Terminal polymerization mechanism

Chain reactions do not go on forever. The fog may clear and the improved visibility ends the succession of accidents. Neutron-scavenging control rods may be inserted to shut down a nuclear reactor. The chemical reactions which terminate polymer chain reactions are also an important part of the polymerization mechanism. Killing off the reactive intermediate that keeps the chain going is the essence of these termination reactions. Some unusual polymers can be formed without this termination these are called living polymers. 

The polymerization mechanism continues to include initiation, termination, and propagation steps. This time, however, there are four distinctly different propagation reactions ... 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

A living radical polymerization mechanism was proposed for the polymerization of MMA23 -240 and VAc241 initiated by certain aluminum complexes in the presence of nilroxides. It was originally thought that a carbon-aluminum bond was formed in a reversible termination step. However, a more recent study found the results difficult to reproduce and the mechanism to be complex.242... 

The polymerization rate equations are based on a classical free radical polymerization mechanism (i.e., initiation, propagation, and termination of the polymer chains). 

When R-X and NiX have high reactivities for chain transfer and/or primary radical termination (Eqs. 36 and 37), and the C-X bond at the chain end further reacts with Ni° by redox reaction (Eq. 38), the polymerization proceeds via a living radical polymerization mechanism. In this polymerization, the polymerization which has R and X groups at both chain ends is produced ... 

The polymerization of MA with 7 was carried out in the presence of 13, i.e., 7 and 13 were used as two-component iniferters [175]. When an identical amount of 13 to 7 was added to the system, the polymerization proceeded according to a mechanism close to the ideal living radical polymerization mechanism. Similar results were also obtained for the polymerization of VAc. These results indicate that the chain end of the polymer was formed by the competition of primary radical termination and/or chain transfer to bimolecular termination, and that it could be controlled by the addition of 13. 

The Fischer-Tropsch synthesis follows a polymerization mechanism where a Q unit is added to the growing chain. A simplified representation of the reaction network is shown in Fig. 1, where the key points are termination by either H-abstraction to give a-olefins or by hydrogenation to give w-paraffins. 

In conclusion, phase transfer catalyzed Williamson etherification and Wittig vinylation provided convenient methods for the synthesis of polyaromatics with terminal or pendant styrene-type vinyl groups. Both these polyaromatics appear to be a very promising class of thermally reactive oligomers which can be used to tailor the physical properties of the thermally obtained networks. Research is in progress in order to further elucidate the thermal polymerization mechanism and to exploit the thermodynamic reversibility of this curing reaction. 

The synthesis and proposed cure mechanisms of this resin are described in reference 2. While the cure mechanism of the BCB terminated resin is not yet known, it is speculated that it reacts via one of two different routes. Initially the strained four member ring of the benzocyclobutene undergoes a thermally Induced ring opening. The opened rings then react with one another by a linear type addition to form a network type of structure or by cycloaddition to form linear polymer chains. An Illustration of the proposed polymerization mechanism of benzocyclobutene (BCB) terminated resins is shown below. 

The majority of papers published in the field of template polymerization deal with the systems in which both template and monomer are dissolved in a proper solvent and initiation occurs according to the chain mechanism.It is generally accepted that, for chain processes, there are at least three elementary processes initiation, propagation and termination. The mechanism of the addition radical polymerization can be schematically written as follows ... 

In consideration of the kinetic law obtained, Rp i0 of magnitude range, one can conclude that the common polymerization mechanism, based on bimolecular termination reactions, is no longer valid for these multifunctional systems when irradiated in condensed phase. Indeed, for conventional radical-induced polymerizations, the termination step consists of the interaction of a growing polymer radical with another radical from the initiator (R), monomer (M) or polymer (P) through recombination or disproportionation reactions ... 

In order to explain the field effects observed for the cationic polymerizations, we have earlier proposed a kinetic scheme based on the two-state polymerization mechanism and on the field-facilitated dissociation hypothesis (11). Though the assumptions involved in the proposed interpretation turn out to be partly invalid in the light of the experimental data accumulated most recently (15), it is still necessary to give an outline of the scheme. We assumed that, by the initiation reaction between initiator molecules (C) and monomer molecules (M), active species of an ion-pair type (My) are produced, a portion of which dissociates into active species of a free ion type (Mf) and gegenions (C ). The propagation, monomer transfer and termination can be effected by the free ions and ion pairs. A dissociation equilibrium is established between the free ions and ion pairs, which can be characterized by a dissociation constant K. Then we have ... 

Many questions remain about the initiation, propagation, and termination steps of the ethene polymerization mechanism. The most important models proposed to date are the Cossee model, which requires a vacant coordination site on the metal center in the position adjacent to the growing alkyl chain, where ethene is coordinated before insertion into the chain (628), and the Green-Rooney model, which requires the presence of a metal-carbene species and a vacant site where ethene is coordinated prior to insertion (629). 

A typical polymerization mechanism by free radicals includes the following steps initiation, propagation, chain transfer, formation of chain defect structure, termination and inhibition, as listed below ... 

Batch Reactors. One of the classic works in this area is by Gee and Melville (21), based on the PSSA for chain reaction with termination. Realistic mechanisms of termination, disproportionation, and combination, are treated with a variety of initiation kinetics, and analytical solutions are obtained. Liu and Amundson (37) solved the simultaneous differential equations for batch and transient stirred tank reactors by using digital computer without the PSSA. The degree of polymerization was limited to 100 the kinetic constants used were not typical and led to radical lifetimes of hours and to the conclusion that the PSSA is not accurate in the early stages of polymerization. In 1962 Liu and Amundson used the generating function approach and obtained a complex iterated integral which was later termed inconvenient for computation (37). The example treated was monomer termination. 

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