Rate of copolymerization

Addition of dialkyl fumarates to DAP accelerates polymerization maximum rates are obtained for 1 1 molar feeds (41). Methyl aUyl fumarate [74856-71-6] (MAF), CgH QO, homopolymerizes much faster than methyl aUyl maleate [51304-28-0] (MAM) and gelation occurs at low conversion more cyclization occurs with MAM. The greater reactivity of the fumarate double bond is shown in copolymerization of MAF with styrene in bulk. The maximum rate of copolymerization occurs from monomer ratios, almost 1 1 molar, but no maximum is observed from MAM and styrene. Styrene hinders cyclization of both MAF and MAM. 

Reactivity ratios for the copolymerization of AN with 7 in methanol at 60 °C, proved to be equal to rx AN= 3,6 0,2 and r%n = 0 0,06, i.e., AN is a much more active component in this binary system. The low reactivity of the vinyl double bond in 7 is explained by the specific effect of the sulfonyl group on its polarity26. In addition to that, the radical formed from 7 does not seem to be stabilized by the sulfonyl group and readily takes part in the chain transfer reaction and chain termination. As a result of this, the rate of copolymerization reaction and the molecular mass of the copolymers decrease with increasing content of 7 in the initial mixture. 

The rate of copolymerization often shows a strong dependence on the monomer feed composition. Many theories have been developed to predict the rate of copolymerization based on the terminal model for chain propagation (Section 7.3.1.1), This usually requires an overall rate constant for termination in copolymerization that is substantially different from that observed in homopolymerization of any of the component monomers. 

Values of 0 required to fit the rate of copolymerization by the chemical control model were typically in the range 5-50 though values <1 are also known. In the case of S-MMA copolymerization, the model requires 0 to be in the range 5-14 depending on the monomer feed ratio. This "chemical control" model generally fell from favor wfith the recognition that chain diffusion should be the rate determining step in termination. 

This model provides a better description of the rate of copolymerization for some systems but has been criticized as having too many adjustable parameters.174... 

If the chains are long, the composition of the copolymer and the arrangement oi units along the chain are determined almost entirely by the relative rates of the various chain propagation reactions. On the other hand, the rate of polymerization depends not only on the rates of these propagation steps but also on the rates of the termination reactions. Copolymer composition has received far more attention than has the rate of copolymerization. The present section will be confined to consideration of the composition of copolymers formed by a free radical mechanism. 

The rate of copolymerization in a binary system depends not only on the rates of the four propagation steps but also on the rates of initiation and termination reactions. To simplify matters the rate of initiation may be made independent of the monomer composition by choosing an initiator which releases primary radicals that combine efficiently with either monomer. The spontaneous decomposition rate of the initiator should be substantially independent of the reaction medium, as otherwise the rate of initiation may vary with the monomer composition. 2-Azo-bis-isobutyronitrile meets these requirements satisfactorily. The rate Ri of initiation of chain radicals of both types Ml and M2 is then fixed and equal to 2//Cd[7], or twice the rate of decomposition of the initiator I if the efficiency / is equal to unity (see Chap. IV). The relative proportion of the two types of chain radicals created at the initiation step is of no real importance, for they wall be converted one into the other by the two cross-propagation reactions of the set (1). Melville, Noble, and Watson presented the first complete theory of copolymerization suitable for handling the problem of the rate. The theory was reduced to a more concise form by Walling, whose procedure is followed here. 

Fig. 27.—The rate of copolymerization of styrene and methyl methacrylate at 60°C in the presence of azo-bis-isobutyronitrile (1 g./l.) plotted against the mole fraction of styrene. Broken line has been calculated from Eq. (26) assuming < = 1. Solid line represents calculated curve for 0 = 13. (Walling. q...

In the low catalyst concentration range, polymerization rate is increased with increased amounts of catalyst however, the exact rate dependence on catalyst concentration has not been established. In general, the rate of copolymerization of butadiene with styrene is increased with increased polymerization temperature, increased Ba/Mg mole ratio, increased buta-diene/styrene comonomer feed ratio, and increased dielectric constant of the polymerization solvent. 

Bajoras and Makuska investigated the effect of hydrogen bonding complexes on the reactivities of (meth)acrylic and isotonic acids in a binary mixture of dimethyl sulfoxide and water using IR spectroscopy (Bajoras and Makuska, 1986). They demonstrated that by altering the solvent composition it was possible to carry out copolymerization in the azeotropic which resulted in the production of homogeneous copolymers of definite compositions at high conversions. Furthermore, it was shown that water solvent fraction determines the rate of copolymerization and the reactivity ratios of the comonomers. This in turn determines the copolymer composition. 

