Poly , frontal polymerization

Isothermal frontal polymerization (IFP) is a self-sustaining, directional polymerization that can be used to produce gradient refractive index materials. Accurate detection of frontal properties has been difficult due to the concentration gradient that forms from the diffusion and subsequent polymerization of the monomer solution into the polymer seed. A laser technique that detects tiny differences in refractive indices has been modified to detect the various regions in propagating fronts. Propagation distances and gradient profiles have been determined both mathematically and experimentally at various initiator concentrations and cure temperatures for IFP systems of methyl methacrylate with poly(methyl methacrylate) seeds and wilh the thermal initiator 2,2 -azobisisobutryonitrile. 

Savostyanov, V. S. Kritskaya, D. A. Ponomarev, A. N. Pomogailo, A. D. 1994. Thermally Initiated Frontal Polymerization of Transition Metal Nitrate Acrylamide Complexes, J. Poly. Sci. Part A Poly. Chem. 32, 1201-1212. 

Lewis and Volpert continue the discussion of the isothermal form of frontal polymerization in Chapter 5. Isothermal frontal polymerization is also a localized reaction zone that propagates but because of the autoacceleration of the rate of free-radical polymerization with conversion. A seed of poly(methyl methacrylate) is placed in contact with a solution of a peroxide or nitrile initiator, and a front propagates from the seed. The monomer diffuses into the seed, creating a viscous zone in which the rate of polymerization is faster than in the bulk solution. The result is a front that propagates but not with a constant velocity because the reaction is proceeding in the bulk solution at a slower rate. This process is used to create gradient refractive index materials by adding the appropriate dopant. 

M.R. Jr. (2004) Photoinduced cationic ring-opening frontal polymerizations of oxetanes and oxiranes. J. Poly. Sci., Part A Polym. Chem., 42, 1630-1646. 

With the development of DESs, they have been applied on some polymerization reactions that are carried out at relatively high temperatures. In these synthetic processes, the eutectic mixtures can act as true solvent-template reactant systems, and thus the DES is at the same time the precursor, the template, and the reactant medium for the fabrication of the desired polymeric material with a defined morphology or chemical composition. Monte and co-workers have reported i) the frontal polymerizations carried out in the eutectic mixtures ChCl/Acrylic acid and ChCl/Mac, " ii) the synthesis of poly(octane-diol-co-citrate) elastomers using eutectic mixtures of 1,8-octanediol and lidocaine at temperatures below 100 C (see Figure 20.3.4), " and iii) the synthesis of poly(acrylic acid)-carbon nanotube composites in the eutectic mixture ChCl/Aciylic acid. " ... 

The most pernicious convective instability occurs with monomers that produce a molten polymer at the front, such as n-butyl acrylate, styrene and methyl methacrylate. A Rayleigh-Taylor instability (2, 25 ), which also appears as fingers as the more dense molten polymer streams down from the reaction zone and destroys the front (Figure 16). The only currently available methods to study frontal polymerization with thermoplastics are to add a crosslinking monomer to produce a thermoset or to increase the viscosity with a viscosifier such as ultrafine silica gel (CAB-O-SBL). To prepare pure poly(n-butyl acrylate) frontally, Pojman et al resorted to performing the reaction under weightless conditions of a sounding rocket (26 ). 

Most work has been with free-radical systems but other chemistries can be used. Begishev etal. studied frontal anionic polymerization of e-caprolactam [18, 19], and epoxy chemistry has been used as well [20-23]. Mariani ctal. demonstrated frontal ring-opening metathesis polymerization [17]. Fiori et al. produced polyacrylate-poly(dicydopentadiene) networks frontally [24], and Pojman etal. studied epoxy-acrylate binary systems [25]. Polyurethanes have been prepared frontally [13,14, 26]. Frontal atom transfer radical polymerization has been achieved [16] as well as FP with thiol-ene systems [27]. Recent work has been done using FP to prepare microporous polymers [28-30], polyurethane-nanosilica hybrid nanocomposites [31], and segmented polyurethanes [32]. 

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