Polymerization time

It will be remembered from Sec. 5.3 that a progressively longer period of time is required to shift the reaction to larger values of p. In practice, therefore, the effects of side reactions and monofunctional reactants are often not compensated by longer polymerization times, but are accepted in the form of lower molecular weight polymers. 

Fig. 4. Activity vs polymerization time. Polymerization occurs in hexane at 343 K (70°C) and 0.7 MPa (7 bat) with a superactive third-generation catalyst...

Although examples in the Kureha patent Hterature indicate latitude in selecting hold times for the low and high temperature polymerization periods, the highest molecular weight polymers seem to be obtained for long polymerization times. The addition of water to PPS polymerizations has been reported to effect polymer stabilization (49), to improve molecular weight (50,51), to cause or enhance the formation of a second Hquid phase in the reaction mixture (52), and to help reprecipitate PPS from NMP solution (51). It has also been reported that water can be added under pressure in the form of steam (53). 

To provide for suitable timing of the pH reduction over the wide range of temperatures that may be encountered, the instant films may use polymeric timing layers in which permeabiUty to alkaU varies inversely with temperature. In the integral films, where all components are retained within the film unit after processing and the moisture content remains high for several days, care must be taken to avoid materials that could migrate or initiate unwanted reactions even at reduced pH. 

Neutralization to terrninate processing was effected by the polymeric acid layer of the covet sheet the onset of this reaction was controlled by the rate of permeation of the overlying polymeric timing layers. MobiUty of the transferred dyes was also reduced by reaction with a mordant contained in the image-receiving layer. A development inhibitor released from one of the timing layers by the alkaline hydrolysis of its precursor assisted in restraining further development and consequent additional dye release. 

The catalysts are primarily DCPD-soluble derivatives of tungsten and molybdenum and the activators are aluminum alkyls (63—64). Polymerization is accompHshed by mixing equal amounts of Hquid DCPD (at >32° C), one part of which contains the catalyst and the other of which contains the activator. The mixture is rapidly injected into a mold, where the polymerization takes place. Polymerization times are from under 30 seconds to several minutes, depending on the size of the part, mold temperature, and modifiers added to the polymerizate. 

In the multistep swelling and polymerization method, polymerization time, temperature, and initiator concentration have marginal effects only. The ratio of monovinyl to divinyl monomer in the polymerization mixture, along with the composition of the porogenic systems, is known to be the most important factor in controlling the ultimate macroporous structure. The average molecular... 

Figure 1 The typical tendencies for the variation of monomer conversion by the polymerization time and for the variation of polymerization rate by the monomer conversion in the ideal emulsion polymerization process.

All these effects increase the overall polymerization rate and decrease the degree of polymerization. The effect of polymerization temperature on the variation of monomer conversion with the polymerization time is exemplified in Fig. 8 for the emulsion polymerization of styrene. 

According to the other kinetic model proposed for the soapless emulsion process, the growing macroradicals may also form micelle structures at earlier polymerization times since they have both a hydrophilic end coming from the initiator and a hydrophobic chain [74]. 

Ftgure 11 The electron micrographs of the final products and the variation of the monomer conversion with the polymerization time at different initiator concentrations in the dispersion polymerization of styrene. Initiator concentration (mol%) (a) 0.5, (b) 1.0, (c) 2.0. The original SEM photographs were taken with 2600 x, 2000 x, and 2600 x magnifications for (a), (b), and (c), respectively, and reduced at a proper ratio to place the figure. (From Ref. 93. Reproduced with permission from John Wiley Sons, Inc.)... 

We have also examined the effect of stabilizer (i.e., polyacrylic acid) on the dispersion polymerization of styrene (20 ml) initiated with AIBN (0.14 g) in an isopropanol (180 ml)-water (20 ml) medium [93]. The polymerizations were carried out at 75 C for 24 h, with 150 rpm stirring rate by changing the stabilizer concentration between 0.5-2.0 g/dL (dispersion medium). The electron micrographs of the final particles and the variation of the monomer conversion with the polymerization time at different stabilizer concentrations are given in Fig. 12. The average particle size decreased and the polymerization rate increased by the increasing PAAc concentra-... 

Figure 4 (a) Polymerization time versus degree of swelling (in toluene) curves for styrene-PEG crosslinked copolymers initiated by macroinimers ( ) MAIM-200, ( ) MAIM-400, (O) MAIM-600, (A) MAIM-1000, and ( ) MAIM-1500. (b) Polymerization time versus degree of swelling (in water) curves for styrene-PEG crosslinked copolymers initiated by ( ) MAIM-1500 and ( ) MAIM-400. Source Ref. 50. 

Fig. 4. Change of polymerization rate and number of propagation centers with polymerization time. Catalyst Cr ( —CjH6)j/Si02 ethylene pressure 6 kg/cm2, temperature 50°C. The symbols A, A, X, O correspond to different polymerization runs. Arrows show the moment of injection of C140.

Efficiencies Related to Composite Films Electrogenerated from 1 M Pyrrole plus 10 2 M Sodium Polyacrylate Aqueous Solution by Polarization of a 1-cm2 Platinum Electrode at 800 mV for Different Polymerization Times... 

The advantage of suspension processes over mass processes is the excellent temperature control that can be obtained through the suspending medium, water. This allows for rapid heat removal and shorter polymerization times. It reduces or eliminates hot spots or heat-kicks characteristic of mass reactors. It also allows the polymerization to be driven very close to completion so that no devolatilization step is normally required. 

Figure 7. Monomer conversion vs, polymerization time in the helical tubular reactor laminar flow regime...

The parameter Z in Equation 17 is equivalent to the fractional activity at an infinite polymerization time. B and v can also be Interpreted in a similar way through Equation 18, where a has the same meaning as Z. 

It is worth noticing that in the first spectra of the series shown in Pig. 4 the two methylenic bands at 2920-2851 cm appear slightly asymmetric, with a broad tail at higher frequencies. This feature becomes less evident at increasing polymerization times, since the intensity of the CH2 bands increases. At least two different explanations can be advanced, (i) Methylene groups next to a low valent chromium would be influenced by the presence of the chromium itself and thus exhibit a distinct difference in the stretching frequency with respect to that of a methylene group in the middle of the... 

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