Critical monomer conversion

The first three benefits are a direct consequence from the extremely low tendency of the Nd-catalyst to form branches and gel. Because of this remarkable feature, Nd-catalysts allow monomer conversions up to 100%. Therefore, the polymerization reaction does not have to be shortstopped below a critical monomer conversion in order to avoid gel. In addition, polymerization temperature does not have to be controlled within a well-defined temperature range. As the maximum polymerization temperature (at complete monomer conversion) can be as high as 120 °C the polymerization process can be performed in a fully adiabatic manner. In this case energy costs for cooling and for the removal of low molar mass residuals can be very low. Another benefit of the Nd-catalyst is the low tendency to catalyze the Diels-Alder dimerization of BD to vinyl cyclohexene. 

As a result of the above mechanism, the polymerizing VCM droplet loses its viscous characteristics at relatively low monomer conversions, while at larger monomer conversions (i.e., X > 30%) it behaves like a rigid sphere due to the presence of the continuous polymer skeleton. In fact, above a critical monomer conversion (i.e., x 30%) the volume contraction of the polymerizing particles stops, which partially explains the appearance of internal particle porosity. Note that the polymer density is approximately 40% higher than the monomer density. 

Three main parameters were used to evaluate the efficiency of the polymerization, namely monomer conversion (Cmma), initiation efficiency of the reaction (/ = Mn theo/3 n,SEc), and polydispcrsity index (PDI). These results are depicted in Fig. 2. It is obvious that the Cu(I)-catalyzed systems are more effective than the Fe(II)-catalyzed systems under the studied conditions. It was concluded that a bipyridine based ligand with a critical length of the substituted alkyl group (e.g., dHbpy) shows the best performance in Cu(I)-mediated systems. Besides, Cu(I) halide-mediated ATRP with 4,5 -Mbpy as the ligand and TsCl as the initiator was better controlled than that with dMbpy as the ligand, and polymers with much lower PDI values were obtained in the former case. 

Figures 6, 7 and 8 show experimental verification of Eq.(40) in batch emulsion polymerization of styrene ( 14). The number of polymer particles was measured by electron micrscopy, not at finite but at 1 hour after the start of polymerization. Figure 6 represents the effect of lowering the initial monomer concentration, Mq on the number of polymer particles formed at fixed initial initiator and emulsifier concentrations. The number of polymer particles formed is constant even if M is lowered to the critical value Mc. This is because normal°condition that micelles disappear before the disappearance of monomer droplets is satisfied in the range of monomer concentration above Mc. The value of Mc can be calculated by the following equation obtained by equating XMc, the monomer conversion where micelles disappear, to XMc2, the monomer conversion where monomer droplets disappear.

The critical properties of GMC-II produced from both laboratory and pilot systems are listed in Table 5. In comparing a 0.055-kg batch (62.8% monomer conversion) from laboratory synthesis and a 1.16-kg batch (54.1% monomer conversion) from the pilot synthesis, the material properties responsible for lithographic response are nearly identical. 

Order of Addition of Benzoyl Peroxide. Block polymerization proceeds best when polar monomers are added before the peroxide. When benzoyl peroxide is added before the monomers, monomer conversion is reduced drastically. The longer the time lapse between addition of peroxide and addition of monomers, the more dramatic is the effect (Table III). When monomers are added first, the time lapse before addition of peroxide is not critical. 

Figure 16. Effect of monomer conversion on critical surface tension of polybutadiene grafted with styrene, doubly precipitated...

Crosslinking polymerization of comonomers in a diluent diiiers from the above model in that the monomers themselves represent a good solvent for the emerging polymer, but their concentration, as well as that of the polymer and of the crosslinks, steadily changes with monomer conversion and remains unknown. These factors dramatically limit the practical usefulness of Eq. [3.6]. Still, it is worthwhile to analyze which of these four parameters are really critical for the phase separation to occur during cross-linking copolymerization. 

Statistical methods were developed for the prediction of gelation. These actually predict gelation at a lower level than does the Carothers equation shown above. As an example we can use a reaction of three monomers. A, B, and C. We further assume that the functionality of two monomers, /a and, is equal to two, while that of/c is greater than two. The critical reaction conversion can then be written as... 

Isodesmic polymerization is conceptually the most simple type of suprcunolecular polymerization. Through the cotrrse of an isodesmic supramolecular polymerization, a linear decrease in the Gibbs free energy of the polymerization is observed as a funaion of monomer conversion to polymer (p) from zero to one (p = 0 —> 1). This verifies that the affinity of a monomer to the growing polymer chain is irrespective of the length of that polymer chain. The absence of a critical concentration or... 

Specifically for the PS bead suspension process, Villalobos et al. [9] reported that the end of the first stage occurred at approximately 30% monomer conversion, corresponding to a critical viscosity of about 0.1 Pa s. They also found that the second stage extended up to about a 70% monomer conversion. In the vinyl chloride monomer (VCM) powder polymerization, it has been shown that, at monomer conversions around 10-30%, a continuous polymer network is commonly formed inside the polymerizing monomer droplets that significantly reduces the drop/particle coalescence rate [10]. CeboUada etal. [11] reported that the PSD was essentially estabhshed at monomer conversions of about 35-40% (i.e., end of the second stage). 

