Activated monomer mechanism propagation step

Mechanisms of the above type are very plausible but two points should be considered. Firstly, all these transition states are equally plausible for butadiene and isoprene whereas butadiene gives a mixed cis-trans product with lithium alkyls in hydrocarbons. Secondly, it is not certain that these carbon-lithium bonds are essentially covalent in hydrocarbons. There is evidence that the lithium compounds of conjugated monomers still exist as charge delocalized ion-pairs in the associated state in hydrocarbons (48). The characteristic ultra-violet absorption band attributable to this kind of anion pair persists almost unchanged in different solvents and alkali metals. The monomeric form active in the propagation step could possibly contain a more covalent carbon-lithium bond but we cannot be sure of this. 

Figure 5.9 outlines the steps for the chain polyaddition mechanism involved in the coordination polymerizations for any kind of active species initiated through different cocatalysts. The counteranion species was suppressed for practical representation of the active site. Once the cationic species is created, it starts the growth of the polymeric chain through continuous addition of monomer. The propagation step is forward described in Figure 5.9 according to the most accepted reaction cycle proposed by Cossee and Arlman, which is known as the Cossee-Arlman mechanism [51]. 

Ionic chain polymerisations refer to chain mechanisms in the course of which the propagation step consists of the insertion of a monomer into an ionic bond. The strength of this ionic bond can vary, depending on the nature of the species, the temperature and the polarity of the solvent, between a closed ionic pair in contact up to free ions (see Figure 23). Final polymer microstructure (configuration,...) and molecular mass distribution depend on the actual nature of the active ionic species. 

Both the initiation step and the propagation step are dependent on the stability of the carbocations. Isobutylene (the first monomer to be commercially polymerized by ionic initiators), vinyl ethers, and styrene have been polymerized by this technique. The order of activity for olefins is Me2C=CH2 > MeCH=CH2 > CH2=CH2, and for para-substituted styrenes the order for the substituents is Me—O > Me > H > Cl. The mechanism is also dependent on the solvent as well as the electrophilicity of the monomer and the nucleophi-licity of the gegenion. Rearrangements may occur in ionic polymerizations. 

More evidence has been accumulated [see e. g. ref. (55)] to show that the polymerisation yielding high molecular weight polypeptides proceeds in two steps — initial self-accelerated reaction followed by an apparently first order reaction. It seems that the growing species slowly reach their stationary concentration and in this period the reaction appears to be auto-catalytic. In the terms of Bamford s mechanism this behaviour is easily explained by postulating slow initiation and rapid propagation. The initiation results from an attack of an activated monomer on a non-aetivated NCA. The propagation results from a... 

The polymerization undergoes a coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as a hydride, alkyl, amide or alkox-ide group, on the carbonyl carbon atom of a lactide or lactone to form a new lanthanide alkoxide species via acyl-oxygen cleavage. The continued monomer coordination and insertion into the active metal-alkoxo bond formed completes the propagation step as shown in Figure 8.50. 

The initiation step is normally fast in polar solvents and an initiator-free living polymer of low molecular weight can be produced for study of the propagation reaction. The propagation step may proceed at both ends of the polymer chain (initiation by alkali metals, sodium naphthalene, or sodium biphenyl) or at a single chain end (initiation by lithium alkyls or cumyl salts of the alkali metals). The concentration of active centres is either twice the number of polymer chains present or equal to their number respectively. In either case the rates are normalized to the concentration of bound alkali metal present, described variously as concentration of active centres, living ends or sometimes polystyryllithium, potassium, etc. Much of the elucidation of reaction mechanism has occurred with styrene as monomer which will now be used to illustrate the principles involved. The solvents commonly used are dioxane (D = 2.25), oxepane (D = 5.06), tetrahydropyran D = 5.61), 2-methyl-tetrahydrofuran (D = 6.24), tetrahydrofuran (D = 7.39) or dimethoxy-ethane D = 7.20) where D denotes the dielectric constant at 25°C. 

With the continual discovery of new reactions leading to the formation of polymers, and with the increased understanding of the mechanisms of those reactions, it is natural that certain definitions of descriptive terms will change with time. The term polycondensation is used here to denote those polymerization reactions which proceed by a propagation mechanism in which an active polymerization site disappears every time one monomer equivalent reacts. This may be illustrated for one propagation step in polyesterification by... 

Example 4-8 An ideal continuous stirred-tank reactor is used for the homogeneous polymerization of monomer M. The volumetric flow rate is O, the volume of the reactor is V, and the density of the reaction solution is invariant with composition. The concentration of monomer in the feed is [M]o. The polymer product is produced by an initiation step and a consecutive series of propagation reactions. The reaction mechanism and rate equations may be described as follows, where is the activated monomer and P2, . . , P are polymer molecules containing n monomer units ... 

The propagation step postulated In the "a.m." mechanism Involves reaction of the carbamate Ions In VII or larger n-mers (e.g., VIII) with NCA to generate additional "activated monomers." Is Is then proposed to react as a nucleophile with acyl NCA moieties of n-mers to yield carbamate Ions of (n + l)-mers, and so forth. 

Figure 9.11 Bound-ion-coordinadon mechanism for polymerizadon on a catalyst surface with growth from a single active site and replenishment of monomer from the liquid phase. Consecutive propagation steps are represented in (a), (b), and (c). (After Ref. 29.)...

Chain-growth polymerization proceeds by one of three mechanisms radical polymerization, cationic polymerization, or anionic polymerization. Each mechanism has three distinct phases an initiation step that starts the polymerization, propagation steps that allow the chain to grow, and termination steps that stop the growth of the chain. We will see that the choice of mechanism depends on the structure of the monomer and the initiator used to activate the monomer. 

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