Free-radical polymerisation propagation

Only very few results are available on the variation of DP with temperature, but they indicate that between -63.5° and -95.5° the DP does not vary significantly and hence EDP = 0 2 kcal/mole [76]. The obvious interpretation of the small EDP, and the large positive Er is that Er is essentially , which means that initiation is slow compared with propagation and termination, and that one is dealing here with a system which has kinetics resembling those of free-radical polymerisations. 

Later, other authors utilized the differences found in the optical activity of monomer and polymer to carry out kinetic investigations on the free-radical polymerisation (70,72,120) and copolymerization (71), and tried to achieve the steric control of the propagation step of free-radical polymerization and copolymerization (13, 14, 39, 73, 98) using optically active monomers and initiators. 

The theory of compartmentalised free-radical polymerisation reactions of the type considered in this paper is of interest primarily because it is believed that most of the polymer which is formed in the course of an emulsion polymerisation reaction is formed by way of reactions of this type. The objective of the theory is to calculate the relative proportions of the reaction loci which at any instant contain 0, 1, 2,. .., r,. .. propagating radicals, and also such properties of the locus population distribution as the average number of propagating radicals per reaction locus, and the variance of the distribution of locus populations. 

Free-radical polymerisation combines initiation and propagation into one process. This method is used... 

Within the framework of this concept, initiation of polymerisation initially leads to accnmnlation of the number of propagating chains and makes the dependence of the nnmber of chains more profound. On the contrary, crosslinking reduces the number of chains, and, at the end of the process, the quantity dN/dt decreases to zero. This approach makes it possible to take into account the unsteady character of the polymerisation. However, becanse the mechanisms involved in the propagation of polylignol chains [3] are markedly different from the classical mechanism of free radical polymerisation, this variant hardly pertains to lignin formation. 

First-order Markov processes are therefore defined by two independent addition probabilities. Although the propagation steps shown above depict free radical polymerisation, the statistical models are equally applicable to other types of chain growth as found, for example, in ionic and Ziegler-Natta polymers (see section 2.3.4). 

Most emulsion polymerisations are free radical processes (318). There are several steps in the free radical polymerisation mechanism initiation (324), propagation and termination (324, 377, 399). In the first step, an initiator compound generates free radicals by thermal decomposition. The initiator decomposition rate is described by an Arrhenius-type equation containing a decomposition constant ( j) that is the reciprocal of the initiator half-life (Ph). The free radicals initiate polymerisation by reaction with a proximate monomer molecule. This event is the start of a new polymer chain. Because initiator molecules constantly decompose to form radicals, new polymer chains are also constantly formed. The initiated monomeric molecules contain an active free radical end group. 

The propagation and termination reactions in free radical polymerisations can be represented by... 

In free-radical polymerisations, rates are controlled by the processes of initiation, propagation, transfer and combination. Although these same processes operate in an emulsion polymerisation, the kinetics in an emulsion polymerisation particle are in general different... 

Free-Radical Addition. In free-radical addition polymerisation, the propagating species is a free radical. The free radicals, R-, ate most commonly generated by the thermal decomposition of a peroxide or aso initiator, 1 (see Initiators, free-RADICAl) ... 

Chain polymerisation typically consists of these three phases, namely initiation, propagation, and termination. Because the free-radical route to chain polymerisation is the most important, both in terms of versatility and in terms of tonnage of commercial polymer produced annually, this is the mechanism that will be considered first and in the most detail. 

Chain polymerisation necessarily involves the three steps of initiation, propagation, and termination, but the reactivity of the free radicals is such that other processes can also occur during polymerisation. The major one is known as chain transfer and occurs when the reactivity of the free radical is transferred to another species which in principle is capable of continuing the chain reaction. This chain transfer reaction thus stops the polymer molecule from growing further without at the same time quenching the radical centre. 

Chain polymerisation involves three major steps (i.e., initiation, propagation and termination). This process of chain polymerisation can be brought about by a free radical, ionic or coordination mechanism. 

Chain polymerisation is characterised by three steps namely initiation (Eq. 5.1), propagation (Eq. 5.2) and termination (Eq. 5.3) where I, R and M refer to the initiator molecule, free radical and monomer respectively and kj, kp and Iq are the respective rate constants for the processes. 

In the hrst step, a redox reaction occurs between Ce(IV) and the -CH2OH end group of PEO, generating a free radical in a-position of the -OH group of PEO. In a consequent step, the radical is transferred from the PEO chain to the vinyl monomer. The radicals formed initiate the actual polymerisation reaction (propagation) ... 

Metallocenes such as Cp2TiCl2 and Cp2ZrCl2 alone are capable of polymerising styrene to an atactic polymer (involving a free radical propagation mechanism) [97]. The same metallocenes activated with methylaluminoxane form active catalysts for the polymerisation of styrene their productivity and syn-diospecificity, however, are not very high. In contrast, when activated with aluminium alkyls, these metallocenes do not afford catalysts that might be active in the polymerisation of styrene [98,99]. 

The free-radical activity is conceived as being generated by an "internal redox" reaction, in which the central metal ion is reduced to a lower valency state (in this case cobalt(II)) by an electron transfer from the ligand, the latter then acquiring a free-radical site by electronic rearrangement. Polymerisation then proceeds by propagation from the free-radical site so generated. 

Conversely, when the rate of propagation is faster than chain transfer, products arising from telomerisation and polymerisation are formed in greater concentration. In this section, free-radical addition to fluoroalkenes will be dealt with first, in order to establish... 

Although the generation of free radicals may be faster by photolytic homolysis the propagation rate should be slower due to the low temperature of polymerisation ( — 20 to -l-15°C as compared to ca. 60°C when using thermochemical initiation). 

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