Monomer reactivity ratio alternation tendency

The rates of addition to the unsubstituted terminus of monosubstituted and 1,1-disubstiluted olefins (this includes most polymerizable monomers) are thought to be determined largely by polar Factors.2 16 Polymer chemists were amongst the first to realize that polar factors were an important influence in determining the rate of addition. Such factors can account for the well-known tendency for monomer alternation in many radical copolymerizations and provide the basis for the Q-e, the Patterns of Reactivity, and many other schemes for estimating monomer reactivity ratios (Section 7.3.4). 

The parameters rA and rs are known as monomer reactivity ratios representing the ratio of rate constants for a radical to add to its own type polymer vs. rate constants for a radical to add to the other type polymer. When kAA = 0 and ksB = 0, it can be seen that rA = 0, re = 0, and each radical reacts exclusively with the other monomer. Rel. (2.3.20) is then reduced to d[P ]/d[P ] = 1, and the monomers alternate regularly along the chain of the copolymer, regardless of the composition of the monomer feed (an excess of one monomer may remain unreacted). This is an ideal case, but copolymers such as that made from (a) styrene and (b) diethyl fumarate (rA = 0.3, re = 0.07) can be close to the ideal case. The styrene/diethyl fumarate polymerization has the tendency to lead to an azeotropic copolymer with 57 mole percent styrene, regardless the feed composition. When the initial composition of the monomers is different from 57 mole percent, the alt-copolymer is formed until one of the materials is finished and the remaining monomer forms a homopolymer. 

Based on these reactivity ratios, the tendency toward alternation is much greater with vinyl acetate as the curing monomer. Thus the polymer resulting from cure of the polyester with vinyl acetate will... 

Distribution of the monomer units in the polymer is dictated by the reactivity ratios of the two monomers. In emulsion polymerization, which is the only commercially significant process, reactivity ratios have been reported (4). IfMj = butadiene andM2 = acrylonitrile, then = 0.28, and r2 =0.02 at 5°C. At 50°C, Tj = 0.42 and = 0.04. As would be expected for a combination where = near zero, this monomer pair has a strong tendency toward alternation. The degree of alternation of the two monomers increases as the composition of the polymer approaches the 50/50 molar ratio that alternation dictates (5,6). Another complicating factor in defining chemical stmcture is the fact that butadiene can enter the polymer chains in the cis (1), trans (2), or vinyl(l,2) (3) configuration ... 

In this copolymerization, the reactivity ratios are such that there is a tendency for S and the acrylic monomers to alternate in the chain. This, in combination with the above-mentioned specificity in the initiation and termination steps, causes chains with an odd number of units to dominate over those with an even number of units. 

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 small reactivity ratio for AN indicates that a growing AN radical is reluctant to react with an AN monomer, but rather will react with a styrene monomer. On the other hand, even when a growing styrene radical reacts rather with an AN monomer, the tendency is not as marked. In the limiting case, if both monomer reactivity rations are going to zero, this effects the formation of strictly alternating polymers. The composition of the polymer can be controlled by the ratio of monomers in the monomer feed. In particular, since one of the monomers will be consumed faster that the other in a discontinuous process, the monomer feed can be adjusted accordingly in the course of polymerization. Also in a continuous process, in a cascade of reaction vessels, monomer can be fed into certain stages. 

When the product of monomer relative reactivity ratios is approximately one r x r2 = 1), the last inserted monomeric unit in the chain does not influence the next monomer incorporation and Bernoullian statistics govern the formation of a random copolymer. When this product tends to zero (r xr2 = 0), there is some influence from the last inserted monomeric unit (when first-order Markovian statistics operate), or from the penultimate inserted monomeric units (when second-order Markovian statistics operate), and an alternating copolymer formation is favoured in this case. Finally, when the product of the reactivity ratios is greater than one (r x r2 > 1), there is a tendency for the comonomers to form long segments and block copolymer formation predominates (or even homopolymer formation can take place) [448],... 

We have already seen that, depending on the values of the reactivity ratios, there is a tendency to get random, alternating, blocky, etc., types of copolymers. Probability theory allows us to quantify this in terms of the frequency of occurrence of various sequences, like the triads AAA or ABA in a copolymerization of A and B monomers. The value of this information is that such sequence distributions can be measured directly by NMR spectroscopy, thus allowing a direct probe of copolymer structure and an alternative method for measuring reactivity ratios. As mentioned above, there are problems, as some spectra can be too complex and rich for easy analysis, as we will see in Chapter 7. 