The rate of copolymerization, unlike the copolymer composition, depends on the initiation and termination steps as well as on the propagation steps. In the usual case both monomers combine efficiently with the initiator radicals and the initiation rate is independent of the feed composition. Two different models, based on whether termination is diffusion-controlled, have been used to derive expressions for the rate of copolymerization. The chemical-controlled termination model assumed that termination proceeds with chemical control, that is, termination is not diffusion-controlled [Walling, 1949]. But this model is of only historical interest since it is well established that termination in radical polymerization is generally diffusion-controlled [Atherton and North, 1962 Barb, 1953 Braun and Czerwinski, 1987 North, 1963 O Driscoll et al., 1967 Prochazka and Kratochvil, 1983] (Sec. 3-10b). 

It was also discovered at Phillips. that the four rate constants discussed above can be altered by the addition of small amounts of an ether or a tertiary amine resulting in reduction or elimination of the block formation. Figures 13 and 14 illustrate the effect of diethyl ether on the rate of copolymerization and on the incorporation of styrene in the copolymer. Indeed, random copolymers of butadiene and styrene or isoprene and styrene can be prepared by using alkyllithium as initiator in the presence of small amounts of an ether or a tertiary amine. 

Figure 13. Effect of amount of diethyl ether on rate of copolymerization at...

It has been emphasized in the copolymerization of styrene with butadiene or isoprene in hydrocarbon media, that the diene is preferentially incorporated. (7,9,10) The rate of copolymerization is initially slow, being comparable to the homopolymerization of the diene. After the diene is consumed, the rate increases to that of the homopolymerization of styrene. Analogously our current investigation of the copolymerization of butadiene with isoprene shows similar behavior. However, the... 

At the reaction temperature prevailing, 3 is partially thermolised and acts as an initiator. This might explain that the rate of copolymerization increases with the proportion of 3 in the monomer mixture. The simultaneous decrease in M can be explained in terms of the intervention of hydrazyl radicals, or simply by the well-known relation Mn 1/[I] /2. 

It has been shown that the final copolymer usually shows a molar composition different from that of the initial mixture of the monomers, and that sometimes it may even be impossible to synthesize some copolymers with a predetermined composition moreover, the behaviour of olefins during the copolymerization differs very often from that during their homopolymerization, as well with respect to their copolymerization tendency, as for their rate of copolymerization. 

In contrast to the non-catalyzed reaction, the base-initiated copolymerization was found to be a specific reaction 35,36,39 -45) and the consumption of both monomers, epoxide and anhydride, is the same. The initiator not only affects the rate of copolymerization but also suppresses the undesirable homopolymerization of the epoxide. At equimolar ratio, epoxide and anhydride are strictly bifunctional. 

Hilt et al. 42,541 found by conductivity measurements that the copolymerization of epoxides with cyclic anhydrides initiated by alkali salts is of ionic character. Luston and Manasek 36,57), who used ammonium salts, came to the same conclusion. The rate of copolymerization increases linearly with rising initiator concentration (Fig. 1, the slope of the curve log k vs log c,n is unity) but only up to a limit which depends on the solubility of the initiator in the reaction system54). A rise in the copolymerization rate is accompanied by an increase in the conductivity of the reaction... 

Solvent polarity influences the rate of copolymerization. Thus with increasing dielectric constant of the solvent, the copolymerization rate rises as a result of the increase in the dissociation constants of the active species. The apparent rate constant for the copolymerization of 2-hydroxy-4-(2,3-epoxypropoxy)benzophenone with phthalic anhydride, initiated by hexadecyltrimethylammonium bromide56), increases from 4.65 x 10 4 s 1 in o-xylene to 6.84x 10 4 s-1 in nitrobenzene. Hilt et al.S4) proposed a suitable model illustrating the effect of solvent polarity in the copolymerization of phthalic anhydride with ethylene glycol carbonate in a mixture of nitrobenzene and trichlorobenzene (Table 4). With increasing fraction of the more polar nitrobenzene, the rate of copolymerization increases. 

