Propagation in polymerization

FIGURE 1111 Cham propagation in polymerization of styrene The growing polymer chain has a free radical site at the benzylic carbon It adds to a molecule of styrene to extend the chain by one styrene unit The new polymer chain is also a benzylic radical it attacks another molecule of styrene and the process repeats over and over again... 

J.S. Lee and G.G. Fuller, The spatial development of transient couette flow and shear wave propagation in polymeric liquids by flow birefringence, J. of Non Newt. Fluid Mech., 26, 57 (1987). 

The Sj 2 mechanism of propagation in polymerization of heterocyclic monomers was generally accepted. 

At much higher frequencies, bulk longitudinal waves can be propagated in polymeric liquids, and these may yield indirectly some information about shear properties. Since their behavior is dominated by the bulk compressional- viscoelasticity, however, discussion of them will be postponed to Chapter 8. 

Since depropagation is the exact reverse of propagation, the activation energy of the depropagation step Ei is expected to equal the sum of the activation energy of the propagation in polymerization E plus the heat of poljunerization AHp, as shown in Figure 4. 

Ah these polymerizations proceed only in the absence of oxygen or water, which react with the highly reactive propagating species. Polymerization is usuahy carried out in an inert, hydrocarbon solvent and under a nitrogen blanket. Under these conditions, polymers with narrow molecular-weight distributions and precise molecular weights can be produced in stoichiometric amounts. 

Other reasons for a wide propagation of polymerization in water include (1) reduction of energy consumed to separate the initial monomer in crystal form (acrylamide is produced and used in the aqueous solution form), which, in addition, is associated with the probability of its spontaneous polymerization, and (2) recovery of the organic solvents, which results in less environmental pollution and the elimination of the stage of solution of polymer reagents used, as a rule, in the form of the aqueous solutions. 

The nature of the gegen ion is probably of the greatest importance in determining the rate of propagation in ionic polymerization. However, it is not clear whether the presence of the gegen ion enhances or inhibits the propagation. One may argue that the... 

In catalytic polymerization the reactivity of the propagation center depends on the catalyst composition. Therefore, the dependence of the molecular structure of the polymer chain mainly on the catalyst composition, and less on the experimental conditions, is characteristic of catalytic polymerization. On the other hand, in polymerization by free-radical or free-ion mechanisms the structure of a polymer is determined by the polymerization conditions (primarily temperature) and does not depend on the type of initiator. 

The following stage of the propagation center formation occurs through the reduction of Cr(VI) to the lower oxidation state. The compounds of Cr(II) seem to be active in polymerization in the solution of bis-triphenyl-silyl-chromate (109). For the formation of these compounds the following scheme taking into account the results (110) concerning the study of the reaction of bis-triphenylsilyl-chromate with olefins was considered (109) ... 

To determine the number of propagation centers in one-component catalysts, in principle the same methods used to study two-component catalysts of olefin polymerization may be applied Qsee (18, 160, 160a) ]. The most widely used methods for the determination of the number of propagation centers in polymerization catalysts are ... 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

Table 4.11 Kinetic Parameters for Propagation in Selected Radical Polymerizations in Bulk Monomer...

Diallyl monomers find significant use in cyclopolymerization (Section 4.4.1). Transfer to monomer is of greater importance in polymerizations of allyl than it is in diallyl monomers.184 This might, in part, reflect differences in the nature of the propagating species [e.g. a secondary alkyl (115) v.v a primary alkyl radical (116)]. Electronic factors may also play a role,185... 

An attractive feature of using the solvent as an agent to control propagation in solution polymerization is that solvents when used are usually present in very large excess in relation to any radical species. Of course, economic, solubility, toxicity, waste disposal, and other considerations limit the range of solvents that can be employed in an industrial polymerization process. 

The data also bear on the validity of the two models that have been proposed to describe the mechanism of ionic chain propagation in the gas phase. In review, Lampe, Franklin, and Field (23) have proposed that the polymerization proceeds through the reactions of long-lived, undissociated, intermediate reaction complexes,... 

The mechanism of the polymerization of NCA with tertiary amine is still controversial. Mori and Iwatsuki claim that the true initiator is the primary amino group formed by hydrolysis of the NCA with contaminated water and that tertiary amine forms a complex with the NCA and accelerates the addition reaction37 . Harwood et al. confirmed the propagating carbamate by NMR in polymerization initiated with a strong base37 . The successive addition of NCA to the polymer end catalyzed with a strong base affords an alternative procedure for the synthesis of block copolypeptides. Block copolypeptides of poly(oxyethylene) were prepared by triethyl amine catalyzed polymerization of NCA in the presence of poly(oxyethylene)bis-eMoroformate38 . 

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