Chain propagation mechanism

Analogous principles should apply to ionically propagated polymerizations. The terminus of the growing chain, whether cation or anion, can be expected to exhibit preferential addition to one or the other carbon of the vinyl group. Poly isobutylene, normally prepared by cationic polymerization, possesses the head-to-tail structure, as already mentioned. Polystyrenes prepared by cationic or anionic polymerization are not noticeably different from free-radical-poly-merized products of the same molecular weights, which fact indicates a similar chain structure irrespective of the method of synthesis. In the polymerization of 1,3-dienes, however, the structure and arrangement of the units depends markedly on the chain-propagating mechanism (see Sec. 2b). 

To elucidate the chain propagation mechanism of alternating copolymerization of TBSM with MA and to quantitatively estimate the contribution of complex-bound monomers to the propagation reaction, a kinetic approach described in Ref.91) was employed. 

Termination is by the recombination of H and Cl. There is complete corroboration of this mechanism from photochemical studies. This is undoubtedly a free-radical chain propagation mechanism. 

Consider now the case of a combined action of two inhibitors, one of which (InH) breaks the chains by reacting with peroxyl radicals and the other (S) breaks down ROOH [9]. The radical In formed in the reaction of InH with R02 is trapped by the peroxyl radical and, therefore, does not contribute to chain propagation (mechanism III, Chapter 14). If kSi -C k3, the inhibitor InH is consumed with a rate of v-Jf, where the initiation rate is Vi = Vio + A 3[ROOH]. After the time ts = ( s[S]o)-1> oxidation becomes quasistationary with the quasistationary hydroperoxide concentration (see Chapter 14)... 

We have synthesized such a material, which is called perfectly imperfect polyethylene, where each branch is a methyl group and its frequency along the backbone is controlled by the nature of the symmetrical diene used in the ADMET polycondensation reaction [37]. Equation 12 illustrates the chemistry used to produce polyethylene by a step polycondensation route rather than a chain propagation mechanism. 

Now consider that a particular straight-chain propagating reaction ensues, that the initial chain particle concentration is simply 1, and that lmol or 1019 molecules/cm3 exist in the system. Thus all the molecules will be consumed in a straight-chain propagation mechanism in a time given by... 

At the risk of being redundant, let me summarize conditions that shift chain propagation mechanisms in lipid oxidation ... 

Free radical trap theories. Fire-retardant chemicals release free radical inhibitors at pyrolytic temperatures that interrupt the chain propagation mechanism of flammability. 

DFT calculations combined with molecular mechanics methods have been used to study the first (R = Me) and the second (R = propyl) insertion of the ethylene monomer into the Ti-R bond of (CpSiMe2NBut)(R)Ti(/t-Me)B(C6F5)3. The influence of the counterion and the solvent effects on the energetic profile of the polymerization have been evaluated. Theoretical investigations have also been directed at mechanistic aspects of olefin polymerizations catalyzed by mono-Cp titanium complexes. The chain propagation mechanism, the chain termination and... 

In plasma state polymerization, the polymer is formed by the repeated stepwise reaction described above. It should be noted that plasma-induced polymerization does not produce a gas phase by-product, because the polymerization proceeds via the utilization of a polymerizable structure. The process can be schematically represented by tiie following chain propagation mechanism ... 

From the Ah-NMR analysis of poly(3,8-d2-styrene oxide) obtained using ZnEt2/H20 as initiator Figure 2), the formation of the partly crystalline fraction can be described by first order Markov statistics, while that for the amorphous fraction follows Bemoulllan statistics. Different chain propagation mechanisms are, therefore, responsible for the formation of the two different polymer fractions obtained from this particular catalyst. Consequently, the existence of two different active centers, responsible for the two polymerization mechanisms and for formation of fractions I and II, are clearly indicated. 

Interfere with the free-radical chain propagation mechanism. 

Polymerization reactions are classified in two categories, which discriminate different chain propagation mechanisms propagation by monomer combination and such by polymer combination. The latter is represented by polyaddition and polycondensation reactions, while the classical representative of the former is radical polymerization. 

C-13 NMR of copolymers provides information regarding composition of a copolymer, monomer sequence distribution and end chain propagation mechanism (22-24). In the present studies, TCA-ST copolymers were studied at different mole compositions in order to see the effect of mole composition on the splitting pattern. An alkyl ester group of a copolymer displayed certain influence on the chemical shift of the carbonyl and quaternary carbon resonances. Quarternary carbons in ST-ST-ST triad always resonate at higher field than that of TC-TC-TC triad. 

Chain propagation. Herein, we review the theoretical achievements based on the formation of Cj and C3 products. According to the choice of the propagation monomer, there are two mechanisms CH -CHy recombination and CO insertion. In the preceding section, the recent DFT investigations on the chain propagation mechanism will be presented in two aspects. 

Plasma-induced polymerization may be schematically represented by a chain propagation mechanism as follows ... 

Polyepichlorohydrin is manufactured by a continuous solution polymerization process using aluminum-based catalysts with base monomers of epichlorohydrin (ECH), ethylene oxide (EO), and AGE. A cationic coordination mechanism is suggested as the chain propagation mechanism. The commercial polymer is 97%-99% head-to-taU and has been shown to be stereo random and atactic. The CO, GCO, ECO, and GECO polymers are amorphous and linear [1]. 

Here again we have an example of the unpairing action of oxygen in the redox reaction (23) whereby free radicals are generated. A chain propagation mechanism (Eq. 25) accounts for the high speed of reaction in the presence of oxygen or peroxide. 

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