Homopropagation reaction

For the copolymerization of two monomers by an anionic mechanism, the copolymer composition equation (7.11) or (7.18), derived in Chapter 7 is applicable with the monomer reactivity ratios defined in the same way as the ratios of rate constants r = k /ki2 and T2 = /s22/ 2i> where k and /C22 are the rate constants for the homopropagation reactions ... 

Implicit in this equation is the assumption that crosspropagation reactions produce the same heat as the corresponding homopropagation reactions namely that AH, .=AH, .=AH [24]. 

Using molecular orbital concepts,the 7r-extension model, and the various transition states in the copolymerization process, it was clearly established that the cross-propagation reactions were very much preferred over the homopropagation reactions.The rr-extension model was shown to be sufficient to account for the alternating copolymerization of the monomer pair to give 11. ° It would be of value to find out if the rr-extension model could also be used to account for other alternating copolymerizations of MA with electron-donor monomers. 

It is apparent from Table 10.20 that Es values for the cross-propagation reactions are noticeably higher than those for the homopropagation reactions. Therefore, cross propagation occurs to give alternating poly(styrene-co-MA). 

The reactions with rate constants /taa and A bb are known as homopropagation reactions and those with rate constants A ab and / ba are called cross-propagation reactions. Hence the rate of consumption of monomer A is given by... 

As the product Ta b decreases there is an increasing tendency towards alternation in the additions of monomer molecules to the propagating chains. The extreme case of azeotropic copolymerization is Ta = a b = 0 and always produces perfectly alternating copolymers, irrespective of the value of /a (i.e. Fa = 0 50 for 0[Pg.123]

Thus, how should block copolymers between styrene and a vinyl ether be prepared Starting with styrene or with a vinyl ether In the former system, the propagating styryl cation is intrinsically more reactive but present at much lower concentration. A rough estimate of the ratio of cation reactivities is = 103 but the ratio of carbocations concentrations is = I0 S. Thus, the ratio of apparent rate constants of addition is 10-2. Macromolecular species derived from styrene should add to a standard alkene one hundred times slower than those derived from vinyl ethers. Thus, one cross-over reaction St - VE will be accompanied by =100 homopropagation steps VE - VE. Therefore, in addition to a small amount of block copolymer, a mixture of two homopolymers will be formed. Blocking efficiency should be very low, accordingly. 

In homopolymerization, the former monomer propagates irreversibly, the homopropagation of THF is highly reversible. It has been pointed out that, for heterocyclic monomers, it is the nature of penultimate unit that governs the reversibility of a given reaction step [304]. Thus, addition of THF to BCMO active center is irreversible, because the backward reaction would require the closure of 4-membered ring. On the other hand, addition of BCMO to THF active centers is reversible ... 

There is another question that has to be discussed the direction of an attack in the S), 2 reaction vs. the position of the anion in the onium salts. Indeed, the simple and attractive picture proposed by Szwarc in order to explain the differences between the reactivities of maaoanions and macroion pairs in the anionic homopropagation of styrene is based on the assumption that an ion pair has to dissociate partially when the transition state of propagation is reached ). Szwarc, after observing a similar reactivity of the polystyryl anion and polystyryl cesium ion pair, also assumed that no partial dissociation was needed for the large Cs cation. The... 

Thus the observed order of reactivities in homopropagation is parallel to the order of reactivities in reaction of standard monomer with different active species and reverse to that observed for reaction of different monomers with standard active species. 

Such reactions disturb the alternating arrangements of the units— Mn-Me—in the products. The reactivity of the monomers determines whether homopropagations occur as well. Alternating propagation depends upon dipole-dipole interactions between Mn and Me monomers in... 

Given the experimental evidence for the existence of radical-solvent complexes and their influence on free-radieal addition reactions such as homopropagation, it is likely that... 

If monomer loss to start, termination or transfer reactions is small, then the degrees of polymerization will be large. The monomers will only be consumed in the two homopropagation and two cross-propagation reactions. 

Each of the four propagation reactions has its own rate constant [ky], where subscript i refers to the nature of the propagating radical chain-end and subscript j denotes the nature of the adding monomer. In the mathematical description of copolymer composition as a function of comonomer feed composition, the individual propagation rate constants are not used. Instead, it is common practice to use so-called reactivity ratios. These reactivity ratios are defined as the ratio between the rate constant for homopropagation and that for aosspropagation. The definition of reactivity ratios is mathematically represented as shown in eqn [5] ... 

The EPUE is encotmtered when rn r2i and/or r22 ri2, which basically means that the full PUM as shown in eqns [16]-[23] is operational. This turned out to be rather exceptional. The prime example is probably the copolymerization of STY and acrylonitrile. Also the copolymerization of STY and maleic anhydride (MAnh) needs elements of the EPUE to describe composition and sequence distribution. This copolymerization is an exceptional case in another respect, since MAnh does not homopropagate. In other words, under the assumption that MAnh is monomer 2, reactions [19], [22], and [23] either have a rate constant of 0 or are irrelevant. 

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