The propagation reaction

The fundamental mechanism of the oxidation of polymers was proposed by Bolland and Gee [83, 85, 86], and has been presented in detail by Norling and Tobolsky [454] and Tryon and Wall [619]. 

The propagatiOTi takes place via tertiary oxoitium ions [37, 38] A0 

A cyclic oligomer forms in some instances in addition to the polymer [40], For instance, in polymerizations with BF3 in methylene chloride at low temperatures a cychc tetramer forms, probably by a backbiting process [40]. 

The activation energy of polymerizations of oxetane monomers is higher that that of tetrahydro-furan (see next section). This indicates that the orientation of the cyclic oxonium ion and the monomer is looser in the Sn2 transition state [42]  

In principle, stereospecificity should be possible in substitute polyoxycyclobutanes, such as 2-methyl, 3-methyl, and others. The 2-methyl derivative however, yields amorphous polymers. This is due to the monomer s unsymmetrical structure [33] NMR studies of the microstructure of polymers from 3,3-dimethyloxetane [44] and 2-methyloxetane [43] led to no conclusions about the manner of ring opening. The predominant head to tail structures may result from attacks at either the methylene or the methine carbons next to the oxonium ions of the propagating species. 

Oxetane compounds also polymerize with the aid of aluminum trialkyl-water acetylacetone catalysts [45, 46]. The reactions can take place at 65°C in heptane and yield very high molecular weight polymers. These polymerizations, however, are ten times slower that similar ones carrier out with propylene oxide, using the same catalyst. The reaction conditions and the high molecular weights of the products led to assumption that coordinated mechanisms of polymerizations take place [46]. 

The propagation takes place via tertiary oxonium ions  

The oxonium exchange reactions may occur with the polymer ether linkages as well as with cyclic tetramers that form, as shown above. The concentrations of the oxonium ions of the ether group on the polymer and on the cyclic tetramers, however, are very small. Polymerizations with PF5, on the other hand, or with (C2H5)30PF6 either in bulk or in methylene chloride solutions, yield no significant amounts of cyclic oligomers.  

---

The electron-releasing R group helps stabilize this cation. As with anionic polymerization, the separation of the ions and hence the ease of monomer insertion depends on the reaction medium. The propagation reaction may be written as... 

If the initiation reaction is much faster than the propagation reaction, then all chains start to grow at the same time. Because there is no inherent termination step, the statistical distribution of chain lengths is very narrow. The average molecular weight is calculated from the mole ratio of monomer-to-initiator sites. Chain termination is usually accompHshed by adding proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide. 

Termination. The conversion of peroxy and alkyl radicals to nonradical species terminates the propagation reactions, thus decreasing the kinetic chain length. Termination reactions (eqs. 7 and 8) are significant when the oxygen concentration is very low, as in polymers with thick cross-sections where the oxidation rate is controlled by the diffusion of oxygen, or in a closed extmder. The combination of alkyl radicals (eq. 7) leads to cross-linking, which causes an undesirable increase in melt viscosity. 

Radical Scavengers Hydrogen-donating antioxidants (AH), such as hindered phenols and secondary aromatic amines, inhibit oxidation by competing with the organic substrate (RH) for peroxy radicals. This shortens the kinetic chain length of the propagation reactions. 

The propagation reactions use a methyl radical and generate another. There is no net consumption, and a single initiation reaction can result in an indefinite number of propagation reactions. 

Model based on the variation of the number of active" coordination sites at the catalyst surface. The growth of tubules during the decomposition of acetylene can be explained in three steps, which are the decomposition of acetylene, the initiation reaction and the propagation reaction. This is illustrated in Fig. 14 by the model of a (5,5) tubule growing on a catalyst particle ... 

We shall now attempt to explain, from the chemical bond point of view, the propagation reaction at the basis of tubule growth. A growth mechanism for the (5n,5n) tubule, the (9 ,0) tubule and the (9tt,0)-(5tt,5tt) knee, which are the three fundamental tubule building blocks, is also suggested. 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

Formation of block polymers is not limited to hydrocarbon monomers only. For example, living polystyrene initiates polymerization of methyl methacrylate and a block polymer of polystyrene and of polymethyl methacrylate results.34 However, methyl methacrylate represents a class of monomers which may be named a suicide monomer. Its polymerization can be initiated by carbanions or by an electron transfer process, the propagation reaction is rapid but eventually termination takes place. Presumably, the reactive carbanion interacts with the methyl group of the ester according to the following reaction... 

At the present time the concept of catalytic (or ionic-coordination ) polymerization has been developed by investigating polymerization processes in the presence of transition metal compounds. The catalytic polymerization may be defined as a process in which the catalyst takes part in the formation of the transition complexes of elementary acts during the propagation reaction. 

