Initiator and catalyst concentrations

L/mol, respectively. We have used these optimized values for comparing model predictions with experimental data obtained from adiabatic polymerization to study the effect of initial polymerization temperature and effects of initiator and catalyst concentrations [24]. 

Kuster, B. F. M. and Vanderbaan, H. S., Dehydration of D-fructose (formation of 5-hydroxymethyl-2-furalde-hyde and levulinic acid). 2. Influence of initial and catalyst concentrations on dehydration of D-fructose. Carbohydrate Res 1977, 54 (2), 165-176. 

Polyol Functionality in Relation to Method of Preparation— Initiator and Catalyst Concentrations (All Reactions 4hrs. reaction time Ratio of Propylene Oxide Lignin 10 1 Temperature 220-250°C)... 

The initiators which are used in addition polymerizations are sometimes called catalysts, although strictly speaking this is a misnomer. A true catalyst is recoverable at the end of the reaction, chemically unchanged. Tliis is not true of the initiator molecules in addition polymerizations. Monomer and polymer are the initial and final states of the polymerization process, and these govern the thermodynamics of the reaction the nature and concentration of the intermediates in the process, on the other hand, determine the rate. This makes initiator and catalyst synonyms for the same material The former term stresses the effect of the reagent on the intermediate, and the latter its effect on the rate. The term catalyst is particularly common in the language of ionic polymerizations, but this terminology should not obscure the importance of the initiation step in the overall polymerization mechanism. 

The initial rate of polymerization of methyl methacrylate initiated by chromium allyls (12) in toluene showed identical dependences on monomer and catalyst concentrations, as Zr(benzyl)4 initiated polymerization of styrene. Some data for the monomer dependence are shown in Fig. 14. 

The stereoregularity of butadiene based polymers prepared in cyclohexane with Ba-Mg-Al catalysts depends on polymerization temperature and catalyst concentration. Trans-1,4 content increases nonlinearly with a decrease in polymerization temperature over the range of 80° to 30°C (Figure 11) and/or a decrease in the initial molar ratio of butadiene to dialkyl-magnesium from 3400 to 400 (Figure 12). For polybutadienes prepared with relatively large amounts of catalyst at 30°C, the trans-1,4 content approaches a limiting value of about 907.. 

Depending on the monomer, one needs to adjust the components of the system as well as reaction conditions so that radical concentrations are sufficiently low to effectively suppress normal termination. The less reactive monomers, such as ethylene, vinyl chloride, and vinyl acetate, have not been polymerized by ATRP. Acidic monomers such as acrylic acid are not polymerized because they interfere with the initiator by protonation of the ligands. The car-boxylate salts of acidic monomers are polymerized without difficulty. New ATRP initiators and catalysts together with modification of reaction conditions may broaden the range of polymerizable monomers in the future. 

We have used an autocatalytic model originally proposed by Malkin et al. [62]. Bolgov et al. [61] found that the originally proposed autocatalytic model [62], which was valid for equal concentration of initiator and catalyst during the anionic polymerization of caprolactam, can be modified for unequal concentration of the initiator and catalyst by an autocatalytic equation of type... 

The existing data show that the rate of cationic homopolymerization is increased in most cases by the application of an electric field. Fig. 1 shows typical results, in which RPE and Rpo are the initial rates of polymerization Rp) with and without electric field, respectively (5). Figs. 2 and 3 represent the dependences of Rp on monomer and catalyst concentrations, respectively (9,10). It is evident that Rt is increased under an electric field, whereas the concentration dependences (the reaction orders) are not influenced. It is likely therefore that the rate enhancement is not due to a new reaction mechnism but rather to an increase... 

With the neutral [(RCN)2PdCl2] pro-catalyst system (Fig. 12.3, graph iv), computer simulation of the kinetic data acquired with various initial pro-catalyst concentrations and substrate concentrations resulted in the conclusion that the turnover rates are controlled by substrate-induced trickle feed catalyst generation, substrate concentration-dependent turnover and continuous catalyst termination. The active catalyst concentration is always low and, for a prolonged phase in the middle of the reaction, the net effect is to give rise to an apparent pseudo-zero-order kinetic profile. For both sets of data obtained with pro-catalysts of type B (Fig. 12.3), one could conceive that the kinetic product is 11, but (unlike with type A) the isomerisation to 12 is extremely rapid such that 11 does not accumulate appreciably. Of course, in this event, one needs to explain why the isomerisation of 11 now proceeds to give 12 rather than 13. With the [(phen)Pd(Me)(MeCN)]+ system, analysis of the relative concentrations of 11 and 13 as the conversion proceeds confirmed that the small amount of... 

However, a recent kinetic study188 has shown unequivocally that chain initiation proceeds via the usual metal-catalyzed decomposition of the hydroperoxide. Thus, the rate of initiation of the autoxidation of cumene was, within experimental error, equal to the rate of production of radicals in the (Ph3P)4Pd-catalyzed decomposition of tert-butyl hydroperoxide in chlorobenzene at the same temperature and catalyst concentration. Moreover, long induction periods were observed (in the absence of added tert-butyl hydroperoxide), when the cumene was purified by passing it down a column of basic alumina immediately prior to use. Autoxidation of cumene purified by conventional procedures showed only short induction periods. These results further demonstrate the necessity of using highly purified substrates in kinetic studies. 

