Propagation centres

Considerable success has been achieved in controlling the MWD of the products of polymerization where the life-times of propagating centres are long. These studies and others using computer controlled reactors will be reported elsewhere. 

The catalyst surface is assumed to be comprised of propagating centres, whose varying activities are represented by a distribution. For species i the rate of polymerization is described by... 

A polymer-forming chain reaction requires at least one rate-constant, namely that for propagation, k, for its complete specification in this simplest case there is only one type of propagating centre, all the centres are formed in a time which is negligible compared to the duration of the reaction, their concentration remains constant throughout the reaction (Stationary State of the Second Kind), and there is no transfer. 

As far as the polymerisation of heterocyclic compounds with one hetero-atom is concerned (cyclic ethers and their analogues) there seems little doubt at present that the propagation involves a displacement at the positive propagating centre. The ring which is part of this -onium ion is opened between the charged atom and a carbon atom next to it, and this becomes attached to the hetero-atom of the monomer ... 

The anions A2H may also be involved. The two forms of propagating centre, (III) and (IV), may well co-exist, their relative kinetic importance depending upon solvent, temperature, concentrations, age of the reaction and - in the general case - the nature of the acid. The transition states for both the reactions (iii) and (iv) can best be visualised as cyclic and 6-centred [2c, 7a, 29]. 

Coleman and Fox published an alternative mechanism [82], According to these authors, the propagating centres exist in two forms, each of which favours the generation of either the m or r configuration. When both centres are in equilibrium, and when this equilibrium is rapidly established, the chain structure can be described by a modified Bernoulli statistics [83, 84]. The configurations of some polymers agrees better with this model than with first-or even second-order Markov models [84, 85]. 

A comparison of the degrees of polymerization and of the molecular mass distribution curves of the products may help to reveal the nature of the reactions which occur. Another kind of degradative transfer was described by Scott and Senogles [12]. It involves intramolecular transfer with the formation of a non-propagating centre and occurs during the generation of chains with a tendency to form five- and six-membered cyclic structures as a transition stage... 

Cationic polymerization shares several of the features discussed above for anionic polymerization, but the requirements in terms of the monomer structure will be opposite, i.e. the vinyl monomers should have an electron-donating side group since the propagating centre carries a positive charge. Alternatively, the presence of an aromatic ring can stabilize the cation by making available resonance structures. Examples of these are alkyl vinyl ethers, isobutylene, isoprene, styrene and a-methyl styrene (Sauvet and Sigwalt, 1989). 

Addition polymerization, where the growth occurs by reaction of monomer with some active propagating centre =SiX which might be an oxonium ion48,61 or a discrete silicenium ion49,62. 

Many examples of living polymerizations are now available. Commonly they are classified by the nature of the propagating centre or polymerization mechanism. An overview of the most prominent living polymerizations is given in Table 2. 

From a chain structure standpoint, the stereochemical nature of the propagating species and the role that the propagating centre plays in deter-... 

Figure 1.7 The propagating centre in a free-radically initiated polymerization is sp planar whereas the carbon atoms bearing the group X within the chain are sp tetrahedral. The precise mechanism of the propagation step determines the stereochemical arrangement of the repeat units...

Polytetrafluoroethene (Teflon) is etched, giving an active surface which can then be bonded strongly to an epoxy resin. With a source of acidic hydrogen such as water or an alcohol, sodium naphthalene affords 1,4-dihydronaphthalene. Polymers with functional groups at each end of the chain ( telechelic polymers) can be made by polymerization of styrene with sodium naphthalene. The anionic radical which is produced in the initiation step dimerizes to a dianion containing two propagating centres. 

The role of the organometallic cocatalyst has been discussed in detail by Yermakov et al. in terms of formation of active centres and of equilibrium chemisorption reactions at the newly-formed centres for TiCla, TiCla-AlEtaCl, and TiCls-AlEtaCl systems. In a later publication it is concluded from Cp values and C-n.m.r. spectra that the catalytic properties (reactivity and stereospecificity) of the propagation centres, are not directly influenced by the cocatalyst. However the cocatalyst indirectly influences the overall rate of polymerization and the polymer isotacticity due to the complexation reactions altering the number of propagating centres and the ratio of stereospecific to non-stereospecific centres. 

Initiation of styrene polymerization by certain Bronsted adds and their derivatives (c.g., trifluoromethanesulphonic acid, acetyl perchlorate, tri-fluoroacetic acid ) gives rise to products with bimodal molecular weight distributions. Based on the available evidence there appears to be little doubt that at least two distinct types of propagating centre account for the consumption of monomers. 

Hydrocarbon Manmners.— Extermination of the nature of the propagating centres in cationic polymerizations of hydrocarbon monomers such as isobutylene and butadiene has been accomplished through model compound studies and detailed physico-chemical analyses of the polymerized products. 

Of the cationically-polymerizable cyclic acetals, 1-3-dioxolan has received the most attention. By means of an ion-trapping technique, quantitative measurements of the concentration of active centres have been made and correlated with initiator incorporation to confirm the living characteristics of the linear polymers under certain conditions. The oxycarbenium ion nature of these propagating centres has been verified by C-n.m.r. spectroscopy. Methyl substitution alters the polymerizability of the homologous 1,3-dioxepanes significantly and steric hindrance in the initiator can influence both the homopolymerization and copolymerization of cyclic acetals. ... 

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