Reversibility of propagation

Any deviation from the living mechanism broadens the distribution. Transfers and termination caused by residual impurities and insufficient mixing are the main causes of improvable broadening of distribution curves. Internal reasons are inherent to two factors, namely to reversibility of propagation and to propagation on centres with various reactivity. 

The reversibility of propagation, or more specifically, the position of the equilibrium as determined by the ratio of the rate constants of propagation and depropagation is also independent of the mechanism. The equilibrium monomer concentration of monosubstituted alkenes such as styrenes and vinyl ethers are so low ([M] < 10-6 mol/L) at temperatures used for carbocationic polymerizations that the reversibility of polymerization can be neglected. 

Highly strained 3- and 4-membered rings polymerize practically irreversibly but polymerization of 5-, 6-, 7-, and higher member rings, important from both a basic and practical point of view, is highly reversible. Thus, in these systems, reversibility of propagation step introduces an additional factor which should be taken into account in kinetic and mechanistic studies and which has practical consequences discussed in more detail in Section II.B.l. 

Cationic polymerization of THF fulfills all the requirements of living polymerization. With several initiators, the initiation is relatively fast and quantitative, and propagation proceeds without transfer or termination. The dependence DP = ([M]0 - [M)e)/[I)0 holds up to high polymerization degrees. The only limitation is that, due to the reversibility of propagation, the molecular weight distribution is broadened and reaches the value of MjMn = 2 in equilibrium. Polymers with narrower MWD were obtained by terminating the polymerization at lower conversion [56]. 

If R = R (bifunctional polymers), reaction (119) does not affect the functionality but leads to the broadening of the molecular weight distribution, which is occurring anyway, due to the reversibility of propagation. Thus, several bifunctional polymers of 1,3-dioxolane were prepared and used, for example, to form the networks containing degradable and hydrolyzable polyacetal blocks (cf., Section IV.B). Reaction (119), however, may effectively prohibit the preparation of monofunctional polymers, e.g., macromonomers. Indeed, two recent attempts to prepare macromonomers by cationic polymerization of cyclic acetals led to nearly statistical... 

Another important factor affecting molecular weight distribution arises from reversibility of propagation. Its influence was discussed earlier in the section dealing with the thermodynamics of propagation, see p. 25 and Ref. 205. 

Thus, we first briefly describe the theory relative to copolymer microstructure and then discuss systems obeying Scheme (15-1), to show the influence of monomer structure on reactivity. Subsequently we describe other systems that do not conform to Scheme (15-1) because of reversibility of propagation or multiplicity of active centers. 

The proportions of the triads vary with the monomer feed. As shown in Fig. 15.2, the proportions of the experimentally found triads agree well with the values calculated from Eq. (15-16a) using r obtained from the content of comonomers in the copolymer (method from Ref. 6)). This agreement means that in this system the penultimate effect, reversibility of propagation, interconversion of active species without propagation [Eq. (15-4)] and redistribution of the initially formed copolymer can be neglected and the process can be described by two reactivity ratios. 

We have given here an extensive treatment of the copolymerization with reversibility of propagation, because this phenomenon is still insufficiently recognized and there are still papers in which reversibility should have been considered schemes that ignore reversibility lead to wrong conclusions. Thus the r values given in these papers do not have the meaning they pretend to have, and various authors have obtained different values for the same pair of monomers for the supposed-to-be reactivity ratios 98). 

Propagating radicals which undergo depolymerization exclusively are relatively stable. For example, the radically initiated polymerizations methyl methacrylate and a-methyl styrene are both reversible (see Figure 1.22). This stems largely from the fact that the active radical resides on a tertiary carbon atom which is more stable than the corresponding secondary radicals involved in the polymerization of methacrylate and styrene, the reversal of propagation being accompanied by many side reactions in each of these cases. 

When copolymers are prepared under conditions where the propagation step is reversible [25, 24, their structures can be significantly different than when the terminal copolymerization model adequately represents the copolymerization process. O Driscoll and Izu [2 ] have calculated the triad distributions to be expected for a-methylstyrene-methyl methacrylate copolymers prepared at 60°, where the propagation steps are essentially irreversible and at higher temperatures, where reversibility of propagation steps is significant. They have kindly provided us with copolymers prepared at 60° and 114°. Fig. 10 shows H-R... 

Reversibility of propagation is often observed in RO polymerization systems, including copolymerization. Although attaining equilibrium is rather infrequently the aim of copolymerization, it can be used as a tool not only for obtaining copolymers of thermodynamically defined properties but also for other purposes. The term equilibrium copolymerization means not only the equilibrium in copolymerization but also copolymerization in which one or more of homo- or... 

Tertiary oxonium ions that are active species in CROP of cyclic ethers are inherently stable (trialkyloxonium salts with stable counterions are commercially available and may be stored without special precautions for prolonged periods of time). Thus, if basic impurities are avoided in CROP of cyclic ethers, there is essentially no irreversible termination. Those polymerization are, however, not classified as living because reversibility of propagation and reversible chain transfer to polymer cause deviations from the ideal situation observed for, for example, anionic vinyl polymerization in which DP = [M]o/[I]o/ molecular weight distribution is close to Poisson distribution and the nature of end-groups may be strictly controlled. As will be discussed in subsequent sections, conditions of living polymerization may be more closely approached if polymerization proceeds by the AM mechanism. 

Practical consequences of reversibility of propagation have been already discussed (cf. Section 4.08.1.1). They may be summarized as follows ... 

The other characteristic feature of CROP of cyclic acetals is reversibility of propagation and the high extent of chain transfer to polymer. 

Since depropagation is the exact reverse of propagation, the activation energy of the depropagation step Ei is expected to equal the sum of the activation energy of the propagation in polymerization E plus the heat of poljunerization AHp, as shown in Figure 4. 

If the temperature is increased to a sufficiently high level the reverse of propagation, known as depropagation may occur. Instead of addition taking place... 

---