Addition polymerization, elementary

The kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

The addition polymerization invariably proceeds by a chain-reaction mechanism involving three elementary steps, i.e. initiation, propagation and termination (Fig. 2). The preferred mode of monomer addition to the growing chain de-... 

The specificity of the reaction mechanism to the chemistry of the initiator, co-initiator and monomer as well as to the termination mechanism means that a totally general kinetic scheme as has been possible for free-radical addition polymerization is inappropriate. However, the general principles of the steady-state approximation to the reactive intermediate may still be applied (with some limitations) to obtain the rate of polymerization and the kinetic chain length for this living polymerization. Using a simplified set of reactions (Allcock and Lampe, 1981) for a system consisting of the initiator, I, and co-initiator, RX, added to the monomer, M, the following elementary reactions and their rates may be... 

A new qualitative quantum-chemical concept of the elementary act of addition polymerization has been proposed as the development of the polymerization theory. An extensive set of various data on the kinetics and the mechanism of polymer structure controlling has been found to have a new explanation from an uniform viewpoint. This concept is developed in the framework of the axiomatic approach to the general polymerization theory and is based on five postulates, namely the principle of the intermediate, the principle of intermediate cyclicity, the principle of correspond-... 

From the chemical viewpoint, all polymerization processes are only a subclass of a wider dass of addition reactions. There is nothing spedfically macromolecular in the polymerization elementary act itself. However, from the viewpoint of investigations of the propagation mechanism, the chain being formed in the course of the process is actually a material store of information, a kind of natural memory device fixing the final results of the periodically breaking out electron storm accompanying each act of monomer insertion in... 

The aim of the next part of the paper is to demonstrate the validity of the above concept of the elementary act of addition polymerization of non-cyclic monomers by comparing some of its conclusions with the available experimental data. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

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 ... 

Secondary reactions usually proceed in addition to template polymerization of the system template-monomer-solvent. They influence both kinetics of the reaction and the structure of the reaction products. Depending on the basic mechanism of reaction, typical groups of secondary reactions can take place. For instance, in polycondensation, there are such well known reactions as cyclization, decarboxylation, dehydratation, oxidation, hydrolysis, etc. In radical polymerization, usually, in addition to the main elementary processes (initiation, propagation and termination), we have the usual chain transfer to the monomer or to the solvent which change the molecular weight of the product obtained. Also, chain transfer to the polymer leads to the branched polymer. 

Quantitative conversion is one of the essential preconditions to achieve a significant molecular weight in stepwise polymerization process. Consequently, an iron-catalyzed Michael reaction would be a suitable elementary step for a polyaddition. Bis-donor 24c and bis-acceptor 41b, readily accessible from common starting materials [69], were converted with FeCl3-6H20 to yield a poly-addition product... 

In addition to the formation of active centres and participation in elementary processes, the discussion of which forms the main topic of this volume, monomers very often react with some component(s) of the polymerizing medium under complex formation. This reaction is very important. Complex formation lowers the effective monomer concentration, and changes in the polymerization rate usually occur. When the complex is much more active than the monomer, it may react preferentially with the active centre. This, of course, changes the addition mechanism and kinetics. When the monomer and complex also compete, the macrokinetics need not necessarily change. Usually, however, the mechanism of the whole process is greatly complicated, and a kind of copolymerization occurs. 

Because the elementary reactions of cationic alkene polymerizations are directly related to the organic chemistry of carbocations, Chapter 2 will investigate electrophilic additions to double bonds, nucleophilic substitution, electrophilic aromatic substitution, and elimination reactions. 

AU four of the elementary reactions in a cationic polymerization involve electrophilic or cationic intermediates. Thus, initiation, propagation, transfer, and termination may be classified as either nucleophilic substitution, electrophilic addition, elimination, rearrangement, or possibly as a pericyclic reaction. Initiation occurs in alkene polymerizations by either addition of acid to the alkene, or by ionization of a covalent initiator followed by addition of the resulting carbocationic intermediate to an olefin s double bond. Although initiation is an electrophilic addition (AdE) reaction in... 

The rate constants of nearly all of the elementary reactions in trityl-initi-ated polymerizations of cyclopentadiene [216], p-methoxystyrene [186], vinyl ethers [217], and a-methylstyrene [218] were determined by kinetic measurements, sometimes combined with conductometric measurements. Monomer conversion was followed by either dilatometry, spectroscopy, or calorimetry. Initiation was followed by the decrease in the 410-nm absorption of the trityl carbenium ions (e = 36,000 mol- L em-1), caused by their reaction with monomer by either direct addition or hydride abstraction. The initiator was assumed not to be consumed in any other reactions. The reaction orders (usually first order in each reagent) and rate constants of initiation were then determined by plotting the rate of initiation versus the initial monomer and initiator concentrations according to Eq. (52). 

The elementary reactions of carbocationic polymerizations can be separated into three types. Deactivation of carbenium ions with anions and transfer to counteranion are ion-ion reactions, propagation and transfer to monomer are ion-dipole reactions, and ionization is a dipole-dipole reaction [274]. Ion-ion and dipole-dipole reactions with polar transition states experience the strongest solvent effects. Carbocationic propagation is an ion-dipole reaction in which a growing carbenium ion adds electro-philically to an alkene it should be weakly accelerated in less polar solvents because the charge is more dispersed in the transition state than in the ground state [276]. However, a model addition reaction of bis(p-methoxyphenyl)carbenium ions to 2-methyl- 1-pentene is two times faster in nitroethane (e = 28) than in methylene chloride (e = 9) at - 30° C [193]. However, this is a minor effect which corresponds to only ddG = 2 kJ morit may also be influenced by specific solvation, polarizability, etc. [276,277]. 

A complete mechanistic picture of a polymerization should include structures of the active species participating in all elementary reactions (initiation, propagation, transfer, and termination), a mechanism (in the organic chemist s sense) of all of these reactions with a special emphasis on propagation which is responsible for the construction of nearly the entire macromolecule (except end groups), and an explanation of various structural effects in the monomer, active centers, additives, medium, etc., which affect rates, molecular weights, and MWDs. 

The way in which a plasma polymer is formed has been explained by the rapid step growth polymerization mechanism, which is depicted in Figure 5.3. The essential elementary reactions are stepwise recombination of reactive species (free radicals) and stepwise addition of or intrusion via hydrogen abstraction by impinging free radicals. It is important to recognize that these elementary reactions are essentially oligomerization reactions, which do not form polymers by themselves on each cycle. In order to form a polymeric deposition, a certain number of steps (cycle) must be repeated in gas phase and more importantly at the surface. The number of steps is collectively termed the kinetic pathlength. 

The action of elementary fluorine on organic compounds has been reviewed by Bigelow (81, 82). The chemical changes which can be brought about are addition, substitution, fragmentation, dimerization, and polymerization. 

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