Monomer reactivity ratios, free radical

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. 

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... 

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. 

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. 

The ratio kp/A( will appear frequently in the equations we develop for radical polymerization. The polymerizability of a monomer in a free radical reaction is related to k /rather than to kp alone. From Eq. (6-30) or (6-29), it can be seen that a given amount of initiator will produce more polymer from a monomer with a higher kp/k J ratio. Thus, at 60°C, the kp values for acrylonitrile and styrene are approximately 2000 and 100 liter/mol sec, respectively. The former monomer does not polymerize 20 times as fast as styrene under the same conditions at 60, however, because the respective k, values are780x 10 liter/mol sec. and70x 10 liter/mol sec. ThenArp/A , for acrylonitrile is 0.07 liter / /mol / sec /, which is just six times that of styrene. Styrene forms the less reactive radical in this case... 

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... 

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... 

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... 

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. 

Obviously, because of the difference in the reactivity of styrene and DVB, the networks prepared by free radical copolymerization do not relate to such an ideal system with uniform distribution of DVB units and constant chain lengths between the junction points. Also, it was not possible to eliminate this serious defect by an anionic copolymerization of the comonomers. The anionic copolymerization has often been initiated by n- or sec-hutyl lithium [110-112]. Under such conditions, styrene is consumed faster than p-DVB, the monomer reactivity ratios being ri = 1.5S and r2 = 0.32. Therefore, DVB-enriched domains wUl form toward the end of the anionic process. On the other hand, the styrene—m-isomer reactivity ratio (r = 0.65 and r2 = 1.20) points to the local incorporation of m-DVB crosslinks into the initially formed copolymer [113, 114]. In addition, the anionic process is also accompanied by intramolecular cycUzation, similar to radical styrene DVB copolymerization [115,116]. 

For example, monomer reactivity ratios for styime and methyl methacrylate in a free-radical copolymerization are r, = 0.5, rj = 0.44. This represents a statistical copolymerization. Contrast this with the anionic reaction, where r = 0.12 and 2 = 6.4, or the cationic reaction where r = 10.5 and Z2 = 0.1. Obviously, the propagation rates are no longer similar, and this is represented in Figure 5.3, where it can be seen that the anionic technique produces a copolymer rich in methyl methacrylate, whereas the cationic system leads to a copolymer with a high styrene content. 

TABLE 2.11 Monomer Reactivity Ratios for Free Radical Copolymerization with Styrene (M ) ... 

Copolymers are made to produce unique or functional properties in the polymeric product. The properties of step copolymers can be understood and, in some cases, predicted from an analysis of the chain length and functional groups in the monomers. The composition and composition-dependent properties of a free radical, chain reaction copolymer can be predicted from monomer reactivity ratios, a property first correctly quantified in 1944 (11-14). These ratios have been extensively measured and tabulated (15). They allow, by use of differential equations, the calculation of the monomer content in a copolymer as a function of time during the reaction. Reactivity ratios have also been measured for cationic chain reactions (16). Anionic chain reactions in monomer mixtures are generally so fast and indiscriminate that reactivity ratios are meaningless. 

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). 

The sequence distribution of two copolymerizing monomers depends on the catalyst or initiator used, the method of pol5merization, and the concentration and reactivities of the monomers. Reactivity ratios for many monomer pairs have been measured for free-radical, anionic, and coordination polymerization of butadiene (128). 

Copolymerization. In free-radical copolymerization (qv), the composition of the copolymer is controlled by the comonomer reactivity ratios (23). The monomer reactivity ratio is defined as the quotient of the rate constants for chain homopropagation to the rate constant for chain cross-propagation. 

Temperature has a greater influence on the ri and V2 values of ionic copolymerizations than in free-radical copolymerization because of the greater spread of activation energies for the propagation reactions involving ions. There is no general trend observable and the monomer reactivity ratios may increase or decrease with temperature in the isobutylene-styrene copolymerization ri increases by a factor of 1.5 and V2 increases by a factor of 3 when going from —94°C to —30°C... 

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. 

In free-radical copolymerization of two monomers the relationship between the composition of the copolymer and the initial monomer mixture is ruled by the monomer reactivity ratios ri and r2- These ratios are related to an individual system of given comonomers, initiator, and temperature [103]. They are summarized in Ref. [104] for numerous systems. 

Online H NMR measurements were r rted by Abdollahi et al. [171] in a recent study of the free-radical copolymerization of vinyl acetate (VA) and Me acrylate (MA) in ben-zene-d at 60°C, with benzoyl peroxide (BPO) as initiator. A significant composition drift in the comonomer mixture was observed as the reaction progressed. The comonomer reactivity ratios were calculated using the data collected only from one sample via online monitoring of the comonomer mixture and copolymer compositions at different reaction time intervals up to medium overall monomer conversions. The results were in good agreement with the fiteratme data reported for this system, indicating the accuracy of the monomer reactivity ratios calculated by the procedme developed in the study. 

For conventional free-radical copolymerizations, polar effects of growing polymer radicals on the approaching monomer is expressed by the Alfrey-Price Q — e scheme, where the copolymerization tendency, i.e., product of monomer reactivity ratios, may be expressed, Eq. (20), in terms of e values. 

The CTC and neutral-monomer reactivity ratios may change both by dilution and with the nature of the solvent. For example, in the p-dioxene-MA-acrylonitrile system in benzene or toluene, the acrylonitrile content in the polymer increased to a maximum and then decreased as the amount of solvent increased.The observed solvent effects support the concept that when a CTC becomes solvated with a 7r-electron-rich solvent the CTC becomes stable to free-radical attack. 

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 ... 

However, the method of initiation has a pronounced effect on the rate constants. There are large differences in reactivity ratio s between the same monomers polymerised using free radical initiation and those where the polymerisation is conducted using ionic initiation. 

Nitrogen is not required as the vapour pressure of vinyl acetate monomer ensures that an oxygen free atmosphere exists above the reactants. The reactivity ratio s for vinyl acetate and methyl methacrylate indicate that MMA is 20-40 times more reactive towards free radicals than is VA. 

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