Termination and Transfer Processes

The initiation and propagation steps in the cationic polymerization of heterocycles have been described in previous sections. The chemistry of these steps has been reasonably well established, mostly by spectroscopic (particularly NMR) methods, and a number of corresponding rate constants have been determined. 

Chain transfer and termination reactions leave their imprint primarily on the resulting structure of the end groups. Unless this structure is known any speculation about these two processes will be premature. The chemical nature of the end groups has, however, been studied only in few systems. 

The borderline between transfer und termination is not very sharp in the Uter-ature and we shall use the following kinetic distinctions for trand er it has no direct kinetic effect, growing species are fully restored, every act of transfer forms one dead macromolecule. We drall also disoiss separately the special case of temporary termination being a reversible termination in which an active center becomes temporarily converted into its inactive (dormant) or much less active, isomeric counterpart. Termination forms one dead maaomolecule and annihilates one active species. 

Under certain conditions, irreversible chain-breaking reactions are absent and cationic ROPs of cyclic ethers proceed as living polymerizations. These conditions are found for polymerizations initiated with acylium and l,3-dioxolan-2-ylium salts containing very stable counterions such as AsFg, PFg, and SbClg or with very strong acids (fluorosulfonic and 

Cationic ROP of ethylene oxide is not useful for the synthesis of linear polymer, but is used to produce crown ethers. Propylene oxide gives less cyclic dimer than does ethylene oxide for steric reasons cyclic tetramer predominates. 

7-2b-3-b Activated Monomer Polymerization. Cyclic oligomer formation can be greatly suppressed and even eliminated by carrying out cationic polymerization as an activated monomer (AM) polymerization by using an acid initiator such as BF3 or triflic acid in the presence of an alcohol [Biedron et al., 1990 Kubisa and Penczek, 1999 Kubisa et al., 2000 Penczek et al., 1986 Wojtania et al., 1986]. 

Activated monomer polymerization competes with conventional cationic ROP. Initiation in conventional ROP involves the reaction of protonated monomer with unprotonated monomer (Eq. 7-31a), whereas initiation in AM ROP involves the reaction of protonated monomer with alcohol (Eq. 7-3 lb). 

There is competition between conventional and AM ROPs. Initiation in conventional ROP is first-order each in protonated monomer and unprotonated monomer. AM ROP is first-order each in protonated monomer and alcohol. The ratio of the rates of AM-to-conventional ROP depends on [ROH]/[M] and the ratio of the rate constants for the two reactions. Assuming that the two rate constants are comparable, AM ROP becomes the dominant process at high [ROH] and low [M], Thus, AM ROP is carried out under monomer-starved conditions. The instantaneous monomer concentration is very low, but monomer is continuously added to the reactor at a rate equal to its rate of consumption. 

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Termination and transfer processes have been neglected in this discussion since these have no effect on polymer composition. Termination and transfer will be discussed later in relation to polymer molecular weight. 

Thus, living polymerization does not allow termination and transfer processes. In real systems, these reactions may take place. However, if transfer and termination cannot be detected under certain reaction conditions using currently available instrumentation, essentially living systems might be achieved. 

Initiation of Poly merization of Vinyl Monomers Propagation Reactions Termination and Transfer Processes Kinetics of Cationic Polymerization of Olefins Temperature Effects... 

Termination and Transfer Processes Macrocyclization. End-Biting and Back-Biting... 

Termination and Transfer Processes. Clear evidence about the mechanism of termination and chain transfer processes can be obtained from the polymer chain end structure. One polymer chain end is controlled by the initiation mechanism, and the second one is controlled by termination and/or chain transfer. The chemical structure of the end groups has been studied in a few cases only (J ). Several peculiarities of these reactions will be outlined here. 

In order to evaluate DP , the termination and transfer processes must be known. In the case of propylene, for example, these can be described by Eqs. (9.9)-(9.12). The corresponding rates are given by... 

The nomenclature poly (M1-6-M2) is used where Mj and M2 are the monomer names for example poly (styrene-b-butadiene). To make block copolymers, the polymer chains must have the ability to propagate [living polymers) when the first monomer is replaced by the second. In conventional addition polymerisation the chain termination and transfer processes make the lifetime of a growing polymer chain too short. Consequently, special ionic polymerisation catalysts were developed. A fixed number of di-anions such as [C6H5CHCH2CH2CHC6H5] are introduced into an inert solvent. These propagate from both ends if a suitable monomer is introduced. As there are no termination or transfer reactions, once the first monomer has been consumed, a second monomer can be introduced to produce a triblock copolymer such as styrene-butadiene-styrene. Each block has a precisely defined molecular weight. These materials undergo phase separation (Chapter 4) and act as thermoplastic rubbers. 

Kennedy and Squires investigated termination and transfer processes in isobutylene polymerization initiated by AICI3 at -78°C in n-pentane (160) and devised a scale of termination or poison and transfer coefficients. Most compounds acted as both poisons and chain-transfer agents. In industry, chain-transfer agents are often deliberately used to control polymerization processes (161). 

To be able to evaluate n , termination and transfer processes must be known. These have been studied and the following termination and transfer processes for propylene have been reported. 

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