Limiting monomer conversion

Fradet and co-workers reported on the thermal ROP of y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolac-tam (compare Scheme 7). Both monomers were polymerized in bulk at 250 °C. In both cases, the authors observed that monomer conversion was limited and did not exceed a plateau value of 0.53 (after a reaction time of 3 h) or 0.57 (after 30 min) for y-carboxyethyl- s-caprolactam and y-aminoethyl- s-caprolactam, respectively. The limiting monomer conversion was ascribed to ring-chain equilibria in both cases. The polymerizations could be accelerated by the addition of polyamidation catalysts, such as phosphorous and hypo-phosphoric acids, but no change of the maximum monomer conversion was observed. In a control experiment, 4-aminoethyl-1,7-heptanedioic acid was polymerized via thermal polymerization however, this only resulted in a low molecular mass compound. This was attributed to the much faster rate of the intra- versus the intermolecular amidation reaction. Cross-linked material was obtained, when both monomers were heated for a prolonged time, and loss of NH3 was observed, which was ascribed to amidine formation and deamination. 

Rodriguez R, Barandiaran MJ, Asua JM. Polymerization strategies to overcome limiting monomer conversion in sUi-cone-acrylic miniemulsion polymerization. Polymer 2008 49 691-696. 

Figure 8.3 shows the profiles of the monomer conversion as a function of time for the experiments with different initiator concentrations. Other experimental conditions were kept constant in this series of polymerizations. The only adjustable parameter is the termination rate constant at the very beginning of polymerization and it has a reasonable value of 7 x 10 liter moT s As expected, the polymerization rate increases with increasing initiator concentration. The kinetic model predicts the polymerization rate data reasonably well. The predicted limiting monomer conversion is lower than the experimental data. 

Polymerization. Chloroprene is normally polymerized with free-radical catalysts in aqueous emulsion, limiting the conversion of monomer to avoid formation of cross-linked insoluble polymer. At a typical temperature of 40°C, the polymer is largely head-to-taH in orientation and trans in configuration, but modest amounts of head-to-head, cis, 1,2, and 3,4 addition units can also be detected. A much more regular and highly crystalline polymer can be made at low temperature (11). Chloroprene can also be polymerized with cationic polymerization catalysts, giving a polymer with... 

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. 

Fig. 38. A Degradation experiments with pregel polymers isolated prior to the onset of macrogelation in 1,4-DVB polymerization [209] Variation of Mw ( ) and dz (O) with the time of ultrasonic degradation. The polymer sample was prepared at 5 g/100 mL monomer concentration and its initial Mw was 2.2 X106 g/mol. The dotted horizontal line shows Mw of zero conversion polymers ( individual microgels ). B Variation of Mw with the polymerization time t and monomer conversion x in 1,4-DVB polymerization at 5 g/100 mL monomer concentration. The region 1 in the box represents the limiting Mw reached by degradation experiments. [Reprinted with permission from Ref. 209,Copyright 1995, American Chemical Society].

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). 

Fig. 9. Kinetic gelation model prediction (heterogeneous limit),-, of monomer conversion vs...

Afterwards, the PP powder is fluidized by a mixture of ethylene, propylene and inert gas (propane) in the fluid bed reactor, FBR, which is cooled by convective gas flow. The monomer conversion per gas circulation is limited to a few percent, because of the adiabatic temperature rise of more than 10 K per % conversion. Neglecting the heat loss through the reactor wall and through the product withdrawal, and assuming a well mixed reactor, the heat removal capacity of a steady-state FBR limits the productivity and can be expressed by... 

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,... 

In traditional liquid solvents, the polymerization reaction rates are often limited by the local increase in viscosity during the process, as this lowers the mass transfer rate of the monomer to the reaction site. A lower viscosity and a higher diffusion coefficient in SCFs each contribute to overcome this limitation, however, allowing the polymerization rate to be significant up to high value of monomer conversion. 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

In conclusion, the number of reports on the impact of the monomer/cata-lyst-ratio ( M/ Nd) on Mn are rather limited. Studies in which Mn was determined are only available for binary catalyst systems of the type NdCl3 3TBP/TIBA and for allyl Nd compounds. The data reported on these catalyst systems are in full compliance with requirement No. 3 for a living polymerization Linear dependence of Mn on monomer/catalyst-ratio at constant monomer conversion . Comparable studies with Nd carboxylates and other Nd precursors are missing. 

In the literature many studies on LDPE tubular reactors are found (2-6).All these studies present models of the tubular reactor, able to predict the influence, on monomer conversion and temperature profiles, of selected variables such as initiator concentration and jacket temperature. With the exception of the models of Mullikin, that is an analog computer model of an idealized plug-flow reactor, and of Schoenemann and Thies, for which insufficient details are given, all the other models developed so far appear to have some limitations either in the basic hypotheses or in the fields of application. 

Gel point. The Carothers equation can also be used to estimate the conversion needed to reach the so-called gel point in condensation polymerization involving monomers with functionalities higher than 2. The gel point is defined as the state of conversion at which gel formation caused by crosslinking begins to become apparent. With the assumption that this occurs when practically all molecules of the limiting monomer have reacted, the requisite fractional conversion of functional groups can be estimated with a rearranged form of the Carothers equation ... 

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