Kinetic cross-linked polymers

Dusek, K, Diffusion Control in the Kinetics of Cross-Linking, Polymer Gels and Networks 4, 383, 1996. 

CHOW Deformation Kinetics of Cross-Linked Polymers... 

The resistance to fluid flow is a measure of the physical structure of the foam. In order to control the flow through a foam, ceU size, degree of reticulation, density, and other physical factors must be controlled. The control of these physical factors, however, is achieved through the chemistry and the process by which the foam is made. The strength of the bulk polymer is measured by the tensile test described above, but it is clear that the tensile strengths of the individual bars and struts that form the boundaries of an individual cell determine, in part, the qualities of the cells that develop. A highly branched or cross-linked polymer molecule will possess certain tensile and elongation properties that define the cells. The process is also a critical part of the fluid flow formula, mostly due to kinetic factors. As discussed above, the addition of a polyol and/or water to a prepolymer initiates reactions that produce CO2 and cause a mass to polymerize. The juxtaposition of these two reactions defines the quality of the foam produced. Temperature is the primary factor that controls these reactions. Another factor is the emulsification of the prepolymer or isocyanate phase with the polyol or water. 

Polymeric supports can also be used with advantage to form monofunctional moieties from difunctional (Hies. Leznoff has used this principal in the synthesis of sex attractants on polymer supports (67). Starting from a sheap symmetrical diol he blocked one hydroxyl group by the polymer. Functionalization of cross-linked polymers is mostly performed by chloromethylation (65). A very promising method to introduce functional groups into crosslinked styrene-divinylbenzene copolymers is the direct lithiation with butyllithium in presence of N,N,N, N -tetramethyl-ethylenediamine (TMEDA) (69, 70). Metalation of linear polystyrene with butyl-lithium/TMEDA showed no exchange of benzylic hydrogen and a ratio of attack at m/p-position of 2 1 (71). In the model reaction of cumene with amylsodium, a kinetic control of the reaction path is established. After 3h of treatment with amyl-sodiuni, cumene is metalated 42% in a-, 39% m-, and 19% p-position. After 20h the mixture equilibrates to affort 100% of the thermodynamically more stable a-prod-uct (72). 

Abstract. An overview of the synthesis and applications of microgels and coreshell particles is provided, with emphasis on work originating from the author s laboratory. Microgels, which are cross-linked polymer latex particles, can be used for selective uptake of ions or polymers, or the controlled release of various compounds. Various methods for the synthesis of core-shell particles are described such as interfacial polymerization, layer-by-layer deposition, colloidosomes , internal phase separation, and silica shells. The release kinetics for controlled (sustained or triggered) release purposes is discussed. 

Braun et al. studied the polymerization kinetics of symmetrical alkenes, particularly of 4-vinylbenzoic acid anhydride. The cross-linked polymer could be transformed to linear macromolecules by hydrolysis [359]. 

Most of the polymeric reagents which have been developed so far make use of an insoluble cross-linked polymer as the backbone. Investigations on the reaction rates and kinetic course in solid-phase synthesis revealed that the reaction sites within the polymeric matrix are chemically and kinetically not equivalent, making quantitative conversions almost impossible. Furthermore, the difficulty in the preparation and accessibility of insoluble polymeric reagents appears to limit a more... 

In view of Eq. (6.26) for ideal polymerization kinetics one would normally expect the reaction rate to fall with time, since the monomer and initiator concentrations decrease with conversion. However, the exact opposite behavior is observed in many polymerizations where the rate of polymerization increases with time. A typical example of this phenomenon is shown in Fig. 6.10 for the polymerization of methyl methacrylate in benzene solution at 50°C [49], At monomer concentrations less than about 40 wt% in this case, the rate (slope of conversion vs. time) is approximately as anticipated from the ideal kinetic scheme described in this chapter, that is, the rate decreases gradually as the reaction proceeds and the concentrations of monomer and initiator are depleted. An acceleration is observed, however, at higher monomer concentrations and the curve for the pure monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel used here is different than the usage in Chapter 5 as it refers only to the sharp increase in viscosity and not to the formation of a cross-linked polymer.) The autoaccelerative gel effect is also known as the Tromsdorff effect or Norrish-Smith effect after pioneering workers in this field. It should be noted that the gel effect is observed under isothermal conditions. It should thus not be confused with the acceleration that would be observed if a polymerization reaction were carried out under nonisotherraal conditions such that the reaction temperature increased with conversion due to exothermicity of the reaction. 

Eom, K., Makarov, D. E., and Rodin, G. J. 2005. Theoretical studies of the kinetics of mechanical unfolding of cross-linked polymer chains and their implications for single-molecule pulling experiments, Phys Rev E Stat Nonlin Soft Matter Phys 71,021904. 

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