Cationic polymerization chain propagation

The theoretical molecular weight distributions for cationic chain polymerizations are the same as those described in Sec. 3-11 for radical chain polymerizations terminating by reactions in which each propagating chain is converted to one dead polymer molecule, that is, not including the formation of a dead polymer molecule by bimolecular coupling of two propagating chains. Equations 2-86 through 2-89, 2-27, 2-96, and 2-97 withp defined by Eq. 3-185... 

Cationic poiymerization. In cationic chain polymerization the propagating species is a carbocation. Cationic polymerizations require monomers that have electron-releasing groups such as an alkoxy, phenyl, or a vinyl group (Table 14.20). 

As we have seen earlier, the propagating species in cationic chain polymerization is a positively charged carbon species. The older term for this trivalent, trigonal, positively charged carbon ion is carbonium ion which we have used up to this point. Olah [10] proposed that the term carbenium ion be used instead of carbonium ion, the latter being reserved for pentavalent, charged carbon ions, and the term carbocation for both carbonium and carbenium ions. Since the term carbenium ion is not universally followed, to avoid the controversy we will henceforth refer to the propagating species as carbocations. Most text and journal references use the term carbocation and the term carbocation polymerization is used synonymously with cationic polymerization in the literature. 

The determination of the various rate constants (ki, kp, kt, kts, ktr) for cationic chain polymerization is much more difficult than in radical chain polymerization (or in anionic chain polymerization). It is convenient to use Rp data from experiments under steady-state conditions, since the concentration of propagating species is not required. The Rp data from non-steady-state conditions can be used, but only when the concentration of the propagating species is known. For example, the value of kp is obtained directly from Eq. (8.143) from a determination of the polymerization rate when [M J is known. The literature contains too many instances where [M" "] is taken equal to the concentration of the initiator, [IB], in order to determine kp from measured Rp. (For two-component initiator-coinitiator systems, [M" ] is taken to be the initiator concentration [IB] when the coinitiator is in excess or the coinitiator concentration [L] when the initiator is in excess.) Such an assumption holds only if Ri > Rp and the initiator is active, i.e., efficiency is 100%. Using this assumption without experimental verification may thus lead to erroneous results. 

The theoretical molecular weight distributions for cationic chain polymerizations (see Problem 8.30) are the same as those described in Chapter 6 for radical chain polymerizations terminating by disproportionation, i.e., where each propagating chain yields one dead polymer molecule. The poly-dispersity index (PDI = DP /DPn) has a limit of 2. Many cationic polymerizations proceed with rapid initiation, which narrows the molecular weight distribution (MDI). In the extreme case where termination and transfer reactions are very slow or nonexistent, this would yield a very narrow MDI with PDI close to one (p. 681). 

The theoretical molecular weight distributions for cationic chain polymerizations (see Problem 8.25) are the same as those described in Chapter 6 for radical chain polymerizations terminating by disproportionation, i.e., where each propagating... 

The kinetic picture of cationic chain polymerization varies considerably. Much depends upon the mode of termination in any oarticular system. A general scheme for initiation, propagation, and termination is presented below. ° By representing the coinitiator as A, the initiator as RH, and the monomer as M, we can write ... 

Rate Constants for Propagation and Termination. For cationic chain polymerizations, the rate of polymerization is generally accepted to be proportional... 

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. 

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). 

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. 

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. 

Monomers, such as ethylene, propylene, isobutylene, and isoprene, containing the carbon-carbon double bond undergo chain polymerization. Polymerization is initiated by radical, anionic or cationic catalysts (initiators) depending on the monomer. Polymerization involves addition of the initiating species R, whether a radical, cation, or anion, to the double bond followed by its propagation by subsequent additions of monomer... 

Gandini and Plesch concluded that in these systems the chain-carriers are not ionic. Since they are certainly highly polar and in many respects behave as if they were ionic, we called the polymerizations propagated by them pseudo-cationic. Admittedly, in retrospect our original evidence for the non-ionic nature of the chain-carriers looks less convincing, but since that time many other phenomena have been found which support our view very forcibly [18] the case for the reality of pseudo-cationic polymerizations has been presented in detail [7], and therefore the argument need not be repeated here. 

The situation is quite different in chain polymerization where an initiator is used to produce an initiator species R with a reactive center. The reactive center may be either a free radical, cation, or anion. Polymerization occurs by the propagation of the reactive center by the successive additions of large numbers of monomer molecules in a chain reaction. The distinguishing characteristic of chain polymerization is that polymer growth takes place by monomer reacting only with the reactive center. Monomer does not react with monomer and the different-sized species such as dimer, trimer, tetramer, and n-trier do not react with each other. By far the most common example of chain polymerization is that of vinyl monomers. The process can be depicted as... 

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 cationic chain propagation occured in 7.8% of the dimer and 30% of the trimer, it is concluded that normal polymerizations of alpha... 

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