Kinetics cationic chain polymerization

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 ... 

Scheme 5.5 Kinetic scheme of cationic chain polymerization.

The discussion below follows closely to that of Szwarc and applies to a cationic chain polymerization, but the principle is general. A traditional nonliving carbocation polymerization system might be characterized by the following typical kinetic parameters ... 

The Tafel slopes obtained under concentrations of the chemical components that we suspect act on the initiation reaction (monomer, electrolyte, water contaminant, temperature, etc.) and that correspond to the direct discharge of the monomer on the clean electrode, allow us to obtain knowledge of the empirical kinetics of initiation and nucleation.22-36 These empirical kinetics of initiation were usually interpreted as polymerization kinetics. Monomeric oxidation generates radical cations, which by a polycondensation mechanism give the ideal linear chains ... 

On the other hand copolymer with a trioxane unit at the cationic chain end (Pi+) may be converted intp P2+ by cleavage of several formaldehyde units. These side reactions change the nature of the active chain ends without participation of the actual monomers trioxane and dioxo-lane. Such reactions are not provided for in the kinetic scheme of Mayo and Lewis. In their conventional scheme, conversion of Pi+ to P2+ is assumed to take place exclusively by addition of monomer M2. Polymerization of trioxane with dioxolane actually is a ternary copolymerization after the induction period one of the three monomers—formaldehyde— is present in its equilibrium concentration. Being the most reactive monomer it still exerts a strong influence on the course of copolymerization (9). This makes it impossible to apply the conventional copolymerization equation and complicates the process considerably. 

In the homopolymerization of dioxolane below 30°C. tertiary oxonium ions exist exclusively (2, 5). Otherwise hydride transfer would occur (carbonium ions abstract hydride from monomeric cyclic formats) (II, 16). In trioxane polymerization, however, at least some of the active chain ends are carbonium ions they cause hydride transfer and elimination of formaldehyde (9, II, 13). Thus, in copolymerization we must expect two different kinds of structures for cationic chains with terminal trioxane unit. Oxonium ions (I) and carbonium ions (II) may have different reactivity ratios in the copolymerization, but hopefully this does not cause severe disturbance since I and II seem to be in a fast kinetic equilibrium with each other (3). Hence, we expect [I]/[II] to be constant under similar reaction conditions. 

Cationic polymerization of tetrahydrofuran is one of the few systems in cationic ring polymerization in which chain transfer to polymer may be practically avoided. The reasons for that are of purely kinetic nature. 

Free-Radical Chain Polymerization. In contrast to the typically slow stepwise polymerizations, chain reaction polymerizations are usually rapid with the initiated species rapidly propagating until termination. A kinetic chain reaction usually consists of at least three steps, namely, initiation, propagation, and termination. The initiator may be an anion, cation, free radical, or coordination catalyst. 

Chain Transfer and Termination There are a variety of reactions by which a propagating cationic chain may terminate by transferring its activity. Some of these reactions are analogous to those observed in cationic polymerization of alkenes (Chapter 8). Chain transfer to polymer is a common method of chain termination. Such a reaction in cationic polymerization of tetrahydrofuran is shown as an example in Fig. 10.1. Note that the chain transfer occurs by the same type of reaction that is involved in propagation described above and it leads to regeneration of the propagating species. Therefore, the kinetic chain is not affected and the overall effect is only the broadening of MWD. 

In comparison to the previously discussed monomers, the polymerization of N-substituted aziridines is easier to control since the side reactions by proton transfer are eliminated because of the absence of primary and secondary amines. Nonetheless, termination by nucleophilic attack of the cationic propagating chain end into polymeric tertiary amines results in the formation of unreactive quaternary ammonium groups, that is, termination. As a result, the polymerization of N-substituted aziridines usually stops at limited conversion as was first demonstrated for A-methylaziridine by Jones [129]. Detailed evaluation of the polymerization kinetics as well as the evolution of molar mass during the polymerization revealed that termination mainly occurs via intramolecular backbiting... 

Cationic surfactants, in contrast to anionic surfactants, usually reduce both the number of particles involved in the polymerization and the rate of polymerization. The nature of the stabilizing emulsifier has a marked effect on the polymerization kinetics. For example, addition of a non-ionic stabilizer [e.g., poly(vinyl alcohol), a block copolymer of carbowax 6000 and vinyl acetate, or ethylene oxide-alkyl phenol condensates] to a seed polymer stabilized by an anionic surfactant decreased the rate of polymerization to 25% of the original rate. The effect was as if the nonionic stabilizer (or protective colloid) acted as a barrier around the seed particles to alter the over-all kinetics. It may be that the viscosity of the medium in the neighborhood of the nonionic surfactant coating of the polymer particle is sufficiently different from that of an anionic layer to interfere with the diffusion of monomer or free radicals. There may also be a change in the chain-transfer characteristics of the system [156]. 

It is important to emphasize that this kinetic treatment is valid for any chain polymerization mechanisms, i.e., free radical, cationic, anionic, and coordination. However, in the case of the ionic mechanisms, the type of initiator used and the nature of the solvent medium may influence the ri and r2 values. This is due to the fact that the growing chain end in ionic systems is generally associated with a counterion, so that the structure and reactivity of such chain ends can be expected to be affected by initiator and the solvent. This will be discussed in Section 2.8.3. 

CHAIN POLYMERIZATION BY CATIONIC MECHANISM 2.7.1 Mechanism and Kinetics... 

Cationic polymerizations proceed through cations acting as kinetic chain carriers. Such cations may be carbocations or onium ions. 

An ionic polymerization in which the kinetiC Chain carriers are cations, chain carrier... 

For a long time the only known steady-state processes involved initiation balanced by termination. This was the first postulate of Bodenstein (see Section 3.02.3) when in the reaction GI2 + H2, Gl is formed in the initiation step by GI2 dissociation and either 2G1 GI2 and Gl + H HGl or 2H H2 terminates the kinetic chains. A large number of reactions of inorganic or organic compounds have been analyzed in this way. This approach has also been adapted for the chain polymerizations. There were several attempts to analyze not only radical polymerizations but also ionic polymerizations by using this assumption, for example, cationic... 

Often the transferred reactive species i.e., radical, cation, anion, etc.) rapidly re-initiates monomer. In this case, the polymerization kinetics are unaffected, and each chain transfer event creates one additional polymer chain. Thus, the total number of chains in the system, P]tot, is given by ... 

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