In radical copolymerization

The values of the reactivity ratios r = kn/ku and r2 = k22/k2 involved in the first of formulas (Eq. 79) are available in the literature for hundreds of monomeric pairs which are employed in radical copolymerization [78]. 

Toropov AA, Kudyshkin VO, VoropaevaNL, Ruban IN, Rashidova S.Sh. (2004c) QSPR modeling of the reactivity parameters of monomers in radical copolymerizations J. Struct. Chem. 45 ... 

Monomer reactivity ratios are generally but not always independent of the reaction medium in radical copolymerization. There is a real problem here in that the accuracy of r values is often insufficient to allow one to reasonably conclude whether r or rx varies with changes in reaction media. The more recent determinations of r values by high-resolution NMR are much more reliable than previous data for this purpose. It has been observed that the... 

The monomer reactivity ratios for many of the most common monomers in radical copolymerization are shown in Table 6-2. These data are useful for a study of the relation between structure and reactivity in radical addition reactions. The reactivity of a monomer toward a radical depends on the reactivities of both the monomer and the radical. The relative reactivities of monomers and their corresponding radicals can be obtained from an analysis of the monomer reactivity ratios [Walling, 1957]. The reactivity of a monomer can be seen by considering the inverse of the monomer reactivity ratio (1 jf). The inverse of the monomer reactivity ratio gives the ratio of the rate of reaction of a radical with another monomer to its rate of reaction with its own monomer... 

TABLE 6-2 Monomer Reactivity Ratios in Radical Copolymerization ... 

Another characteristic feature of ionic copolymerizations is the sensitivity of the monomer reactivity ratios to changes in the initiator, reaction medium, or temperature. This is quite different from the general behavior observed in radical copolymerization. Monomer reactivity ratios in radical copolymerization are far less dependent on reaction conditions. 

The effect of a substituent on the reactivity of a monomer in cationic copolymerization depends on the extent to which it increases the electron density on the double bond and on its ability to resonance stabilize the carbocation that is formed. However, the order of monomer reactivities in cationic copolymerization (as in anionic copolymerization) is not nearly as well defined as in radical copolymerization. Reactivity is often influenced to a larger degree by the reaction conditions (solvent, counterion, temperature) than by the structure of the monomer. There are relatively few reports in the literature in which monomer reactivity has been studied for a wide range of different monomers under conditions of the same solvent, counterion, and reaction temperature. 

Steric effects similar to those in radical copolymerization are also operative in cationic copolymerizations. Table 6-9 shows the effect of methyl substituents in the a- and 11-positions of styrene. Reactivity is increased by the a-methyl substituent because of its electron-donating power. The decreased reactivity of P-methylstyrene relative to styrene indicates that the steric effect of the P-substituent outweighs its polar effect of increasing the electron density on the double bond. Furthermore, the tranx-fl-methylstyrene appears to be more reactive than the cis isomer, although the difference is much less than in radical copolymerization (Sec. 6-3b-2). It is worth noting that 1,2-disubstituted alkenes have finite r values in cationic copolymerization compared to the values of zero in radical copolymerization (Table 6-2). There is a tendency for 1,2-disubstituted alkenes to self-propagate in cationic copolymerization, although this tendency is low in the radical reaction. 

Discuss the general effects of temperature, solvent, and catalyst on the monomer reactivity ratios in ionic copolymerizations. How do these compare with the corresponding effects in radical copolymerizations ... 

It is important to note that the tendency of a monomer towards polymerization and therefore also towards copolymerization is strongly dependent on the nature of the growing chain end. In radical copolymerization the composition of the copolymer obtained from its given monomer feed is independent of the initiating system for a particular monomer pair, but for anionic or cationic initiation this is normally not the case. One sometimes observes quite different compositions of copolymer depending on the nature of the initiator and especially on the type of counterion. A dependence of the behavior of the copolymerization on the used catalyst is often observed with Ziegler-Natta or metallocene catalysts. 

