Adsorption monomer

Copolymerization. The solid phase of the precipitation polymerization also influences copolymer composition, since differential monomer adsorption on the polymer particles considerably modifies the effective reactivity ratios of the comonomers. This problem has been discussed by several authors (22,23,24,25,26). 

Barrett and Thomas (10)proposed that these effects of differential monomer adsorption could be modeled by correcting homogeneous solution copolymerization reactivity ratios with the monomer s partition coefficient between the particles and the diluent. The partition coefficient is measured by static equilibrium experiments. Barrett s suggested equations are ... 

Michaelides A, Alavi A, King DA. 2003. Different surface chemistries of water on RufOOOl From monomer adsorption to partially dissociated bilayers. J Am Chem Soc 125 2746-2755. 

Michaelides A, Ranea VA, de Andres PL, King DA. 2003b. General model for water monomer adsorption on close-packed transition and noble metal surfaces. Phys Rev Lett 90 216102. 

This relationship makes complete the mapping from the original problem of k-mer adsorption on L to an effective Ising-like one (monomer adsorption) on L. F is exactly known [17-19], hence, the exact form of F is obtained... 

Preferential wetting of polymer by monomer was demonstrated qualitatively some years ago 60). Monomer adsorption has been confirmed by Miller [cited in reference 128)], but quantitative information on the extent of adsorption is lacking. Dainton makes the reasonable assumption that the concentration of monomer on the particle is proportional to that in solution. 

No precise information about the olefin polymerisation mechanism has been obtained from kinetic measurements in systems with heterogeneous catalysts analysis of kinetic data has not yet afforded consistent indications either concerning monomer adsorption on the catalyst surface or concerning the existence of two steps, i.e. monomer coordination and insertion of the coordinated monomer, in the polymerisation [scheme (2) in chapter 2], Note that, under suitable conditions, each step can be, in principle, the polymerisation rate determining step [241]. Furthermore, no % complexes have been directly identified in the polymerisation process. Indirect indications, however, may favour particular steps [242]. Actually, no general olefin polymerisation mechanism that may be operating in the presence of Ziegler-Natta catalysts exists, but rather the reaction pathway depends on the type of catalyst, the kind of monomer and the polymerisation conditions. 

It is important to note that the true steady state deposition rate that is independent of the mixing modes (method A or B) was obtained only after more than 100 min operation in the case of styrene-N2. Without monomer adsorption in the case of acetylene-N2 system, the steady-state deposition rate is established much sooner than that in the styrene-N2 system. However, even in this case, it took more than 30 min to reach the steady-state deposition rate under the conditions of method B. 

The addition of nitrogen or water vapor changes the pattern of monomer adsorption, release of sorbed monomer, and the rate of monomer vapor consumption by polymerization considerably, as seen in the similar plots shown in Figures 34.10-34.12. With the addition of water vapor, the hydrophilicity of the porous substrate plays an important role that can be seen in the difference of pressure change observed when reversing the order of exposure to monomer and to water vapor. Namely, the amount of water vapor sorbed by hydrophilic Millipore filter is greater when water vapor is introduced first, whereas hydrophobic polysulfone sorbs more water when relatively more hydrophilic (than polysulfone) 4-vinylpyridine monomer is introduced first. Results shown in Figures 34.11 and 34.12 indicate that excessive water vapor prevents vapor phase polymerization... 

The eqs. (6) and (8) represent the exact solution describing the adsorption of k-mers at multilayer regime on a homogeneous surface in 1-D. In the case of monomer adsorption (when k=l), the equations (6) and (8) reduce to the well-known BET isotherm [1]. For k=2, the dimer isotherm can be written in a simple form ... 

As no completely satisfactory representation of the formation and nature of active species in heterogeneous catalysts has yet been devised this may be an over-simplification. Clearly the first order dependence of rate on monomer concentration is indicative of comparable solvent and monomer adsorption, even with what might be considered more strongly adsorbed monomers, such as butadiene, in comparison with mono-olefins or aliphatic hydrocarbons. The role of the more strongly adsorbed metal alkyl is more difficult to assess. The proportion of active alkylated transition metal atom sites will obviously increase with increase in [A] up to a limiting value. 

Factors influencing the rate of monomer diffusion are surface tension distribution coefficient of monomers between the two phases rate of monomer transfer from phase to phase solvent power of organic phase for the polymer permeability of polymer film to monomers adsorption of monomers by the polymer film viscosity of the system. 

From the earlier discussion it will be amply apparent that block copolymers may be synthesized from NCAs by the use of a preformed poly-0 -amino acid to initiate the polymerization of the NCA of a different a-amino acid. The kinetics of such a reaction would be expected to be determined by features already discussed in general, the rate coefficient for initiation may differ from that of propagation (Section 1) and phenomena attributable to stereoselectivity, polymer gregation and monomer adsorption (Sections 8 and 9) may arise. 