Overberger, Yuki, and Urakawa (50) studied the polymerization of methacrylonitrile with potassium amide in liquid ammonia at — 78° and found that the molecular weight was independent of monomer concentration and catalyst concentration. It was shown that trace quantities of water had no effect on the degree of polymerization, but water in excess gave lower yield and degree of polymerization. Overberger, Pearce, and Mayes (49) found that lithium amide was ineffective as initiator for methacrylonitrile in liquid ammonia at — 78°. 

DP is the average number of monomer (or repeat) units per polymer chain and so is directly related to molecular weight (or viscosity). This relationship shows that we must have control over variables that have a significant effect on propagation, chain transfer, and termination to achieve the desired polymer properties. What are these variables They are the same as those we have discussed throughout this chapter temperature, reactant monomer concentrations, concentrations of chain transfer agents or other impurities that affect polymerization, initiator or catalyst concentration, residence time, etc. 

Initial attempts to obtain Q8M8 using dry toluene as the solvent afforded a product with incomplete substitution of the starting compound Q8M8, as evidenced by a peak in the NMR spectrum attributed to residual Si - H groups. Increased reaction time, temperature and catalyst concentration did not enable the reaction to be completed. In an effort to obtain complete substitution, dry tetrahydrofuran was used as a solvent for the reac-... 

Recent work on the dimerisation of 1,1-diphenylethylene by aluminium chloride produced conclusive evidence that direct initiation does not lead to the total ctmsump-tion of the catalyst. This excellent piece of research diowed that about 2.5 aluminium atoms are needed to give rise to one carbenium ion. Similar indications were reported by Kennedy and Squires for the low temperature polymerisation of isobutene by aluminium chloride. They underlined the peculiar feature of limited yields obtained in flash polymerisations with small amounts of catalyst. The low conversions could be increased by further or continuous additions of the Lewis acid. Equal catalyst increments produced equal yield increments It was also shown that introductions of small amounts of moisture or hydrogen chloride in the quiescent system did not reactivate the polymerisation. This work was carried out in pentane and different purification procedures for this solvent resulted in the same proportionality between polymer yield and catalyst concentration. Experiments were also performed in which other monomers (styrene, a-methylstyrene, cyclopentadiene) were added to the quiescent isobutene mixture. The polymerisation of these olefins was initiated but limited yields were again obtained. Althou the full implications of these observations must await more precise data, we agree with the authors interpretation that allylic cations formed in the isobutene polymerisation, while incapable of activating that monomer, are initiators for the polymerisation of the more basic monomers added to the quiescent mixture. The low temperature polymerisation of isobutene by aluminium chloride was also studied... 

Buffers which have been widely employed are Tris acetate, pH 7.8 and Tris phosphate, pH 7.7 (Loening 1968). (Tris hydrochloride should be avoided because of hypochlorite formation at the anode.) Buffers may be made up as concentrated stock solutions and mixed in the correct proportions with acrylamide, agarose, initiator and. catalyst. 

All of the systems discussed above employed an ATRP condition with high initiation efficiency ([PC]t [Initiator]o) and low polydispersity of primary chains (Mw/Mn < 1.1). ATRP allows further control of the relative initiation rate of initiators and the polydispersity of primary chains via rationally adjusting the structure of initiators, the solubility of catalysts, and the concentration of deactivators. Based on Eq. 3., it is expected that by fixing the initial molar ratio of cross-linker to initiator and their concentrations, a reduced initiation efficiency and/or a broadening distribntion of primary chains would decrease the onset of the experimental gel point based on monomer conversion and accelerate the gelation process. 

The determination of peroxides has two goals one is to monitor peroxide concentration used as initiator and catalysts and the other is to detect formation of hazardous peroxides formed as autoxidation products in ethers, acetals, dienes, and alkylaromatic hydrocarbons. A sample is dissolved in a mixture of acetic acid and chloroform. The solution is deaerated and potassium iodide reagent is added and let to react for 1 h in darkness. The iodine formed in reaction is measured by absorbance at 470 nm and result calculated to active oxygen in the sample. The method can determine hydroperoxides, peroxides, peresters, and ketone peroxides. Oxidizing and reducing agents interfere with flic determination. 

It is important to note that a local polymerisation process has a macroscopic effect on the average MW and MWD dependence on the initial concentration of reactants. The polymer MW, according to the process kinetics chart [27], is fully determined by the chain-to-monomer transfer and therefore, predicts the independence of the MW and MWD on monomer and catalyst concentrations [21]. Catalyst and monomer solutions are usually introduced irregularly into the reactor and do not have enough time for sufficient mixing with the reacting blend due to high polymerisation reaction rates. It favours further destabilisation of the reactor, wider MWD and lower MW of the final product. 

Figure 1.1 demonstrates the diffusion model-based fields of temperature, as well as the monomer and catalyst concentrations during the cationic polymerisation of isobutylene. It is clear that the process and experimental behaviour are close, mainly in the catalyst input areas where it is mixed with the monomer solution. Isobutylene polymerisation is similar to the behaviour of fast chemical processes the temperature and reaction rate in a reaction zone depend on the initial concentration of reactants, the value and the factor K, which is the heat transfer through the reactor wall Kjjt. Although the rate of isobutylene polymerisation is maximal within the catalyst input areas, the reaction occurs sufficiently far in the axial direction to result in a change of output characteristics and polymer properties (molecular characteristics) when moving away from catalyst input area. 

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