Propagation constants styrene derivatives

Butadiene and isoprene have also been studied in tetrahydrofuran (72). At 0° the rates are close to first order in polyisoprenyl or poly-butadienyllithium concentration which indicates that the rate constant for the ion-pair is being measured in the concentration range studied (> 10-s molar). The rate constants at 30°, k9 (butadiene) =1.8 litre/mole sec, kj, (isoprene) = 0.13 litre/mole sec, are appreciably lower than for styrene. For butadiene at —39° a kp value of 6 X 10 2 litre/mole sec can be derived from the results of Spirin (98). This value checks well with that extrapolated from Morton s data. The observed propagation constant for isoprene is rather low and is in fact equal to that of the mono-etherate in solvent mixtures of appreciably lower dielectric constant. At room temperature there is evidence for isomerization of polyisoprenyl-lithium in tetrahydrofuran which becomes particularly marked as the... 

Table 13 tabulates the rate constants of propagation of styrene monomers calculated from UV data assuming that e = 10,000 mol 1L cm 1 for growing styrene and a-methylstyrene carbenium ions, and e = 28,000 mol- L em-1 for carbenium ions based on /i-methoxystyrene derivatives. The former extinction coefficient was determined in superacid media assuming quantitative formation of carbenium ions, and is probably... 

The most important conclusion from stopped-flow studies is that the rate constants of propagation of several styrene derivatives are approximately kp 105-1 mol, L sec I at 0° C, which is relatively high compared with those of radical and anionic systems (average kp == 102 mol 1L-sec 1 at 0° C). Solvent effects are noticeable, with propagation slower in more nucleophilic 1,2-dichloroethane [17] than in CH2Cl2 [18] under comparable conditions. That is, the carbenium ion reactivity is apparently reduced by interaction with more nucleophilic solvents. However, such interactions do not result in formation of chloronium ions, whose spectra would be very different compared to those of the corresponding carbenium ions. 

Several authors have employed Burton and Pepper s scheme to calculate propagation rate constants for systems displaying the rather frequent behaviour of fast initiation-limited yields. Ykeda et al. and Yamamoto et al. have applied this principle to the polymerisation of styrene derivatives by sulphuric acid, and Kohjiyaet al. to the system cyclopentadiene-perchloric acid We wish to caution about the uncritical usage of this procedure, since, as has been shown above, it can lead to gross underestimates. In fact, it should only be used where there is sufficient mechanistic evidence to justify its application. 

A more reactive styrene derivative, p-methoxystyrene (pMOS) is also polymerized through two types of active centre to a product having a bimodal molecular weight distribution under certain conditions. - With iodine initiators in CCI4 solutions a product with a low molecular weight unimodal distribution is formed exclusively and the polymer chains appear to retain their activity throughout the polymerization. /ra/w-jS-Methyl substitution was found to reduce the propagation rate constant of pMOS by a factor of 1000. ... 

From such data it is then possible to derive the absolute values of the propagation rate constants representing the interaction of the growing chain with the incoming monomer unit. These are shown for isoprene and styrene in Table II. Obviously these propagation rate constants are all characterized by a low activation energy as well as an extremely low frequency factor. This is true both for the non-stereospecific polymerization of isoprene in THF as well as for the stereospecific case of isoprene in hexane (although it seems somewhat more extreme in the latter and for the styrene in benzene). 

The overall polymerization rates and the apparent rate constants of propagation (/c/pp = RP/[M][I]0) for the same initiating system are, however, very different for each class of monomers. For example, the same initiating system, that will polymerize a-methylstyrene (aMeSt) in 1 h, will complete polymerization of vinyl ether within less than 1 min but would require a few days to polymerize styrene and isobutene under otherwise identical conditions. This trend is due to the equilibria between dormant and active species. In this case the apparent rate constant of propagation is the product of the rate constant of propagation (weakly depending on monomer structure) and the ionization constant (kpapp = kp + -Kf). This equilibrium constant is much higher for more stable cations derived from vinyl ethers than from aMeSt, than styrene or isobutene. 

Thus, how should block copolymers between styrene and a vinyl ether be prepared Starting with styrene or with a vinyl ether In the former system, the propagating styryl cation is intrinsically more reactive but present at much lower concentration. A rough estimate of the ratio of cation reactivities is = 103 but the ratio of carbocations concentrations is = I0 S. Thus, the ratio of apparent rate constants of addition is 10-2. Macromolecular species derived from styrene should add to a standard alkene one hundred times slower than those derived from vinyl ethers. Thus, one cross-over reaction St - VE will be accompanied by =100 homopropagation steps VE - VE. Therefore, in addition to a small amount of block copolymer, a mixture of two homopolymers will be formed. Blocking efficiency should be very low, accordingly. 

Sawamoto et al. studied the spectroscopic stop-flow behaviour of the polymerisation of styrene and some of its derivatives by triflic acid In ethylene chloride at 30°C. They obtained overall values of the propagation rate constant of about 1 x 10 M s for styrene, p-chlorostyrene and p-methylstyrene. As previously in the same laboratory with p-methoxystyrene, invisible species were held responsible for some or most of the propagation, particularly in media of low dielectric constant, i.e. pseudocationic polymerisation occurs in all these systems and predominates when the polarity of the solvent does not favour ion formation. 

These equations are similar to those assumed for the reactivity ratio determination. In contrast to what has been observed for conventional styrene-MMA copolymers, however, these equations indicate that a substantial proportion of the (SMM+MMS)-type resonance appears to occur in the C-area. The proportion of methoxy resonance observed in the C-area, in fact, exceeds P(SMS) by a substantial amount for many of the copolymers. This can be due to the assumption of an inadequate model for the copolymerization reaction, to the use of incorrect reactivity ratios and cyclization constants for the calculations or to an inadequate understanding of the methoxy proton resonance patterns of S/MMA copolymers. It is possible that intramolecular reactions between propagating radicals and uncyclized methacrylic anhydride units present on propagating chains result in the formation of macrocycles. Failure to account for the formation of macrocycles would result in overestimation of rc and rc and in underestimation of the proportions of MMA units in SMS triads in the derived S./MMA copolymers. This might account for the results obtained. An alternate possibility is that a high proportion (>50%) of the M-M placements in the copolymers studied in this work can be expected to have meso placements (], J2), whereas only a small proportion of such placements ( 20%) are meso in conventional S/MMA copolymers. Studies with molecular models (20) have indicated that the methoxy protons on MMA units centered in structures such as the following can experience appreciable shielding by next nearest styrene units. 

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