Km = equilibrium constant of monomer adsorption, k], kc, kd, km, kp= rate constants of the mechanism steps. 

Further evidence has been forthcoming on metallic sodium as an initiator. This is widely employed to form the tetrameric dianion of a-methylstyrene, which in turn is a highly convenient initiator. Recent work has shown that monomer adsorption onto the metal surface to produce a rotationally mobile species almost certainly plays an important role, and the ubiquitous dianion probably has the structure (16) rather than those previously suggested. 

We applied our approach under the assumption that the monomer is the photoactive species, and compared the results for the case of the dimer to check whether or not another photoactive species exists. The procedure for the calculation is the same with the case of dimer, although we considered only the fee site, which is more stable than the bridge in monomer adsorption. The resonant orbital is n for the free NO molecule, and the resulting action spectra is shown in Figure 20.16 with the experimental and theoretical dimer action spectra. At low photon energy, they agree well, but the increase at co 3.6eV seems to be too rapid in the monomer model. 

Christy s theory of polymerization by electron bombardment contains an important idea which can be applied to the theory of direct glow discharge polymerization (3). Williams and Hayes emphasized the importance of monomer adsorption to the electrode in the direct method (4). Poll also described the process of film formation in the direct method (. Yasuda et al. employed the indirect method, using an electrodeless discharge, and the results... 

This is explained by the equation indicated in Table 1(b). As substrate temperature is increased monomer adsorption decreases and the growth rate of film also decreases. This is predicted by the equation in Table 1(a). At still higher substrate temperatures, the growth rate of film becomes constant as only the 3 term remains in Eq. (4). A value of 3=2.4 was obtained from Fig. 

The variation in the film growth rate for various discharge conditions can be associated with the variation in monomer adsorption. The effect of ion transport can not be neglected. In our experiment at 1 KHz, about 1/4 of the film volume could be attributed to ion transport. The effect of the discharge frequency on film growth rate and film appearance was significant. This is closely related to the number of residual Ions between the electrodes and the degree of polymerization in the vapor. 

An explanation might be found if monomer chemisorption takes place instead. Indeed boehmite dissolution occurs by means of its lateral surfaces [19] monomer chemisorption on lateral boehmite surface would block surface functional sites and thus inhibit at least partially boehmite dissolution. Therefore, at intermediate molybdenum loading, as monomer concentration in solution is not negligible compared to AHM concentration, monomer adsorption on boehmite lateral surfaces should compete with aluminum extraction. Fig. 8 reports boehmite dissolution kinetics carried out at pH=4.8, for two [AHM] concentrations (intermediate and high molybdate loading). This result could support the fact that aluminum dissolution is slowed down at intermediate molybdenum loading and that no substantial moly-bdate adsorption could take place a plateau is reached in the adsorption isotherm (Fig. 1). 

Further evidence of monomer adsorption on a metal surface is provided by the extensive studies of Richards and his co-workers (14). As is well known, alkyl bromides in tetrahydrofuran vigorously react with alkali metals, say lithium, yielding the Wurtz coupling products. The violent reaction slows down on addition of aromatic monomers like styrene, and the nature of... 

In the case of a zip mechanism a conductivity decrease is observed for monomer addition, due to the monomer adsorption on the template and the consequent mobility decrease of the charge carriers. Also, a conductivity increase is recorded when the initiator is added but no subsequent conductivity change is expected since the polymerisation takes place when the monomer is already pre-adsorbed, with reduced mobility, on the template. 

If one does not introduce any corrections to the 0 in the phase coexistence region, results similar to those reported in [1, 104, 105, 119-128] are obtained. In the phase coexistence region (0>0J we then have const and the critical monomer adsorption is less than F, for... 

Adsorption of nonionic surfactants on porous solids has been studied by Huinink et al. in a series of p ers [ 149,150]. They elaborated a thermodynamic approach that accounts for the major features of experimental adsorption isotherms. It is a very well known fact that during the adsorption of nonionic surfactants there is a sharp step in the isotherm. This step is interpreted as a change from monomer adsorption to a regime where micelle adsorption takes place. Different surfactants produce the step in a different concentration range. The step is more or less vertical depending on the adsorbate. The thermodynamic analysis made by Huinink et al. is based on the assumption that the step could be treated as a pseudo first order transition. Their final equation is a Kelvin-like one, which shows that the change in chemical potential of the phase transition is proportional to the curvature constant (Helmholtz curvature energy of the surface). 

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