High monomer conversion

High Monomer Conversion Usually instantaneous Mp tends to fall with conversion increasing Dp. Dp s in the range 2-5 are common but can be much larger. 

Observed monomer concentrations are presented by Figure 2 as a function of cure time and temperature (see Equation 20). At high monomer conversions, the data appear to approach an asymptote. As the extent of network development within the resin advances, the rate of reaction diminishes. Molecular diffusion of macromolecules, initially, and of monomeric molecules, ultimately, becomes severely restricted, resulting in diffusion-controlled reactions (20). The material ultimately becomes a glass. Monomer concentration dynamics are no longer exponential decays. The rate constants become time dependent. For the cure at 60°C, monomer concentration can be described by an exponential function. 

A copper-based ATRP catalyst that is sufficiently stable and active can be used at very low concentrations. However, it is very important to mention that a copper(I) complex is constantly being converted to the corresponding copper(II) complex as a result of unavoidable and often diffusion-controlled radical termination reactions (k=l.0-4.0 x 109 M 1 s 1). Therefore, the deactivator (copper(II) complex) will accumulate as the reaction proceeds resulting in slowing down of the polymerization rate and limiting high monomer conversions. 

There are limitations for all types of LRP. The occurrence of irreversible bimolecular termination of propagating radicals becomes considerable under certain conditions high monomer conversion, polyfunctional initiators, high initiator concentration, and high targeted molecular weight (about >100,000). 

Concerning the use of ATRP with MIPs, the major limitation for this technique in the context of MIP synthesis is the small choice of monomers with suitable functional groups. Typical monomers used for molecular imprinting such as methacrylic acid (MAA) are incompatible, as they inhibit the metal-ligand complex involved in ATRP. With other monomers like methacrylamide [59] and vinylpyridine [60] it is difficult to achieve high monomer conversion. Template molecules also often carry functional groups that may inhibit the catalyst. All this seems to make ATRP not the best choice for molecular imprinting. Nevertheless,... 

The reason why the experimental values of particle number are somewhat lower than the theoretical values seems to be that the time where the number of polymer particles was measured is not at infinite but at only 1 hour after the start of polymerization. Figure 9 shows that the number of polymer particles increases with reaction time. The solid lines represent the theoretical values predicted by the Nomura and Harada model. However, since Nt= 0 when Mq= 0, there would be an optimum value of MQ where the number of polymer particles formed becomes maximum. Unfortunately, it is difficult at present to predict the optimum value of MQ theoretically because any reaction model cannot yet explain perfectly the kinetic behavior at high monomer-conversion range. Therefore, one cannot help determining, at present, the optimum value of MQ experimentally. Figures 7 and 8 also show that Eq.(40) roughly satisfies the experimental results. 

Guang Hui Ma et al. [83] prepared microcapsules with narrow size distribution, in which hexadecane (HD) was used as the oily core and poly(styrene-co-dimethyla-mino-ethyl metahcrylate) [P(st-DMAEMA] as the wall. The emulsion was first prepared using SPG membranes and a subsequent suspension polymerization process was performed to complete the microcapsule formation. Experimental and simulated results confirmed that high monomer conversion, high HD fraction, and addition of DMAEMA hydrophilic monomer were three main factors for the complete encapsulation of HD. The droplets were polymerized at 70 °C and the obtained microcapsules have a diameter ranging from 6 to 10 pm, six times larger than the membrane pore size of 1.4 p.m. 

It has been shown that the xanthate method could be successfully applied in order to graft hardwood high yield bisulfite pulp made of aspen. The optimum grafting condition, which would give almost 100% grafting efficiency (i.e.,100% formation of true graft and no homopolymer) and rather high monomer conversion of 80% are as follows ... 

In spite of the presence of Nd-clusters, partial alkylation and micro heterogeneities the number of active Nd-species seems to be fairly constant during the course of a polymerization. Otherwise neither consistent polymerization kinetics (particularly lst-order monomer consumption up to high monomer conversion) nor linear increases of molar mass during the whole course of the polymerization would be observed in so many studies. It therefore can be concluded that the fraction of active Nd as well as the number of active catalyst species are fixed either at an early stage of the polymerization or even prior to initiation of the polymerization. It can be speculated whether the fixation of the number of active species occurs during catalyst prefor-mation/activation or even during the preparation of the Nd compound. In contrast to this consideration Jun et al. report on the decay of active cen-... 

HIGH MONOMER CONVERSION POLYMERIZATION 2.5.1 Free Radical... 

This behaviour is explained assuming that, when the thiocarbonylthiyl radicals are immobilized on the resin, their termination efficiency is lower, thus favouring the propagation reaction. However, when high monomer conversions are reached, termination of the homopolymer chains by the polymer-bound thiocarbonylthiyl radicals becomes predominant and a high graft yield is obtained. 

At low monomer concentration (that is, at high monomer conversion), entry, propagation, and termination become diffusion-controlled processes. 

At high monomer conversion, the viscosity inside the polymer particles increases sharply and further polymerization becomes diffusion controlled. The particles are referred to as a glassy polymer and the kinetics for a zero-one system is no longer valid. To account for these changes, the propagation rate coefficient can be expressed as follows... 

The emulsion polymerization process has various advantages as compared to bulk or solution polymerization as the reaction proceeds at low viscosity. The low viscosity during the process allows adequate removal of heat of polymerization and the production of higher solid latexes in combination with high monomer conversion and short cycle time. This process is... 

Heat-resistant hyperbranched copolymers of VBC and CMI have also been synthesized by ATRP. Under identical polymerization conditions and after the same reaction time, high monomer conversions occurred near the equimolar feed composition, indicating the formation of charge transfer complexes between VBC (electron-donor) and maleimide (electron-acceptor). As expected, the Tg of the copolymer increased with an increasing content of maleimide in the feed [266]. 

---