Reactivity ratio temperature effects

Acrylamide copolymerizes with many vinyl comonomers readily. The copolymerization parameters ia the Alfrey-Price scheme are Q = 0.23 and e = 0.54 (74). The effect of temperature on reactivity ratios is small (75). Solvents can produce apparent reactivity ratio differences ia copolymerizations of acrylamide with polar monomers (76). Copolymers obtained from acrylamide and weak acids such as acryUc acid have compositions that are sensitive to polymerization pH. Reactivity ratios for acrylamide and many comonomers can be found ia reference 77. Reactivity ratios of acrylamide with commercially important cationic monomers are given ia Table 3. 

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. 

The effect of temperature on r is not large, since activation energies for radical propagation are relatively small and, more significantly, fall in a narrow range such that En Eu is less than 10 kJ mol-1 for most pairs of monomers. However, temperature does have an effect, since E 2 — E is not zero. An increase in temperature results in a less selective copolymerization as the two monomer reactivity ratios of a comonomer pair each tend toward unity with decreasing preference of either radical for either monomer. Temperature has the greatest... 

There are few studies of the effect of temperature on monomer reactivity ratios [Morton, 1983]. For styrene-1,3-butadiene copolymerization by r-butyllithium in rc-hexane, there is negligible change in r values with temperature with r — 0.03, r2 = 13.3 at 0°C and n = 0.04, r% = 11.8 at 50°C. There is, however, a signihcant effect of temperature for copolymerization in tetrahydrofuran with r — 11.0, r2 = 0.04 at —78°C and r — 4.00, r2 = 0.30 at 25° C. The difference between copolymerization in polar and nonpolar solvents is attributed to preferential complexing of propagating centers and counterion by 1,3-butadiene as described previously. The change in r values in polar solvent is attributed to the same phenomenon. The extent of solvation decreases with increasing temperature, and this results in... 

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 ... 

Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

In contrast to thiophene, benzo[6]thiophene is preferentially substituted at the /3-position. The /3 a reactivity ratios and partial rate factors for the electrophilic substitutions of benzo[6]thiophene have been summarized. The reactivity ratio varies over a wide range, depending on the nature of the electrophile and the temperature of the reaction in the case of acetylation, the percentage of the a-substituted product increases with temperature. Also in contrast to thiophene, the extended selectivity treatment applied to the reactions at the a- and /3-positions of benzo[6]thiophene gives a non-linear plot. The effect of fusion of a benzene ring to thiophene is to decrease the reactivity of the a-position and increase the reactivity of the /3-position. 

Asymmetric diisocyanates such as 2,4-TDI are more complex because the initial reactivity of the two isocyanate groups is not equivalent and the substitution effect amplifies the difference. The 4-NCO is about 10-20 times more reactive than the 2-NCO, but the reactivity ratio also depends on temperature (see Chapter 5). This difference also explains why the TDI dimer can be prepared quantitatively (Eq. 2.28). 

Equation (5.1) becomes a rigorous equation only in the case of stepwise polymerizations with equal initial reactivities, absence of substitution effects, and following a single reaction path. It may be also used when the reactivity ratio does not vary with temperature. This is, fortunately, the case of epoxy-amine reactions where the reactivity ratio of the secondary to the primary amine is approximately constant in a broad temperature range. But, even in this case, the parallel polyetherification of epoxy groups must be negligible to keep a single reaction path. 

In order to elucidate the effect of temperature, the authors of Refs. [310,210] determined experimentally the boundary points x x = 0.08 and XgX = 0.65 of the transparency region for the (ST + HA) system at complete conversion, p = 1, when in the course of synthesis the temperature was increased in a given way from 28 °C to 78 °C. Despite a noticeable difference between such a regime and the isothermal one (see Fig. 24), it was found that the regions, in which at p = 1 turbid copolymers were formed, practically coincide. The same could be said about the calculated values of dispersion cr2(l) at the boundary points of the mentioned regions. This might be associated with a rather weak dependence of the reactivity ratios on temperature. A similar practical independence of the turbidity region... 

Mechanistic Aspects of Cationic Copolymerizations The relative reactivities of monomers can be estimated from copolymerization reactivity ratios using the same reference active center. However, because the position of the equilibria between active and dormant species depends on solvent, temperature, activator, and structure of the active species, the reactivity ratios obtained from carbocationic copolymerizations are not very reproducible [280]. In general, it is much more difficult to randomly copolymerize a variety of monomers by an ionic mechanism than by a radical. This is because of the very strong substituent effects on the stability of carbanions and carbenium ions, and therefore on the reactivities of monomers substituents have little effect on the reactivities of relatively nonpolar propagating radicals and their corresponding monomers. The theoretical fundamentals of random carbocationic copolymerizations are discussed in detail and the available data are critically evaluated in Ref. 280. This review and additional references [281,282] indicate that only a few of the over 600 reactivity ratios reported are reliable. 

The effect of temperature on reactivity ratios in free radical copolymerization is small. We can reasonably assume that the propagation rate constants in the reactions (7-2)-(7-5) can be represented by Arrhenius expressions over the range of temperatures of interest, such as... 

These predictions are essentially confirmed by experience. Most free-radical reactivity ratios are measured by convention at temperatures near 60°C, and the effect of changes in conditions in the range 0-90" C is usually assumed to be negligible, compared to the experimental difficulties in detecting the elTects of slight variations in r or f2. 

Our plans also included the exploration of the fundamentals of this copdymer-ization in which one of the monomers, P-pinene, isomerizes prior to propagation. In this sense, this research concerned the investigation of an isomerization copolymerization , a field that has not yet been explored. This phase of our work involved the elucidation of the effect of a variety of initiating systems (counter-ions), solvents, and temperatures on the reactivity ratios of these monomers, on the rate of polymerization, and on the molecular we ts of the products. 

Subsequently the data were treated by tiie Arrhenius concept, ie, the logarithm of the reactivity ratios was fdotted versus the redprocal temperature as drown in Fig. 8. This plot expresses quantitatively the effect of temperature on the reactimty ratios of isobutylene and 0-pinene. Evidently, in the higher temperature range, from —50 ... 

A similar effect of temperature on the reactivity ratios has already been observed in cationic copdymerizations. Thus M. Imoto and K. Saotome (25) found in a brief... 

Fig. 8. The effect of temperature on the reactivity ratios of isobutylene and iS-pinene (EtAlClainEtCI)...

On the other hand presence of B -N dyad in the polymer could only arise from a transesterification reaction where the B was inserted between the -BN-. As shown in Figure 3, NMR displays a small but distinct peak (ca.14%) at the resonance position corresponding to B -N diad. Since the polymerization was run at the same temperature of 245°C forl70 min and to the same degree of polymerization (M n 2252) as the earlier experiment on the reactivity ratios, one can conclude that the role of interchain transesterification is relatively small and that the monomers have approximately equivalent reactivity ratios. The reaction of B with the HBA-HNA dimer was also examined at 225° and 285°C to determine the possibility of a temperature effect The times of reaction were 20 hrs and one hour, respectively,... 

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