Monomer reactivity ratios, free radical copolymerization

After the demonstrations of preparation of stereoregular polymers having novel properties by means of special ionic methods, die possibilities of free radical methods were examined extensively. It must be concluded that in free radical systems the structures of homopolymers and copolymers can be little influenced by specific catalysts and other reaction conditions, but are determined largely by monomer structure. This is consistent with the relative uniformity of comonomer reactivity ratios in radical copolymerizations. However, it has been found possible to obtain somewhat more syndiotactic structure, dldl. than normally obtained by radical reactions, at low temperatures and by selecting solvents. Examples are polyvinyl chlorides of higher than usual crystallinity from polymerizations at low temperature e.g.. —50°C under ultraviolet light... 

Copolymerization. Acrylic and methacrylic acids readily copolymerize free radically with many vinyl monomers. This versatility results from a combination of their highly reactive double bonds and their miscibility with a wide variety of water- and solvent-soluble monomers. Reactivity ratios derived from copolymerizations with many monomers are tabulated in many books on polymerization, for example in Wiley s Polymer Handbook (14) (see also Wiley s Database of Polymer Properties). Q and e values are parameters that may be established for a monomer based on a large number of reactivity ratios with other monomers. These parameters are associated with interactions between the monomer and the growing chain via resonance (Q) and polar effects (e). 

Methylthiophene/styrene copolymers Methyl methacrylate does not homopolymerize or copolymerize if present in the monomer feed during the oxidation of 3-methylthiophene. This is the reason that its copolymer with 3-MT is prepared indirectly as described above. Its homopolymerization is generally initiated by anions or free radicals. Styrene, however, undergoes a random copolymerization when present during the chemical oxidation of 3-methylthiophene initiated with anhydrous FeCls [73]. Monomer reactivity ratios for the copolymerizations in methylene chloride and nitrobenzene at 5°C are reported, but there is considerable scatter in the Fineman-Ross plots. The proposed structure of the 3-MT/stryrene copolymer is shown in Figure 11.16, where R = H. 

Copolymerization Equation-Monomer Reactivity Ratios. During the copolymerization of two comonomers (A and B), the chain can grow by the occurence of the four following reactions that differ from one another by the nature of the free radical and the inserted monomer ... 

Free-radical copolymerization of vinyl acetate with various vinyl siloxane monomers was described 345). Reactions were conducted in benzene at 60 °C using AIBN as the initiator. Reactivity ratios were determined. Selective hydrolysis of the vinyl acetate units in the copolymer backbone was achieved using an aqueous sodium hy-droxide/THF mixture. The siloxane content and degree of hydrolysis were determined by H-NMR. 

Ra, Ra symbol of a-type radical or ion and its concentration kap constant of propagation reaction between Ra and M klap constant of termination reaction between Ra and R rap> rfia reactivity ratios for binary free-radical copolymerization of monomers Ma and M ... 

The polymerization of a mixture of more than one monomer leads to copolymers if two monomers are involved and to terpolymers in the case of three monomers. At low conversions, the composition of the polymer that forms from just two monomers depends on the reactivity of the free radical formed from one monomer toward the other monomer or the free radical chain of the second monomer as well as toward its own monomer and its free radical chain. As the process continues, the monomer composition changes continually and the nature of the monomer distribution in the polymer chains changes. It is beyond the scope of this laboratory manual to discuss the complexity of reactivity ratios in copolymerization. It should be pointed out that the formation of terpolymers is even more complex from the theoretical standpoint. This does not mean that such terpolymers cannot be prepared and applied to practical situations. In fact, Experiment 5 is an example of the preparation of a terpolymer latex that has been suggested for use as an exterior protective coating. 

The free radical polymerization of DADMAC (M,) with vinyl acetate (M2) in methanol proceeds as a nonideal and nonazeotropic copolymerization with monomer reactivity ratios rx=1.95 and r2=0.35 were obtained [75]. The resulting low molar mass copolymers were reported to be water soluble over their whole range of composition. Modification of the vinyl acetate unit by hydrolysis, ace-talization, and acylation resulted in DADMAC products with changed hydrophilic or polyelectrolyte properties [75]. For the copolymerization of DADMAC and AT-methyl-AT-vinylacetamide (NMVA) a nearly ideal copolymerization behavior could be identified [45]. The application properties of the various copolymer products will be discussed in Sect. 8. 

