Polymeric Steric Forces

When a polymer layer is present at the surface of the particles (either adsorbed or chemically grafted), a repulsion force can be created when the layers on two neighboring particles overlq . This happens whenever the polymer molecules would rather become more compact than mix as the two layers are squeezed together 

If polymer molecules are dissolved and moving fieely in the medium, they can favor flocculation rather than stabilization. Whenever the interparticle distance becomes smaller than the dimensions of the polymer molecules, the gap is depleted of these molecules for thermodynamic reasons. As a result, the particles are forced even closer together and become flocculated (depletion 

Interaction potential for polymerically stabilized suspension (Vzj attraction from dispersion forces Vs polymeric repulsion V, total interaction potential). 

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Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

In the above descriptions we concentrated on situations where a polar background solvent was implicitly assumed. In apolar solvents double layer repulsion is diflhcult to achieve because dissociation, leading to charged surface groups, is less likely to occur and it becomes essential to stabilize colloids with polymers as to prevent instabilities. In the first decades after the establishment of the DLVO theory most papers on forces between colloidal particles focused on Van der Waals and double layer interactions. Forces of other origin such as polymeric steric stabilization [17], depletion [40] or effects of a critical solvent mixture [41] gained interest at a later stage. 

All three types of thin liquid films from both ABA and AB polymeric surfactants are stabilized by DLVO-forces at low electrolyte concentrations and by non-DLVO-forces at higher electrolyte concentrations. The latter are steric surface forces of the type brush-to-brush and loop-to-loop interactions (according to de Gennes). These steric forces act in 0/W emulsion films as well, but there transitions to Newton black films (NBF) have also been established. A difference between foam and O/W emulsion films has been observed. The barrier in the ri(h) isotherm for an emulsion film is much lower and the transition to NBF can occur. The NBFs from polymeric surfactants are very stable, as are the emulsions obtained from the same solutions. Actually, two types of bilayer emulsion films are obtained, those stabilized by brush-to-brush or loop-to-loop steric interactions and the others - by short-range interactions, also steric, in a two-dimensional ordered system. The minor difference in the experimentally measured thickness (about 2 nm) is not sufficient to characterize the state of these films. 

AT is intended to include any and all of the effects of the sorption rate of monomer on the surface, steric arrangement of active species, the addition of the monomer to the live polymer chain, and any desorption needed to permit the chain to continue growing. We assume a steady state in which every mole of propylene that polymerizes is replaced by another mole entering the shell from the gas, so that all of the fluxes are equal to Ny gmol propylene reacted per second per liter of total reactor volume. The following set of equations relates the molar flux to each of the concentration driving forces. 

The surface forces apparatus (SEA) can measure the interaction forces between two surfaces through a liquid [10,11]. The SEA consists of two curved, molecularly smooth mica surfaces made from sheets with a thickness of a few micrometers. These sheets are glued to quartz cylindrical lenses ( 10-mm radius of curvature) and mounted with then-axes perpendicular to each other. The distance is measured by a Fabry-Perot optical technique using multiple beam interference fringes. The distance resolution is 1-2 A and the force sensitivity is about 10 nN. With the SEA many fundamental interactions between surfaces in aqueous solutions and nonaqueous liquids have been identified and quantified. These include the van der Waals and electrostatic double-layer forces, oscillatory forces, repulsive hydration forces, attractive hydrophobic forces, steric interactions involving polymeric systems, and capillary and adhesion forces. Although cleaved mica is the most commonly used substrate material in the SEA, it can also be coated with thin films of materials with different chemical and physical properties [12]. 

The driving force in some isomerization polymerizations is relief of steric strain. Polymerization of P-pinene proceeds by the first-formed carbocation XVI rearranging to XVII... 

Syndioselective polymerizations of propene are somewhat less regioselective than the isoselective reactions, with the typical highly syndiotactic polymer showing a few percent of the monomer units in head-to-head placement [Doi, 1979a,b Doi et al., 1984a,b, 1985 Zambelli et al., 1974, 1987]. The mode of insertion is secondary, contrary to what is expected for a carbanion propagating center. Apparently, steric requirements imposed by the counterion derived from the initiator force propagation to proceed by secondary insertion. 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

The dispersion polymerization system is composed of monomer, solvent, initiator, and stabilizer. The combination of monomer, solvent, and stabilizer is essential for particle preparation. That is to say, the stabilizer is chosen to meet the demand of the monomer and solvent. In any system, the stabilizer has affinity or cohesive strength for both the medium and the polymer particles. In a dispersion polymerization, the medium and polymer particles both are organic compounds. Therefore, it is not rational to rely on dispersion stabilization, which comes from the electrostatic repulsion force between particles. The stabilizer for dispersion polymerization that makes interfacial energy low must have affinity for particles due to the same quality and solvation at the surface of particles. It is desired that the stabilizer be a polymer that indicates a steric stabilization effect on the surface (5). 

R = Me because the initial reaction is then more complicated than expressed by equation (85). In dimeric Cr2(NR2)6, steric hindrance prevents chromium(III) from obtaining its preferred octahedral coordination this, the ligand field stabilization in Cr(NR2)4 (tetrahedral, d2), the covalency of the Cr —NR2 bond, and the polymeric structure of involatile Crn(NEt2)2 all contribute to the driving force for reaction (86). The structural changes are represented for R = Et in equation (87). 

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