The rates and degrees of polymerizations in radical copolymerizations conform essentially to the same laws as for radical homopolymerization (see Sect. 3.1). Raising the initiator concentration causes an increase in the rate of polymerization and at the same time a decrease in the molecular weight a temperature rise has the same effect. However, these assertions are valid only for a given... 

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... 

In an apparently homogeneous solution, macromonomers, possibly together with the resulting graft copolymers, may lead to some structure formation such as micelle or looser association, which may in turn change the apparent reactivities due to some specific solvation or partition of the monomers around the active sites. Such a bootstrap effect [52] maybe responsible for some complicated dependency of the apparent reactivities on the monomer concentration and composition in radical copolymerization of 29 with n-butyl acrylate [53]. 

Usually or most widely applied, polymer latexes are made by emulsion polymerization [ 1 ]. Without any doubt, emulsion polymerization has created a wide field of applications, but in the present context one has to be aware that an inconceivable restricted set of polymer reactions can be performed in this way. Emulsion polymerization is good for the radical homopolymerization of a set of barely water-soluble monomers. Already heavily restricted in radical copolymerization, other polymer reactions cannot be performed. The reason for this is the polymerization mechanism where the polymer particles are the product of kinetically controlled growth and are built from the center to the surface, where all the monomer has to be transported by diffusion through the water phase. Because of the dictates of kinetics, even for radical copolymerization, serious disadvantages such as lack of homogeneity and restrictions in the accessible composition range have to be accepted. 

Plotting jc(l - n)/n versus x ln will give a straight line with a slope of -ri and an intercept of r2- The monomer reactivity ratios for some common monomers in radical copolymerization are listed in Table 14.25. 

We expect the reactions complementary to equations (1) and (2), namely electrophilic attacks, to be faster for alkenes than for alkynes. Thus, reactivity ratios (/-ii and rj2) for corresponding alkynes and alkenes (PhC CH, PhCH=CH and BuC CH, BuCH=CH2) in radical copolymerizations favour the alkene over the alkyne . Electrophilic additions of Br, CI2, ArSCl and H3O+ to alkenes are usually much faster than those to alkynes . However, A (C=C)/A (C=C) can vary from 10 to < 1 for the different electrophilic processes and by 10 for one process (Br2 addition) when the solvent is changed from HjO to HOAc . This unexpected trend in reactivity continues undiminished in the rates of acid-catalysed hydration... 

Equation (26) is the ideal copolymer composition equation suggested [203] early in the development of copolymerization theory but which had to be abandoned in favour of eqn. (23) as a general description of radical copolymerization. Only in this particular case are the rates of incorporation of each monomer proportional to their homopolymerization rates. It was shown that the reactivity of a series of monomers in stannic chloride initiated copolymerization followed the same order as their homopolymerization rates [202] and so eqn. (26) could be at least qualitatively correct for carbonium-ion polymerizations and possibly for reactions carried by carbanions. This, in fact, does not seem to be correct for anionic polymerizations since the reactivities of the ion-paired species at least, differ greatly. The methylmethacrylate ion-pair will, for instance, not add to styrene monomer, whereas the polystyryl ion-pair adds rapidly to methylmethacrylate [204]. This is a general phenomenon no reaction will occur if the ion-pair is on a monomer unit which has an appreciably higher electron affinity than that of the reacting monomer. The additions are thus extremely selective, more so than in radical copolymerization. There is no evidence that eqn. (26) holds and the approximate agreement with eqn. (25) results from other causes indicated below. 

A major limitation of a-methylstyrene in free-radical polymerizations is its very low ceiling temperature of 61 °C.347 As a result, AMS is utilized commercially only in radical copolymerization. Nonetheless, it is among the most active CCT monomers with Cc = 9 x 105 at 50 °C for 9a as CCT catalyst.348 This value is relatively unchanged at 40 °C. This high value reflects the low kp = 1.7 M 1 s 1 so that kc = 5 x 105 M-1 s 1. 

