End group model

PA-6 (70) / EP (0-30) / EP-g-MA (0-30) internal mixer at 240°C / SEM / effects of adding phthahc anhydride to consume amine end-groups / model reactions to assess roles of amine- vs. amide-anhydride reactions Marechal et al., 1995... 

EP-g-MA (0-30) adding phthalic anhydride to consume amine end-groups/model reactions to assess roles of amine- vs. amide-anhydride reactions 1995... 

An important step in tire progress of colloid science was tire development of monodisperse polymer latex suspensions in tire 1950s. These are prepared by emulsion polymerization, which is nowadays also carried out industrially on a large scale for many different polymers. Perhaps tire best-studied colloidal model system is tliat of polystyrene (PS) latex [9]. This is prepared with a hydrophilic group (such as sulphate) at tire end of each molecule. In water tliis produces well defined spheres witli a number of end groups at tire surface, which (partly) ionize to... 

One of the most sensitive tests of the dependence of chemical reactivity on the size of the reacting molecules is the comparison of the rates of reaction for compounds which are members of a homologous series with different chain lengths. Studies by Flory and others on the rates of esterification and saponification of esters were the first investigations conducted to clarify the dependence of reactivity on molecular size. The rate constants for these reactions are observed to converge quite rapidly to a constant value which is independent of molecular size, after an initial dependence on molecular size for small molecules. The effect is reminiscent of the discussion on the uniqueness of end groups in connection with Example 1.1. In the esterification of carboxylic acids, for example, the rate constants are different for acetic, propionic, and butyric acids, but constant for carboxyUc acids with 4-18 carbon atoms. This observation on nonpolymeric compounds has been generalized to apply to polymerization reactions as well. The latter are subject to several complications which are not involved in the study of simple model compounds, but when these complications are properly considered, the independence of reactivity on molecular size has been repeatedly verified. 

Howardt describes a model system used to test the molecular weight distribution of a condensation polymer The polymer sample was an acetic acid-stabilized equilibrium nylon-6,6. Analysis showed it to have the following end group composition (in equivalents per 10 g) acetyl = 28.9,... 

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

Most recent work is in accord with mechanism (b). In an effort to distinguish these mechanisms studies on model propagating species have been carried out.IS6 liW For S-MMA polymerization initiated by AIBMe- -13C (Scheme 8.13) it has been established by end group analysis that extremely small amounts of ethyl aluminum sesquichloride (<10 3M with 1.75 M monomers) are sufficient to cause a substantial enhancement in specificity for adding S in the initiation step. This result suggests that complexation of the propagating radical may be sufficient to induce alternating copolymerization but does not rule out other hypotheses. 

The outcome of the polymerization depends strongly on the particular monomer. Polymerizations of S, MMA, MA, VAc and some derivatives have been reported. Studies on model compounds indicate that the primary or secondary dithiocarbamatc end groups arc much less susceptible to photodissociation than benzyl or tertiary derivatives. 

Fiery1 252-254) studied only the last stage of the reactions, i.e. when the concentration of reactive end groups has been greatly decreased and when the dielectric properties of the medium (ester or polyester) no longer change with conversion. Under these conditions, he showed that the overall reaction order relative to various model esterifications and polyesterifications is 3. As a general rule, it is accepted that the order with respect to acid is two which means that the add behaves both as reactant and as catalyst. However, the only way to determine experimentally reaction orders with respect to add and alcohol would be to carry out kinetic studies on non-stoichiometric systems. 

Thomas et al. [72] used a modified force microscope in which a compensatory force was applied to the probe to keep its displacement at zero. With this system they studied interactions between organomercaptan molecules with CH3, NH2, or COOH end groups. All measurements were performed in dry nitrogen. From SEM-measured tip radii and pull-off force they calculated the work of adhesion using the DMT model. They found that the work of adhesion values qualitatively scaled as expected for van der Waals, hydrogen bonding,... 

Figure 1.14 (a) STM image (lOnmx lOnm, tip bias +0.52V, tunneling current 0.5 nA) of a PVBA-induced sawtooth blade in a restructured Ag(l 1 0) surface terrace, (b) Structural model of the chiral kink arrangements induced by lateral interaction of molecular carboxylate end groups with Ag l 0 0 microfacets. (Reprinted with permission from Ref. [48]. Copyright 2004, American Institute of Physics.)... 

The molecular models are in a sense a subset of the geometrical ones - we assume that we know which molecules are present and we assume that we know their geometries (indeed sometimes we assume more than that, such as the usual assumption that thiol end groups lose their protons when forming their asymmetric bond with gold). In this we also necessarily assume that there are no other species, either on the electrode surface or in the surrounding media, that influence the current flow through the system. 

When the end groups of the polymers obtained by radical polymerization using certain iniferters still have an iniferter function, such radical polymerization is expected to proceed via a living radical mechanism even in a homogeneous system, i.e.,both the yield and the molecular weight of the polymers produced increase with reaction time. The generalized model is shown in Eq. (18) [16] ... 

It will be obvious from my account that although I consider myself primarily a physical chemist, my concern was always with finding the most efficient solution to a problem. If that meant doing phase-rule studies with a vacuum line (hardly ever attempted before), or getting infra-red spectra of polymers to identify end-groups when the only IR instrument in the City of Manchester at that time was in the ICI laboratories at Blackley, or synthesising new model compounds or super-pure A1C13, we just overcame the practical obstacles and got on with it. 

However, the chemical changes observed in low molecular weight compounds can be quite misleading as models for polymers. Difficulties include the high concentration of end groups, e.g. COOH in N-acetyl amino acids, which can dominate the radiation chemistry of the models. Low molecular weight compounds are usually crystalline in the solid state and reactions such as crosslinking may be inhibited or severely retarded. 

Figure 14.1 Energy minimized CPK models of generation 0 through 4 PAMAM and PPI dendrimers. EDA ethylene diamine DAB diamino butane G generation number numbers of end groups given in parentheses...

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