The propagation reaction itself is of the first order with respect to the monomer concentration. This was demonstrated by measuring the propagation rate constants Kp at different monomer concentrations (98). 

The value of Kp as a measure of the reactivity of the active centers in the propagation reaction is the most fundamental characteristic of polymerization catalysts. The conclusions on the polymerization mechanism based on the correct values of N and Kp are much more unambiguous than those made when considering only the data on the polymerization activity and molecular weight of a polymer. 

The activation of olefins through the formation of the ir-complex with the transition metal ion at polymerization was postulated as one of the stages of the propagation reaction in many works, beginning with those of Ludlum el at. 184) and Carrick (185) ... 

The theoretical problems of the coordination mechanism of the propagation reaction were considered by Cossee et al. 170,186, 187). 

Also, the rates of the propagation steps are equal to one another (see Problem 8-4). This observation is no surprise The rates of all the steps are the same in any ordinary reaction sequence to which the steady-state approximation applies, since each is governed by the same rate-controlling step. The form of the rate law for chain reactions is greatly influenced by the initiation and termination reactions. But the chemistry that converts reactant to product, and is presumably the matter of greatest importance, resides in the propagation reactions. Sensitivity to trace impurities, deliberate or adventitious, is one signal that a chain mechanism is operative. 

Thus, confirmation of whether the product obtained in an attempted reaction in a true random copolymer is important to clarify the mechanism of the propagation reaction and to correlate structure and reactivity in ring-opening polymerizations. Considering that apparent copolymers may be formed by reactions other than copdymerization, for example, by ionic grafting or by combination of polymer chains, characterization of cross-sequences appears to be one of the best ways to check the formation of random copolymers. 

Polymerization equilibria frequently observed in the polymerization of cyclic monomers may become important in copolymerization systems. The four propagation reactions assumed to be irreversible in the derivation of the Mayo-Lewis equation must be modified to include reversible processes. Lowry114,11S first derived a copolymer composition equation for the case in which some of the propagation reactions are reversible and it was applied to ring-opening copalymerization systems1 16, m. In the case of equilibrium copolymerization with complete reversibility, the following reactions must be considered. 

In the anionic copolymerization of lactams, this exchange reaction is faster than the propagation reaction and the copolymer composition is determined by this reaction and not by the propagation reaction127. A general solution of the copolymerization problem considering this equilibrium has not as yet been obtained. 

The effect of propagation-depropagation equilibrium on the copolymer composition is important in some cases. In extreme cases, depolymerization and equilibration of the heterochain copolymers become so important that the copolymer composition is no longer determined by the propagation reactions. Transacetalization, for example, cannot be neglected in the later stages of trioxane and DOL copolymerization111, 173. This reaction is used in the commercial production of polyacetal in which redistribution of acetal sequences increases the thermal stability of the copolymers. 

In the copolymerization of five- and six-membered oxacyclic monomers, the effective monomer concentration in the propagation reaction decreases because only the monomer in excess of equilibrium is available for copolymerization. However, it is not easy to determine the equilibrium monomer concentration in a copolymerization system. The following equilibrium is expected to exist in the copolymerization of THF. 

The centre of experimental and theoretical investigation on cationic polymerization is the propagation reaction, Eq. (1), and the influence on it. 

The formation of high molecular products during the cationic polymerization depends on whether the propagation reaction, consisting of the interaction of the cationic chain end as a reactive intermediate with the monomer, reproduces the reactive intermediate (see Eq. (1)). For this reason the monomer functions as the agent and as the substrate when in the form of the cation. This means, however, the interaction between the monomer and the cationic chain end is a function of the monomer structure itself when all other conditiones remain the same. 

The initiation and the propagation reactions are the deciding factors for the polymerization. When the relationships mentioned above are strongly simplified, and if the monomer structure is altered, the polymerization tendencies can be traced to the corresponding changes in the rc-electron systems. 

The fact that the cationic polymerization could not be experimentally registered for R = —CN, —COOCHj, could be explained in theory with the high n-energy use for the start reaction in contrast to the energy use of R = —Ph—CH3. The vinyl acetate (R = OCOCH3) does not polymerize cationically. This can be explained by the fact that the propagation reaction is so disadvantageous that the formed ions cannot start the polymerization. 

The propagation reactions of the growing cationic chain end with the monomer ethene have already been discussed in part 4.3. The reaction enthalpies of the corresponding propagation steps show different tendencies for the gas phase and solution, when the cationic chain end is lengthened. However, as the monomer is increased in size and the cationic chain end remains the same, then the tendencies for the gas phase and solution correspond to each other. This is an indication that the solvent influence on the cationic propagation reaction is determined by the nature of the cations in question and their solvation. 

---