Cationic polymerization propagation step

Chain-growth polymerization proceeds by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has three distinct phases an initiation step that starts the polymerization, propagation steps that allow the chain to grow, and termination steps that stop the growth of the chain. We will see that the choice of mechanism depends on the structure of the monomer and the initiator used to activate the monomer. 

By reacting 1,4-butane diol with paraformaldehyde in the presence of sulfuric acid at 150-180" , the seven-membered 1,3-dioxepane is prepared. With other aldehydes, homologous 2-alkyl-substituted 1,3-dioxepanes have also been prepared using a cationic ion exchange resin instead of sulfuric acid. This latter technique was introduced by Astle [42]. The dioxepanes have been converted to polymers in methylene dichloride or in 1,2-dichloro-ethylene. The initiator used was boron trifluoride etherate the reaction temperatures ranged from -10° to +10°. The reactions were carried out under anhydrous conditions by techniques suitable for reaction kinetics studies. The work indicated that, at least for this class of compounds, the polymerization propagation step involves linear alkoxycarbenium ions [47]. 

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. 

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. 

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. 

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 ... 

Chain-reaction mechanisms differ according to the nature of the reactive intermediate in the propagation steps, such as free radicals, ions, or coordination compounds. These give rise to radical-addition polymerization, ionic-addition (cationic or anionic) polymerization, etc. In Example 7-4 below, we use a simple model for radical-addition polymerization. 

The kinetic chain reaction typically consists of three steps (1) initiation, (2) propagation, and (3) termination. The initiators for free radical, anionic, and cationic polymerizations... 

Carbonyl monomers can be polymerized by acidic initiators, although their reactivity is lower than in anionic polymerization. Protonic acids such as hydrochloric and acetic acids and Lewis acids of the metal halide type are effective in initiating the cationic polymerization of carbonyl monomers. The initiation and propagation steps in polymerizations initiated with protonic acids can be pictured as... 

As was stated above, the interpretation that the field affects the dis-sodation state of the growing chain ends was not uniquely substantiated by the experimental data, except those on copolymerizations. Thus it is interesting to investigate the field influence on much simpler systems than cationic homopolymerizations. For this purpose we have chosen living anionic systems in which only propagation steps are involved. The system first studied was a living anionic polymerization of styrene with n-butyllithium in the binary mixtures of benzene and tetrahydrofuran (17,24) and in the binary mixtures of benzene and dimethoxyethane (15). 

The recent study by Overberger and Jarovitzky (37) shows clearly the cationic nature of the propagation step in the polymerization of alpha-D-styrene by Ziegler catalysis. 

Yoshida and coworkers also developed a microreaction system for cation pool-initiated polymerization [62]. Significant control of the molecular weight distribution (Mw/Mn) was achieved when N-acyliminium ion-initiated polymerization of butyl vinyl ether was carried out in a microflow system (an IMM micromixer and a microtube reactor). Initiator and monomer were mixed using a micromixer, which was connected to a microtube reactor for the propagation step. The polymerization reaction was quenched by an amine in a second micromixer. The tighter molecular weight distribution (Mw/M = 1.14) in the microflow system compared with that of the batch system (Mw/M > 2) was attributed to the very rapid mixing and precise control of the polymerization temperature in the microflow system. 

Typical monomers that may be polymerized by cationic methods include styrene, isobutylene, and vinyl ethers. Unlike radical polymerizations, solvent polarity can influence the rate of polymerization. This is due to the presence of the counterion (see Fig. 15.13). For example, more polar solvents can increase the degree of separation between the growing end and the counterion during the propagation step, increasing the rate of propagation.17... 

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