes for the polymerization of p-PIN. 

It is to be noted that N-vinylcarbazole (NVC) undergoes also living cationic polymerization with hydrogen iodide at —40 °C in toluene or at —78 °C in methylene chloride and that in this case no assistance of iodine as an activator is necessary 10d). NVC forms a more stable carbocation than vinyl ethers, and the living propagation proceeds by insertion between the strongly interacting NVC-cation and the nucleophilic iodide anion. 

The term Cationic Polymerization covers a wide field of polymerization processes which are characterized by the propagation of the polymer via cationic chain end according to Eq. (1). 

Calculation of potential energy surfaces should be illustrated in real terms by two simple examples modelling propagation steps of cationic polymerization. To present the potential energy surface graphically the energy can be a function of no more than two variables. The selection of this variable strongly depends on the chosen model. 

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

The formation of high molecular products during the cationic polymerization depends on whether the propagation reaction, consisting of the interaction of the cationic chain end as a reactive intermediate with the monomer, reproduces the reactive intermediate (see Eq. (1)). For this reason the monomer functions as the agent and as the substrate when in the form of the cation. This means, however, the interaction between the monomer and the cationic chain end is a function of the monomer structure itself when all other conditiones remain the same. 

The fact that the cationic polymerization could not be experimentally registered for R = —CN, —COOCHj, could be explained in theory with the high n-energy use for the start reaction in contrast to the energy use of R = —Ph—CH3. The vinyl acetate (R = OCOCH3) does not polymerize cationically. This can be explained by the fact that the propagation reaction is so disadvantageous that the formed ions cannot start the polymerization. 

In total, many possibilities of propagation result for the ions formed by the attack on the C = 0 double bond. According to the calculations, 4 of the structures which can be formed theoretically by interaction of an acrolein chain end with an acrolein monomer possess energetic preference. Two of them are the structures c and d. These results agree with the experimental cationic polymerizability ofacroleine(R = —CHO), as well as with the fact that in the cationically polymerized polyacroleine the following structures alternate with each other88) ... 

Carbocations as reactive intermediates play an essential role in organic reactions and have been thoroughly researched 102, l0J). The individual quality of the cationic polymerization results from the reproduction of the cationic reactive intermediate in every propagation step during the addition of monomers. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

These various structures show characteristic differences of the reactivity during the propagation step. When one observes cationic polymerizations, the propagation via free ions takes place from 10 to 100 times faster than that via ion pairs 1-2). This ratio should be valid for anions from Lewis acids as well as those from protic acids. 

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 ... 

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

Fig ure 2.7 Cationic polymerization initiated by Bronsted-Lowry acid a) initiation, b) propagation, c) termination... 

The kinetic expressions which describe the rate and degree of polymerization in cationic polymerizations are derived in a manner analogous to that for radical polymerization. The results are similar with the main difference being that the direct and inverse dependencies of the rate and degree of polymerization, respectively, on the initiator concentration or initiation rate are both first-order, not half-order as in radical polymerization. The difference arises from cationic termination being mono-molecular in the propagating species instead of bimolecular as in radical polymerization. 

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