Ethene Homopolymerization

The high-pressure CSTRs have been designed with special emphasis on ideal mixing. A technique for the characterization of residence-time behavior under high-pressure conditions and during ethene polymerization has been developed [30]. Effective mixing ensures isothermal conditions within the CSTR which largely facilitates the quantitative kinetic analysis of polymerization reactions carried out in such a reactor. 

It goes without saying that modeling of ethene high-pressure copolymerizations requires a significant number of additional reaction steps to be included. 

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In addition to the cationization (Eq. (20)), the first three propagation steps of the ethene homopolymerization (Eqs. (21)—(23)) should be comparatively investigated. 

Before some detailed remarks about the first propagation step of ethene homopolymerization (Eq. (21)) are given, a general survey of the energetic conditions for the reactions (21)—(23) in the gas phase and in solution (solvent CH2C12) should be provided 131). 

Summarizing the calculations for the ethene homopolymerization with respect to the interaction between cation and monomer leads to the following ... 

Ethene, copolymerization of, 76 111 Ethene homopolymerization, 76 102-103 l-Ethenyl-2-pyrrolidinone. See AT-Vinyl-2-pyrrolidinone... 

Section 4.6.2 illustrates the experimental procedures that have recently been applied toward the study of high-pressure free-radical polymerization processes. Section 4.6.3 presents results of propagation, termination, chain-transfer (to monomer and to polymer), and P-scission rate coefficients for ethene homopolymerization. Recent results from experiments and modeling investigations into high-pressure copolymerizations (with ethene being one of the monomers) are reported in Section 4.6.4, together with information on homopolymerization rate coefficients of the comonomer species. 

Scheme 4.6-1 Kinetic steps in ethene homopolymerizations I = initiator E = ethene R = free radical P = polymer chain CTA = chain transfer agent. The subscripts are l,r,s = chain length sec = secondary, LCB = long-chain branching.

Figure 4.6-5 Rate parameter jt,Oi iis a function of monomer conversion measured via SP-PLP during ethene homopolymerizations at 230 °C and 2550 bar [31].

From an extended set of SP-PLP experiments carried out at temperatures between 190 and 230 °C and at pressures between 1950 and 2900 bar, Schweer [24] derived expressions for the temperature, pressure, monomer conversion (X), and viscosity (t)) dependence of termination and propagation rate coefficients for the ethene homopolymerization, eqs (4.6-2) and (4.6-3). 

As mentioned above, the Goto et al. [35] scheme is frequently used for modeling ethene homopolymerization, and the associated ktr.p and kp coefficients... 

For understanding the nature of the comonomer effect, it is also very important that the rate enhancement takes place in the sequential processes of homo- and copolymerization, i.e., when the ethene homopolymerization is carried out after the a-olefin homopolymerization or ethene/a-olefin copolymerization [122, 123] (Table 6). 

CEF - comonomer effect, Rcop - rate of ethene insertion in ethene/a-olefin copolymerization Rpol -rate of ethene homopolymerization... 

Ri is the specific rate of ethene homopolymerization and R2 the specific rate of ethene homopolymerization on the second step of the two-step process Mixture of dibutylphthalate and ethylbenzoate Phenyltriethoxysilane... 

Finally Fig. 5 shows the dependence of the mixed and the true r parameters on the conversion, in relation to the Al ratios of 5 1 (o, ) and 20 1 (x,A). Here we have tried to investigate a possible dependence of the copolymerization parameters on the overall concentration of the active species. The results in Fig. 5 show that the effect of both AlATi ratios on the r parameter with increasing conversion is the same. In offier words, wiffi increasing the AI/Ti ratio both of the ethene homopolymerization and copolymerization active centres increase shnultaneourdy. 

Scheme 2 Kinetic steps of ethene homopolymerization r, s characterize chain length).

Plotted in Figure 19 is against inverse temperature for ethene homopolymerizations at 2000 bar and very low initiation rates. " Arrhenius-type behavior of Ctr,M is clearly seen. The slope to the straight line yields the difference in the activation energies for CT to monomer and propagation. With EaC p) being known from SchweeTs work, the experimental value of EA(Qr,M) yields EA(fetr,M) = (74 8) kj mol" which is remarkably close to a(Vm) = 83 kjmol" estimated by Heuts et from ab initio quantum mechanical calculations for the transfer reaction between an ethyl radical and ethene. 

The two most prominent examples of catalysts in the late transition metal-catalyzed ethene homopolymerization are the Ni- or Pd-based a-diimine 1 and phosphine sulfonate-based 2 catalysts (Figure 3). 

In principle, such protection using the carboxylate salt is feasible, but an exchange of the metal cation coordinated to the carboxylate salt has to be taken into account. This would lead to a transfer of the protected olefin between the titanocene and the active catalyst. Copolymerization experiments of Tim/Iia with ethene lead to the desired titanocene-protected copolymers in yields comparable to ethene homopolymerization. Hydrochlorination of the protected polymer regenerates the protecting Cp2TiCl and the free carboxylic acid of the polymer (Scheme 19). 

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