The reactivity ratios, rA and rB, express the relative preference of a growing chain for one of the two monomers. In free-radical copolymerizations they are roughly independent of solvent, initiator, and temperature. In ionic copolymerization, however, they depend strongly on the counterion and the solvent. 

An alternative rationale for the unusual RLi (hydrocarbon) copolymerization of butadiene and styrene has been presented by O Driscoll and Kuntz (71). Rather than invoking selective solvation, these workers stated that classical copolymerization kinetics is sufficient to explain this copolymerization. They adapted the copolymer-composition equation, originally derived from steady-state assumptions for free-radical copolymerizations, to the anionic copolymerization of butadiene and styrene. Equation (20) describes the relationship between the instantaneous copolymer composition c/[M,]/rf[M2] with the concentrations of the two monomers in the feed, M, and M2, and the reactivity ratios, rt, r2, of the monomers. The rx and r2 values are measures of the preference of the growing chain ends for like or unlike monomers. 

From the values of the monomer reactivity ratios, the relative reactivity of the monomers toward the growing free radicals derived from MAOThe, MAOA and MAOU (t, a and u, respectively) was estimated (Table 6). As for the growing radical of MAOThe (t), for example, the reactivities of MAOThe and MAOU monomer are equal but higher than that of MAOA monomer in ethanol solution while the reactivities of these monomers are nearly equal in dioxane solution. The copolymerization proceeds predominantly under the influence of base-base pairing between adenine and uracil rings. 

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 ... 

A number of years ago we asked one of our students (Dr. Yun Xu) to free radically copolymerize styrene with a (protected) vinyl phenol monomer. We needed to determine the reactivity ratios and Dr. Xu obtained the following data for monomer M, (which happened to be the vinyl phenol monomer). 

If both reactivity ratios are greater than one, there will be a tendency for formation of sequences of uniform composition in the copolymers. Such reactivity ratio combinations are not known in free-radical copolymerizations when both monomers are present simultaneously in the reaction vessel, but they can be made in other systems. 

Most kinetic studies on copolymerizations using coordination catalysts have been restricted to the determination of monomer reactivity ratios. There are problems both experimentally and in interpretation since the major simplification assumed to hold for most free radical initiated systems, namely that monomer incorporation is determined only by the monomer concentrations and the four rate coefficients, cannot be taken for granted. Further, catalyst activity and selectivity are influenced by the conditions of catalyst preparation including the manner and order of... 

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. 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

A combination of variables controls the outcome of the copolymerization of two or more unsaturated monomers by CCT free-radical polymerization.382 Of course, all of the features that control the outcome of a normal free-radical polymerization come into effect.40 426 429 These include the molar ratio of monomers, their relative reactivity ratios and their normal chain-transfer constants, the polymerization temperature, and the conversion. In the presence of a CCT catalyst, the important variables also include their relative CCT chain-transfer constants and the concentration of the Co chain-transfer agent. The combination of all of these features controls the molecular weight of the polymer and the nature of the vinyl end group. In addition, they can also control the degree of branching of the product. 

A value of unity (or nearly unity) for the monomer reactivity ratio signifies that the rate of reaction of the growing chain radicals towards each of the monomers is the same, i.e. kn ki2 and 22 — A 2i and the copolymerization is entirely random. In other words, both propagating species and M2 have little or no preference for adding either monomer. The copolymer composition is the same as the comonomer feed with a completely random placement of the two monomers along the copolymer chain. Such behavior is referred to as Bemoullian. Free-radical copolymerization of ethylene and vinyl acetate and that of isoprene and butadiene are examples of such a system, but this is not a common case. Random monomer distributions are obtained more generally in a situation where both types of radicals have exactly the same preference for the same type of monomer as represented by the relationship... 

