Monomers reactivity ratios

The copolymer equation provides a means of calculating the amount of each monomer incorporated in the chain from a givrai reaction mixture or feed when the reactivity ratios are known. It shows that if monomer Mj is more reactive than Mj, then Mi will raiter the copolymer more rapidly consequraitly, the feed becomes progressively poorCT in M, and composition drift occurs. The equation is that an instantaneous expression, which relates only to the feed composition at any given time. 

As Tj and Tj are obviously the factors that control the composition of the copolymCT, one must obtain reliable values of r for each pair of monomers (comonomers) if the copolymaization is to be completely understood and controlled. This can be achieved by analyzing the composition of the copolymer formed from a 

A plot of / (l - F)/F against (f lF) should then be linear and yield rj from the slope and Tj from the intercept. Several other linear forms have been suggested for the determination of the reactivity ratios, but it is now much easier to estimate Tj and from a nonlinear least-squares fit to the composition data. 

Some representative values of and rj for a number of comonomers are shown in Table 5.1. These are seen to differ widely. 

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GopolymeriZation Initiators. The copolymerization of styrene and dienes in hydrocarbon solution with alkyUithium initiators produces a tapered block copolymer stmcture because of the large differences in monomer reactivity ratios for styrene (r < 0.1) and dienes (r > 10) (1,33,34). In order to obtain random copolymers of styrene and dienes, it is necessary to either add small amounts of a Lewis base such as tetrahydrofuran or an alkaU metal alkoxide (MtOR, where Mt = Na, K, Rb, or Cs). In contrast to Lewis bases which promote formation of undesirable vinyl microstmcture in diene polymerizations (57), the addition of small amounts of an alkaU metal alkoxide such as potassium amyloxide ([ROK]/[Li] = 0.08) is sufficient to promote random copolymerization of styrene and diene without producing significant increases in the amount of vinyl microstmcture (58,59). 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

AGE-Gontaining Elastomers. The manufacturing process for ECH—AGE, ECH—EO—AGE, ECH—PO—AGE, and PO—AGE is similar to that described for the ECH and ECH—EO elastomers. Solution polymerization is carried out in aromatic solvents. Slurry systems have been reported for PO—AGE (39,40). When monomer reactivity ratios are compared, AGE (and PO) are approximately 1.5 times more reactive than ECH. Since ECH is slightly less reactive than PO and AGE and considerably less reactive than EO, background monomer concentration must be controlled in ECH—AGE, ECH—EO—AGE, and ECH—PO—AGE synthesis in order to obtain a uniform product of the desired monomer composition. This is not necessary for the PO—AGE elastomer, as a copolymer of the same composition as the monomer charge is produced. AGE content of all these polymers is fairly low, less than 10%. Methods of molecular weight control, antioxidant addition, and product work-up are similar to those used for the ECH polymers described. 

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). 

From this scheme it can be seen that the copolymer composition is determined by the values of four monomer reactivity ratios. 

The arrangement of monomer units in copolymer chains is determined by the monomer reactivity ratios which can be influenced by the reaction medium and various additives. The average sequence distribution to the triad level can often be measured by NMR (Section 7.3.3.2) and in special cases by other techniques.100 101 Longer sequences are usually difficult to determine experimentally, however, by assuming a model (terminal, penultimate, etc.) they can be predicted.7 102 Where sequence distributions can be accurately determined Lhey provide, in principle, a powerful method for determining monomer reactivity ratios. 

Finally where both reactivity ratios take the value of zero, the monomers do not react at all, with growing polymer chains terminated in their own kind of monomer unit. This results in alternating copolymerisation. A few typical monomer reactivity ratios are given in Table 2.2. 

Table 2.2 Typical monomer reactivity ratios (reaction temperature 60 °C in each case)...

Thus ri represents the ratio of the rate constants for the reaction of a radical of type 1 with monomer Mi and with monomer M2, respectively. The monomer reactivity ratio similarly expresses the relative reactivity of an M2 radical toward an M2 compared with an Ml monomer. The quantity d[M /d M given by Eq. (5) represents the ratio of the two monomers in the increment of polymer formed when the ratio of unreacted monomers is The former ratio... 