The initiation by aromatic tertiary amines is affected by the type of substituent and its position in the ring. A series of meta-substituted pyridines show a higher activity than ortho- or para-substituted derivatives with electron-releasing substituents. 3-Aminopyridine is much more reactive than 2-aminopyridine and 3-methylpyridine is more effective than 2- or 4-methylpyridine 19). The strong electron-withdrawing substituent of 2-bromopyridine and its steric hindrance eliminate the effect of this amine. A 2,6-Dimethylpyridine was also found to display a steric effect. Thus, in the presence of this amine, the rate of copolymerization decreases by a factor of 4 compared with 3-methylpyridine 69). 

However, the experimental data in the literature do not allow to determine the effect of the structure of the amines on the rate of copolymerization of epoxides with cyclic anhydrides, nor to verify the validity of Eq. (36) for this type of reaction. 

HA compounds is not necessary for the formation of a polyester. Nevertheless, an acceleration effect of HA compounds on the rate of copolymerization was detected later 36 57 74), even for systems in which proton donors are directly bound to monomers 67). This effect is not the sum of the contributions from the tertiary amine and the proton donor but even stronger. Hence, proton donors display a cocatalytic effect. Concerning the effect of HA compounds Tanaka and Kakiuchi 36) established a linear correlation between Hammett s ct constants and the logarithm of the gelation time for various substituted derivatives of benzoic acid, benzyl alcohol and phenol, and positive reaction parameters q were found in all cases. This means that electron-withdrawing substituents increase the effect of HA compounds, or their effect becomes more pronounced with increasing hydrogen atom acidity. 

The induction period is followed by the region of the maximum rate of copolymerization 52). Conversion curves can be correlated in this region 45,74) with the first-order kinetic equation (Fig. 6) and the rate of copolymerization is thus of the first oder with respect to one of the monomers. Also, recalculation of some experimental data found in the literature 35,52,901 gives a satisfactory fit to the first order kinetic equation. If the rate of copolymerization is first-order with respect to the tertiary amine 9, 32.35. 39,40,45,52.65.67,73,74,9s, 97,98 overall rate of copolyme-... 

In our opinion, the area of this type of copolymerization is still open to discussion and research, and this suggestion concerns mainly the mechanism of copolymerization, formation of active centres in the initiation by Lewis bases, the influence of proton donors on the course of copolymerization, and tbe effect of the structure of the initiator on the rate of copolymerization. It is necessary, however, to study the copolymerization on model compounds. 

Dilution with toluene slowed the copolymerization rate, and kinetic measurements were carried out in toluene at 0°-30°C. As reported previously (II), the over-all activation energy of the spontaneous copolymerization of CPT and S02 was calculated to be 16.5 kcal/mole from the Arrhenius plot of the initial rate vs. polymerization temperature. Dependence of the intial rate of copolymerization upon monomer concentration was checked at various monomer concentrations and found to be quite high (II) this could not be explained without participation of the monomer in the initiation step. 

Figure 2. Rate of copolymerization of maleic anhydride and styrene in different solvents...

As shown in Figure 2, the rate of the heterogeneous copolymerization of styrene and maleic anhydride in benzene (8 = 9.2) is faster than the homogeneous copolymerization of these monomers in acetone (8 = 9.9). However, this rate decreases as the solubility parameter values of the solvents decrease in heterogeneous systems. Thus, the rate of copolymerization decreases progressively in xylene (8 = 8.8), cumene (8 = 8.5), methyl isobutyl ketone (8 = 8.4), and p-cymene (8 — 8.2). All of these rates were faster than those observed in homogeneous systems. The solubility parameter of the alternating styrene-maleic anhydride copolymer was 8 = 11.0. 

The slow rate of copolymerization in acetone was related to the ease of termination of macroradicals by coupling. This coupling was hindered by the coiling of the macroradical chains in benzene, but propagation continued to take place since the monomers were able to pene-... 

Finally, the rate of copolymerization should be slow enough. This guarantees that, during the chemical reaction, the equilibrium concentration fields remain approximately constant, and the growing chain has an equilibrium conformation between successive attachments of the monomers. Therefore, the CDSD regime is realized when... 

When butadiene and styrene are mixed in the presence of an organolithium initiator, the resulting copolymerization process and product will be governed by the reaction conditions. The rate of copolymerization, the relative composition of the copolymer, and the distribution of monomer units (i.e., block, random, etc.) will be determined by such factors as solvent, temperature, and monomer feed ratio. 

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