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 ... 

Both the mini- and macroemulsion copolymerizations of pMS/MMA tend to follow bulk polymerization kinetics, as described by the integrated copolymer equation. MMA is only slightly more soluble in the aqueous phase, and the reactivity ratios would tend to produce an alternating copolymer. The miniemulsion polymerization showed a slight tendency to form copolymer that is richer in the more water-insoluble monomer. The macroemulsion formed a copolymer that is slightly richer in the methyl methacrylate than the co-... 

This reasoning predicts that a reactivity ratio or an r V2 product greater than unity will decrease with increasing temperature and vice versa. The tendency for random polymerization will increase and the tendency for monomer alternation will decrease with increasing reaction temperature, so long as the same copolymerization mechanism predominates over the experimental temperature range. 

Rank the following monomers in order of their increased tendency to alternate in copolymerization with butadiene and explain your reasoning vinyl acetate, styrene, acrylonitrile, and methyl methacrylate, Hint Use Q-e values if reactivity ratios are not readily available.)... 

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. 

One potential problem with conventional free-radical copolymerization is that the reactivity ratios of the two monomers tend to be different from one another [6]. On one hand this leads to non-random sequences of the monomers on a single chain (usually the product of the reactivity ratios is less than one so that there is a tendency to form alternating sequences) and, on the other, to substantial composition drift if the polymerization is carried out in bulk to high conversions. Random copolymers with a range of compositions as a result of composition drift may however be useful in practice, allowing a compositionally graded interface to be formed. 

The tendency for these monomers to produce an alternating copolymer is also supported by their reactivity ratios—0 for maleic anhydride and 0.02 for styrene. It has also been suggested that a strong donor monomer such as maleic anhydride and a strong acceptor monomer... 

The kinetics of copolymerization and the microstructure of copolymers can be markedly influenced by the addition of Lewis acids. In particular, Lewis acids are effective in enhancing the tendency towards alternation in copolymerization of donor-acceptor monomer pairs and can give dramatic enhancements in the rate of copolymerization and much higher molecular weights than are observed for similar conditions without the Lewis acid. Copolymerizations where the electron deficient monomer is an acrylic monomer e.g. AN, MA, MMA) and the electron rich monomer is S or a diene have been the most widely studied." Strictly alternating copolymers of MMA and S can be prepared in the presence of, for example, dictliylaluminum scsquichloridc. In the absence of Lewis acids, there is only a small tendency for alternation in MAA-S copolymerization terminal model reactivity ratios are ca 0.51 and 0.49 - Section 7.3.1.2.3. Lewis acids used include EtAlCT, Et.AlCL ElALCL, ZnCT, TiCU, BCl- LiC104 and SnCL. 

Some copolymerization systems are not strictly alternating, but still they show a tendency toward alternation. This occurs when both and r2 < 1. The alternating trend increases as the reactivity ratios approach zero. An interesting feature of these systems is that they present the so-called azeotropic composition, at which Fj = /j. At this composition, the copolymer formed has the same composition as the monomers in the feed and, therefore, systems copolymerizing at this condition do not show compositional drift. It can be shown that a necessary condition that the reactivity ratios have to satisfy in order for a copolymerization system to show an azeotropic point is that either both and r2 < 1 or both and T2 > 1. 

Table 8.5 Product of Reactivity Ratios of Monomers Showing Order of Alternating Tendency... 

Reactivity ratios express the relative tendency for monomer blocking versus monomer alternation along the polymer chain. A value of R greater than 1 indicates a tendency for monomer 1 to incorporate in blocks, while a value of R less than 1 indicates a tendency for monomer 1 to alternate with monomer 2 along the polymer chain. The kinetic behavior of monomer pairs can be classified into the categories described in Table 12.3. Typical reactivity ratios for monomers commonly used in acrylic fiber production are listed in Table 12.4 [71,72]. 

An alternating copolymer is obtained if both comonomers do not homopolymerize, and therefore rir2 = 0. If one of the two reactivity ratios equals 0, but the other one does not, the system usually has a strong tendency to alternate, but nonal-temating sequences (i.e., longer block lengths of the monomer that is able to homopolymerize) are also possible. Hiis class of copolymers will be discussed in more detail below. 

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