Some commercially important examples of random free-radical copolymerizations include styrene (ri = 0.8)-butadiene (r2 = 1.4) for which rir2 = 1.1 and vinyl chloride (ri = 1.4)-vinyl acetate (r2 = 0.65) for which rir2 = 0.9. In these products the proportion of a given monomer in the copolymer depends on the feed concentrations and reactivity ratios [Eq. (7.11)]. 

Free-Radical Copolymerization. Although the mechanism of copolymerization is similar to that described for homopolymerizations, the reactivities of monomers may differ when more than one monomer is present in the feed and lead to polymer chains with varying amounts of the reactant monomers. The difference in the reactivity of monomers can be expressed with reactivity ratios, r. 

The reactivity ratios for the free-radical copolymerization of styrene (rj = 0.4) and acrylonitrile (r2 = 0.04) result in uneven incorporation of each monomer into the copolymer as seen in Figure 3. Thus, most SAN and ABS polymers are made at the crossover point (A in Figure 3) to avoid composition drift. 

In copolymerization chemistry it is often reported that the composition of the copolymer formed differs from the initial input composition because the monomers differ in reactivity toward free radical addition. Thus with less than 100% incorporation of monomers into the polymer, there is a possibility that the concentration ratios between the Immobilines built into the gel will differ from the ratios in the starting solution. This could have serious effects on the pH gradient generated, for instance, change its... 

The free-radical copolymerization of acrylamide with three common cationic comonomers diallyldimethylammonium chloride, dimethyl-aminoethyl methacrylate, and dimethylaminoethyl acrylate, has been investigated. Polymerizations were carried out in solution and inverse microsuspension with azocyanovaleric acid, potassium persulfate, and azobisisobutyronitrile over the temperature range 45 to 60 C. The copolymer reactivity ratios were determined with the error-in-variables method by using residual monomer concentrations measured by high-performance liquid chromatography. This combination of estimation procedure and analytical technique has been found to be superior to any methods previously used for the estimation of reactivity ratios for cationic acrylamide copolymers. A preliminary kinetic investigation of inverse microsuspension copolymerization at high monomer concentrations is also discussed. 

If the nitroxide does leave the vicinity of the propagating chain end then the reactivity ratios for the radicals should also be the same as in conventional radical polymerizations. However, if the capping and uncapping rates for the two monomers are different this would lead to different concentrations of the two types of propagating chain ends relative to what would be present in a conventional radical polymerization. To address this issue, stable free radical copolymerizations of styrene-isoprene and styrene-acrylonitrile were studied in detail to compare the low conversion copolymer compositions to those prepared by conventional radical polymerization. The microstructure of the polymers was also examined. 

Random copolymers of styrene/isoprene and styrene/acrylonitrile have been prepared by stable free radical polymerization. By varying the comonomer mole fractions over the range 0.1-0.9 in low conversion SFRP reactions it has been demonstrated that the incorporation of the two monomers in the copolymer is analogous to that found in conventional free radical copolymerizations. The composition and microstructure of random copolymers prepared by SFRP are not significantly different from those of copolymers synthesized conventionally. These two observations support the conclusion that the presence of nitroxide in the SFR process does not influence the monomer reactivity ratios or the stereoselectivity of the propagating radical chain. Rather, the SFR propagation mechanism is essentially the same as that of the conventional free radical copolymerization process. 

Because of the ease of synthesis and industrial importance of diallyl esters much of the research has dealt with the behavior of the isomeric phthalates. Some other dicarboxylic acid esters have been studied by Simpson and Holt [41]. The kinetics of the poljmierization of the diallyl esters of oxalic, malonic, succinic, adipic, and sebacic acid have also been considered. In previous kinetie studies, no differentiation was made between the behavior of the uncyclized monomer (or its free radical) and of the cyclic free-radicals. A priori, differences should have been presumed, but evidently Matsumoto and Oiwa [46] were the first seriously to attempt a kinetic analysis based on the concept that the linear and the cyclic units are two different species. In effect, these two species copolymerize with each other. However, the analysis has not been carried so far as to determine reactivity ratios. 

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