Alternating tendency

The monomer pair, acrylonitrile—methyl acrylate, is close to being an ideal monomer pair. Both monomers are similar in resonance, polarity, and steric characteristics. The acrylonitrile radical shows approximately equal reactivity with both monomers, and the methyl acrylate radical shows only a slight preference for reacting with acrylonitrile monomer. Many acrylonitrile monomer pairs fall into the nonideal category, eg, acrylonitrile—vinyl acetate. This is an example of a nonideality sometimes referred to as kinetic incompatibiUty. A third type of monomer pair is that which shows an alternating tendency. 

These values indicate strong alternation tendencies that decrease with increasing temperature. Computations show that 1 1 ETFE copolymers obtained at —30 and 65°C should have about 97 and 93%, respectively, of alternating sequences (15). 

Lewis acids (dicthylaluminum chloride, ethyl aluminum scsquichloridc) have been used in conjunction with ATRP to provide greater alternating tendency in S-MMA copolytnerization.519 However, poor control was obtained because of interaction between the catalyst (CuCI/dNbpy) and the Lewis acid. Better results were obtained by RAFT polymerization/10 Copper catalysts, in particular Cu(lI)Br/PMDETA, have been shown to coordinate monomer but this has negligible influence on the outcome of copolymerization/6 ... 

The deviation of riV2 from unity has already been cited as a measure both of alternating tendency and of specificity in the radical-monomer reactions. This product of the reactivity ratios approaches unity only in those cases in which the monomer substituents are similar to one another in their electron-attracting or releasing capacities. Devia-... 

The alternating tendency of the copolymers is advantageous in that the polymerizations can be carried out to high conversions with little or no compositional drift. For random copolymerizations in which there is preferential incorporation of one monomer due to a mismatch in reactivity ratios, the compositional variations with conversion can be substantial. Such compositional heterogeneities in resist materials can lead to severe problems during image development. 

The monomers have been arranged in Table 6-3 in their general order of reactivity. The order of monomer reactivities is approximately the same in each vertical column irrespective of the reference radical. The exceptions that occur are due to the strong alternating tendency... 

Two mechanisms have been proposed to explain the strong alternation tendency between electron-acceptor and electron-donor monomers. The polar effect mechanism (analogous to the polar effect in chain transfer—Sec. 3-6c-2) considers that interaction between an electron-acceptor radical and an electron-donor monomer or an electron-donor radical and... 

Both the cross-propagation and complex mechanisms may be operative in alternating copolymerizations with the relative importance of each depending on the particular reaction system. The tendency toward alternation, with or without added Lewis, acid, is temperature-and concentration-dependent. Alternation decreases with increasing temperature and decreasing total monomer concentration since the extent of complex formation decreases. When the alternation tendency is less than absolute because of high reaction temperature, low monomer concentration, absence of a Lewis acid, or an imbalance in the coordinating abilities of the two monomers, copolymerization proceeds simultaneously by the two mechanisms. The quantitative aspects of this situation are considered in Sec. 6-5. 

The patterns of reactivity parameters, like the Q-e parameters, can be used to analyze reactivity in both copolymerization and homopolymerization. Look at the data in Table 6-4 and compare with the parameters in Table 6-8. The highly reactive radicals are those with lower values of ns- The highly reactive monomers are those with the more positive or less negative values of v. However, v is not the only consideration, polarity is also important. For example, maleic anhydride is a monomer with one of the most positive v values, but it undergoes facile copolymerization only with monomers with which it has a polarity difference. This is the alternation tendency and is given by... 

The alternation tendency is dependent only on the polarity parameters for the monomers and radicals. 

The general characteristics of anionic copolymerization are very similar to those of cationic copolymerization. There is a tendency toward ideal behavior in most anionic copolymerizations. Steric effects give rise to an alternating tendency for certain comonomer pairs. Thus the styrene-p-methylstyrene pair shows ideal behavior with t = 5.3, fy = 0.18, r fy = 0.95, while the styrene-a-methylstyrene pair shows a tendency toward alternation with t — 35, r% = 0.003, i ii 2 — 0.11 [Bhattacharyya et al., 1963 Shima et al., 1962]. The steric effect of the additional substituent in the a-position hinders the addition of a-methylstyrene to a-methylstyrene anion. The tendency toward alternation is essentially complete in the copolymerizations of the sterically hindered monomers 1,1-diphenylethylene and trans-, 2-diphe-nylethylene with 1,3-butadiene, isoprene, and 2,3-dimethyl-l,3-butadiene [Yuki et al., 1964]. 