Details are given of a non-steady-state operation for controlling latex particle size distribution by using a continuous emulsion polymerisation of vinyl acetate. The experiment was conducted in a continuously stirred tank reactor under conditions below the critical micelle concentration of the emulsifier. The mean residence time was switched alternately between two values in the nonsteady-state operation to induce oscillations in monomer conversion in time. The effect of the switching operation on particle size distribution is discussed. 13 refs. 

The enzymatic activity strongly depends on temperature. In case of reaction using Novozyme-435 in toluene, the polymerization rate of e-CL was the fastest at 90 C (7). Since Novozyme-435 can be used below 140 C without deactivation in scCOa as described above section, the reactivity was investigated as a function of temperature below 100 °C. The initial slope of the percent monomer conversion versus time plots was used to calculate the apparent rate constant ( app)- Figure 6 shows k pp depending on reaction temperature. The reaction provided the fastest rate at 80 °C, which is almost the same as the reaction in toluene. Even for the reaction condition at 20 °C lower than critical temperature, the enzyme catalyzed the polymerization of e-CL although the reaction rate was very slow. This indicates that super critical state is not required for polymerization of e-CL catalyzed by Novozyme-435. The dielectric constants at 10 MPa take between 1.54 and 1.12 at 10 C and 80 C, respectively. This range is not so wide that the reactivity wouldn t be affected. Rather the reaction temperature would be a dominant factor to enhance the polymerization. 

The question of end-group fidelity is critical to evaluate the efficiency of any jROP catalyst system. All polymer samples were characterized by NMR spectroscopy (Fig. 28.5), which attested to the presence of the expected termini, -C(CH3)(H)0H together with HC=CCH20-C(0)C(H)(CH3)-, Ci5H 0-C(0)C(H)(CH3)-, or Bn0-C(0)C(H)(CH3)-. MALDI-ToF mass spectrometry corroborated the identity of the polymer end groups and established that intermolecular transesterification reactions occurred at high monomer conversion. 

It was stated at the beginning of this topic that heat management and safety aspects are important for the selection of the most suitable polymerization process. As the effectiveness of heat transfer is largely governed by the viscosity of the reaction mixture it is instructive to compare typical viscosities versus monomer conversion plots for the various polymerization processes that have been described above (Figure 5.3.13). Obviously, the most critical process with respect to heat removal is polymerization in-substance, while suspension and emulsion polymerizations show only very small changes in viscosity and thereby allow proper heat transfer even at high monomer conversions. 

Other than temperature and pressure, some of the more critical state variables during emulsion polymerization are monomer conversion, particle size and molecular weight. The bulk of this paper will be organized around discussions of the continuous monitoring of the above properties, with the exception of molecular weight, since, at the present time, continuous measurement of the molecular weight of a polymer does not appear to be feasible. 

Emulsion polymerization of vinyl chloride is initiated by a water-soluble initiator such as potassium persulfate. Initially in the reactor, monomer droplets are dispersed in the aqueous phase (continuous phase) containing initiator and surfactant (emulsifier). As the reactor content is heated, the initiator decomposes into free radicals. When the surfactant concentration exceeds the critical micelle concentration (CMC), micelles are formed. Free radicals or oligomers formed in the aqueous phase are then captured by these micelles. Vinyl chloride monomer is slightly soluble in water. As the monomer dissolved in water diffuses into micelles containing radicals, polymerization occurs. With an increase in monomer conversion in the polymer particles, separate monomer droplets become smaller and eventually they disappear. The monomer concentration in polymer particles is constant as long as liquid monomer droplets exist. The rate of emulsion polymerization is represented by... 

The pronounced decrease of (kt) in the TD regime is associated with the occurrence of the so-called gel-effect. " Also known as the Trommsdorff, Norrish-Smith or Norrish-Trommsdorff effect, this effect can cause problems within both an industrial and scientific context ranging from a product mixture to reactor explosion, due to its exothermic nature. " " Increasing polymer content induces overlap of polymer chains and decreases the mesh-size in between the polymer chains beyond a critical limit. As a consequence, TD may become the rate-determining step in Scheme 1.21 for the majority of macroradicals, thus (kt) decreases by orders of magnitude in some cases. It is important not to confuse the gel effect with the auto-acceleration that is observed when a polymerization is carried out under non-isothermal conditions, so that the reaction temperature increases with increasing monomer conversion, due to the exothermic nature of the polymerization reaction. The gel effect is observed under isothermal reaction conditions. The cause of the gel effect has been discussed extensively and various theories have emerged which can explain all or part of the experimental data (excellent reviews on the topic can be found in ref. 150 and 151). 

AH iagredients in the polymerization recipe are not always added at the beginning of the process. For example, better latex stabHity can sometimes be achieved by starting with only part of the emulsifier, saving the rest for later addition. Sometimes a portion of the modifier is held out for late addition to aHow higher final conversion without premature consumption of aH of it. OccasionaHy, if a low acrylonitrile product is the objective, part of the acrylonitrile monomer wiH be saved for late addition so that a chemically more uniform copolymer is produced, which can sometimes enhance properties in critical appHcations. 

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