The composition of the increment of polymer formed at a monomer composition specified by /i(= 1 —/2) is readily calculated from Eq. (8) if the monomer reactivity ratios ri and V2 are known. Again it is apparent that the mole fraction Fi in general will not equal /i hence both /i and Fi will change as the polymerization progresses. The polymer obtained over a finite range of conversion will consist of the summation of increments of polymer differing progressively in their mole fractions F. ... 

The reader is referred to Refs. 1, 2, and 3 for comprehensive tabulations of monomer reactivity ratios. 

The effect of temperature on the monomer reactivity ratio is fairly small. In those few cases examined with sufficient accuracy,the ratio nearly always changes toward unity as the temperature increases —a clear indication that a difference in activation energy is responsible, in part at least, for the difference in rate of the competing reactions. In fact, the difference in energy of activation seems to be the dominant factor in these reactions differences in entropy of activation usually are small, which suggests that steric effects ordinarily are of minor importance only. 

Table XXII.—Monomer Reactivity Ratio Products (50 to 80°) (From Mayo and Walling )... 

Quaternary ammonium salts of 1-acryloy 1-4-methyl piperazine can be prepared by methylation with methyl chloride and dimethyl sulfate. These monomers can be polymerized by means of radical polymerization, either alone or with a comonomer [617]. A useful comonomer with appropriate monomer reactivity ratios is acrylamide. 

MB Hughn, MM Rehab. Some observations on monomer reactivity ratios in aqueous and nonaqueous media. Polymer 28 2200-2206, 1987. 

Monomer reactivity ratios and thus comonomer sequence distributions in copolymers can vary with copolymerization reaction conditions. The comonomer distribution could affect the geometry of the adsorbed polymer - mineral complex and the fines stabilization properties. 

We also investigated the copolymerizations of 1-hexene with 4-methyl-l-hexene and of 4-methyl-l-hexene with 5-methyl-l-hexene by the aforementioned techniques (33). The monomer reactivity ratios for these two pairs are shown in Table VII. 

Monomer Reactivity Ratios at 30 C. Catalyst System l AlCl/ -TiClj AA (Al/Ti = 2)... 

Table 5. Monomer reactivity ratios of alkyl acrylates and MMA...

The composition curve In the co-graft polymerization of CTFE and butadiene is shown in Figure 7. From the result, the monomer reactivity ratios were obtained as rQ, j. =0.10 0.06 and rButadiene=16 3 from the Finemann-Ross plot. Since the product of CTFE rButadiene is nearly equal to one, a random co-grafting takes place in the reaction system. The high reactivity of butadiene in the co-grafting results in the remarkable acceleration of the gel formation by the addition of a small amount of butadiene to CTFE (Figure 6). 

Compositionally uniform copolymers of tributyltin methacrylate (TBTM) and methyl methacrylate (MMA) are produced in a free running batch process by virtue of the monomer reactivity ratios for this combination of monomers (r (TBTM) = 0.96, r (MMA) = 1.0 at 80°C). Compositional ly homogeneous terpolymers were synthesised by keeping constant the instantaneous ratio of the three monomers in the reactor through the addition of the more reactive monomer (or monomers) at an appropriate rate. This procedure has been used by Guyot et al 6 in the preparation of butadiene-acrylonitrile emulsion copolymers and by Johnson et al (7) in the solution copolymerisation of styrene with methyl acrylate. 

Equation 6-12 is known as the copolymerization equation or the copolymer composition equation. The copolymer composition, d M /d Mi, is the molar ratio of the two monomer units in the copolymer. rate constant for a reactive propagating species adding tis own type of monomer to the rate constant for its additon of the other monomer. The tendency of two monomers to copolymerize is noted by r values between zero and unity. An r value greater than unity means that Mf preferentially adds M2 instead of M2, while an r value less than unity means that Mf preferentially adds M2. An r value of zero would mean that M2 is incapable of undergoing homopolymerization. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

Different types of copolymerization behavior are observed depending on the values of the monomer reactivity ratios. Copolymerizations can be classified into three types based on whether the product of the two monomer reactivity ratios r rx is unity, less than unity, or greater than unity. 

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