Explain the relative alternating tendencies in these copolymerizations. 

Most of the largest differences between observed and calculated r values in Table VI occur for cumene combinations. Such discrepancies are inevitable if cumene tends to alternate with butadiene and Tetralin but not with styrene. (This distinction could be partly experimental error, which tends to be greatest in combinations of the least reactive hydrocarbon—cumene—with the other hydrocarbons.) These alternation tendencies are measured by the products of the reactivity ratios, the last number in each group of three in Table VI, 1 corresponding to no effect and 0 to inability of one or both peroxy radicals to react with the hydrocarbon from which it is derived. If we take cumene to be 1/40 as reactive as butadiene (instead of 1/30, as in the table), agreement is better for the two styrene-cumene reactivity ratios and poorer for the other cumene ratios. If instead we take cumene to be 1/20 as reactive as butadiene, agreement is much better for the butadiene-cumene and Tetralin-cumene... 

In contrast to the radical-monomer interaction in the transition state proposed by Mayo and Walling (62, 63), the formation of a molecular complex between the electron donor monomer and the electron acceptor monomer—i.e., monomer-monomer interaction—has been proposed as the contributing factor in the free radical alternating copolymerization of styrene and maleic anhydride (8) as well as sulfur dioxide and mono-or diolefins (6, 9, 12, 13, 25, 41, 42, 43, 44, 61, 79, 80, 88). Walling and co-workers (83, 84) did note a relationship between the tendency to form molecular complexes and the alternating tendency and considered the possibility that alternation involved the attack of a radical on a molecular complex. However, it was the presence in the transition state of polar resonance forms resembling those in the colored molecular complexes which led to alternation in copolymerization (84). 

On the basis of their reactivity ratios (Table f 6-5 below), predict what types of microstruc- i tures (e.g., tendency to alternating, tendency to blocky) you would expect in a free radical copolymerization of the following monomer pairs ... 

An examination of reported reactivity ratios (Table 6) shows that the behaviour rj > 1, r2 1 or vice versa is a common feature of anionic copolymerization. Only in copolymerizations involving the monomers 1,1-diphenylethylene and stilbene, which cannot homopolymerize, do we find <1, r2 <1 [212—215], and hence the alternating tendency so characteristic of many free radical initiated copolymerizations. Normally one monomer is much more reactive to either type of active centre in the order acrylonitrile > methylmethacrylate > styrene > butadiene > isoprene. This is the order of electron affinities of the monomers as measured polarographically in polar solvents [216, 217]. In other words, the reactivity correlates well with the overall thermodynamic stability of the product. Variations of reactivity ratio occur with different solvents and counter-ions but the gross order is predictable. 

The alternating tendency in copolymeri/ation was established on a quantitative basis by Frank F<. Mayo (of the Stanford Research Institute) and Cheves Walling (of the University of Utah) while working in the laboratories of the U.S. Rubber Company. Their work was fundamental to the development of free radical chemistry it showed clearly for the first time the dependence of reactivity on the nature of the attacking free radical, and led directly to the concept of polar factors, working not only in copolymerization and other additions of free radicals, but in free radical reactions of all kinds. 

Problem 32.7 (a) Draw structures to account for the strong alternating tendency in copolymerization of butadiene (M ) and acrylonitrile (M2), (b) Toward - Mi acrylonitrile is 2.5 times as reactive as butadiene, but toward —M2 butadiene is 20 times as reactive as acrylonitrile. How do you account for this contrast ... 

Gradient copolymers of St and AN were also prepared [115,116]. Since St and AN have reactivity ratios that are both significantly less than 1, the copolymerization has an alternating tendency along the backbone with an enrichment in AN at high conversions. Alternatively, a gradient along the backbone can be... 

The effect of initial composition of the monomer mixture the copolymerization rate is shown in Figure 13. A maximum rate was observed at the equimolar monomer mixture. This fact suggests that an alternative tendency exists in the copolymerization. 

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