Activation volume copolymerization

By studying the effect of pressure on copolymerizations it is possible to obtain and activation volume difference for various homo- and cross-propagation reactions. For instance, in two of the propagation reactions occurring in the styrene-acrylonitrile polymerization ... 

The variation of reactivity ratios with pressure is small. The activation volume, AV (ri) = -(51n ri/9p)r/ 7, serves as a quantitative measure of this dependence. To derive reliable, accurate AV fn) values, experiments over an extended pressure range are required. Unfortunately, the pressure range of ethene-acrylate copolymerizations is not easily extended (to induce larger changes in n) as polymerization pressure is limited towards high p by the equipment and towards low p by inhomogeneity of the reaction mixture. 

As anticipated for a chemically controlled reaction, CO2 has only a minor influence on the rate coefficient for chain-transfer to DDM and to the MMA tri-mer in MMA and styrene homo- and copolymerizations. Going from bulk polymerization to solution polymerization with 40 wt%> CO2 present enhances Cx by about 10%, but leaves the associated activation volume, AV (Cx), unchanged [48]. As pointed out in the previous section, the observed lowering of kp,app upon increasing CO2 content is no true kinetic effect, and the propagation rate coefficient kp,kin most likely remains unaffected by the presence of CO2. Thus, ktr for DDM and for the MMA trimer should not be significantly varied by the presence of CO2. 

The variation of reactivily ratios with pressure is minor, as it is the difference between the activation volumes for homopropagation and cross-propagation that determines the formal activation volume of r. AV (ri)=-(d In rildp iRT. As a consequence, the accurate measurement of the resulting small activation volumes poses problems, which is especially true for AV (rx), as the comonomer content of the feed is mostly rather low. On the other hand, because of the small size of AV (r), this number need not be known overly accurately. Moreover, the pressure range of ethene copolymerizations is not very extended, as pressure is limited toward high p for technical reasons and toward low p by inhomogeneity of the reaction mixture. 

Equation (A8) is valid if the composition of the initial reaction mixture does not change appreciably with pressure. According to Eq. (A8), an increase in pressure favors the inclusion of monomers, which in homopolymerization show a large negative aetivation volume (e.g., substituted olefins). Values of copolymerization parameters obtained at different pressures and activation volumes of some copolymerization reaetions are listed in Section G. 

Remarkably, the use of a fluorous biphasic solvent system in combination with a [Rh(NBD)(DPPE)]+-type catalyst (NBD = norbornadiene) copolymerized into a porous nonfluorous ethylene dimethacrylate polymer, resulted in an increased activity of the catalyst relative to a situation when only toluene was used as solvent [30]. The results were explained by assuming that fluorophobicity of the substrate (methyl-trans-cinnamate) leads to a relatively higher local substrate concentration inside the cavities of the polymer when the fluorous solvent is used. That is, the polymer could be viewed as a better solvent than the fluorous solvent system. This interpretation was supported by the observations that (i) the increase in activity correlates linearly with the volume fraction of fluorous solvent (PFMCH) and (ii) the porous ethylene dimethacrylate polymer by itself lowers the concentration of decane in PFMCH from 75 mM to 50 mM, corresponding to a 600 mM local concentration of decane in the polymer. Gas to liquid mass transport limitation of dihydrogen could be mled out as a possible cause. 

Acrylic resins, ESCA spectra, 469/ Activation energy, and free volume, 168 Adhesion, resists, 43 Aldehydefs), copolymerization, 401 Aldehyde copolymers electron-beam exposure, 418/... 

In addition to the formation of active centres and participation in elementary processes, the discussion of which forms the main topic of this volume, monomers very often react with some component(s) of the polymerizing medium under complex formation. This reaction is very important. Complex formation lowers the effective monomer concentration, and changes in the polymerization rate usually occur. When the complex is much more active than the monomer, it may react preferentially with the active centre. This, of course, changes the addition mechanism and kinetics. When the monomer and complex also compete, the macrokinetics need not necessarily change. Usually, however, the mechanism of the whole process is greatly complicated, and a kind of copolymerization occurs. 

Graft Copolymers. In graft copolymerization, a preformed polymer with residual double bonds or active hydrogens is either dispersed or dissolved in the monomer in the absence or presence of a solvent. On this backbone, the monomer is grafted in free-radical reaction. Impact polystyrene is made commercially in three steps first, solid polybutadiene rubber is cut and dispersed as small particles in styrene monomer. Secondly, bulk prepolymerization and thirdly, completion of the polymerization in either bulk or aqueous suspension is made. During the prepolymerization step, styrene starts to polymerize by itself forming droplets of polystyrene with phase separation. When equal phase volumes are attained, phase inversion occurs. The droplets of polystyrene become the continuous phase in which the rubber particles are dispersed. R. L. Kruse has determined the solubility parameter for the phase equilibrium. 

One area of Robert Simha s activity that is missing here is work on the kinetics and statistics of chemical reactions such as polymerization, copolymerization, depolymerization, degradation, and sequencing of biomacromolecules (e.g., proteins, polynucleotides, DNA). The decision to omit this topic was based, on the one hand, on its chemical character, and on the other, on the vastness of these topics, which would essentially require an additional volume. 

The particles in Fig. 30 represent an excellent replica of the catalyst particle distribution through the polymer particle distribution the spherical form of the initial particles is retained and the equivalent circle diameters are enlarged from about 90 pm to about 360 pm after 190 min polymerization time. Their dependence on time indicates a very active catalyst system with a fast copolymerization rate and a fast particle expansimi caused by the loosely agglomerated MgQ2 support and the volume increase of the amorphous copolymer. It seems that this copolymerization system follows the multigrain model. 

The international symposium on Recent Developments in Olefin Polymerization Catalysts was held in Tokyo in October 1989. This volume includes 38 i>apers fi"om the 31 lectures and 18 posters presented at the symposium, which covered the following topics Overview of super-active homogeneous and heterogeneous catalysts, kinetic profile of olefin polymerization including copolymerization, characterization of catalysts and polymers, methods for the determination of active center concentration, role of Lewis bases on the catalyst isospecificity, polymerization mechanisms, and synthetic pathways for functionalized polyolefins. We believe the contents are well balanced between fundamental research and application as well as between homogeneous and heterogeneous catalyst systems. 

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