The simple copolymer equation [Eq. (7.11)] has been experimentally verified in innumerable comonomer systems. The equation is equally applicable to radical, cationic, and anionic chain copolymerizations, although the and T2 values for any particular monomer pair can be drastically different in the three types of chain copolymerization. For example, for the monomer pair of styrene (Mx) and methyl methacrylate (M2) the ri and T2 values are 0.52 and 0.46 in radical copolymerization, 10 and 0.1 in cationic polymerization, and 0.1 and 6 in anionic copolyraerization. Methyl methacrylate as expected has higher reactivity in anionic copolymerization and lower reactivity in cationic copolymerization, while the opposite is the case for styrene. Thus the copolymer obtained from an equimolar styrene-methyl methacrylate feed is approximately a 1 1 copolymer in the radical case but is essentially a homopolymer of styrene in cationic copolyraerization and a homopolymer of methyl methacrylate in anionic copolymerization. This high selectivity of ionic copolymerization limits its practical use. Since, moreover, only a small number of monomers undergo ionic copolyraerization (see Chapter 8), the range of copolymer products that can be obtained is limited. On the other hand, almost all monomers undergo radical copolymerization and thus a wide range of copolymers can be synthesized. 

Some representative values of r and T2 in radical copolymerization for a number of monomer pairs are shown in Table 7.1. These are seen to differ widely. The reactivity ratios obtained in anionic and cationic copolymerizations are given and discussed in Chapter 8. 

In radical copolymerization the Alfrey-Price Q-e scheme has been proposed for systematizing a large amount of data and for correlating the reactivity of a monomer to its chemical structure. In this scheme the monomer reactivity ratios are given by the following equations ... 

Matyjaszewski et al. first used initiators bearing an unsaturated group for the ATRP process of styrene. In 1998, they [318] used vinyl chloroacetate as the initiator for the ATRP of styrene. As VAc was unreactive towards styrene in radical copolymerization, vinyl chloroacetate was able to initiate the ATRP of styrene (Scheme 65). The resulting PS macromonomers, with molar mass ranging from 5 x 103 to 15 x 103 gmol, were copolymerized with N-vinylpyrrolidinone. The amphiphilic copolymers obtained were used as hydrogels. 

Figure 11.7. Instantaneous copolymer composition in radical copolymerization of styrene and 2-vinyl thiophene mole fraction of styrene in polymer as function of initial mole fraction and fractional conversion of styrene calculated with reactivity ratios pa = 0.35 and pb = 3.10 (from Mayo and Walling [128]).

While ring-opening polymerization of camphorsultam was attempted futilely to prepare a new polymer containing a bicyclic structure and a new acidic sulfonamide group in the backbone [115b], radical cyclopolymerization was exploited in the synthesis of 193 nm alicyclic polymers (Fig. 79). Transannular polymerization to form polynortricyclene bearing tert-butyl ester was utilized in radical copolymerization with MA (Fig. 79) [275]. Radical cyclopolymer-... 

Fig. 92 Time dependence of feed and copolymer compositions in radical copolymerization of NB with TFMAA [288]...

Fullerene-styrene copolymers have been prepared in radical initiated and thermal polymerization reactions [148-151]. In radical copolymerizations of Cgg and styrene, copolymers with Cgg contents up to 50% (wt/wt) can be obtained [150]. Electronic absorption spectra of the copolymers are very different from that of monomeric C o (Fig. 36). The absorptivities per unit weight concentration of the copolymers j increase with increasing C q contents in the copolymers in a nearly linear relationship (Fig. 37). Fluorescence spectra of the Cgg-styrene copolymers, blue-shifted from the spectrum of monomeric Cgo, are dependent on excitation wavelengths in a systematic fashion [149]. Interestingly, the observed absorption and fluorescence spectral profiles of CgQ-styrene and Cyg-styrene copolymers are very similar, even though the spectra of monomeric CgQ and C70 are very different. The absorption and fluorescence spectra of the fullerene-styrene copolymers are also similar to those of the pendant Cgg-poly-styrene polymer (19) prepared in a Friedel-Crafts type reaction [150,156]. 

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