Polymerization period

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). 

In contrast to the results from previous studies with related monomers, at low temperatures, from —78 to —40°C, no polymerization reaction apparently occurred. However, if the polymerization reactions initiated with either BF3 0Et2 or SnCl were carried out at 0°C and the system was allowed to attain ambient temperature (20°C) over a period of 24 h, or if initiation was done directly at ambient temperature and stirring was continued for 24 h, good yields of low molecular weight polymers, which were insoluble in methanol, were obtained. The latter procedure was found to be the most effective, but at 0°C only viscous residues resulted. However, for shorter polymerization periods, even at 20°C, no products insoluble in methanol were obtained, and the monomer was recovered virtually unreacted. 

Martin and coworkers tried to prepare carbon tubes from the carbonization of polyacrylonitrile (PAN) in the channels of anodic oxide film (10). A commercially available film with a pore diameter of 260 nm was immersed in an aqueous acrylonitrile solution. After adding initiators, the polymerization was carried out at acidic conditions under N2 flow at 40°C. The PAN formed during the reaction was deposited both on the pore walls and on both sides of the film. Then the Film was taken from the polymerization bath, followed by polishing both faces of the film to remove the PAN deposited on the faces. The resultant PAN/alumina composite film was heat-treated at 250°C in air, and then it was heat-treated at 600°C under Ar flow for 30 min to carbonize the PAN. Finally, this sample was repeatedly rinsed in I M NaOH solution for the dissolution of the alumina film. The SEM observation of this sample indicated the formation of carbon tubes with about 50 xm long, which corresponds to the thickness of the template film. The inner structure of these tubes was not clear because TEM observation was not done. The authors claim that it is possible to control the wall thickness of the tubes with varying the polymerization period. 

It may be noticed that during the initial polymerization period (adjust-... 

Abbreviations N, number of particles during ibe constant rate period and areas occupied b> emulsifier molecule in the micelle and saturated particle surface r,. and Tp,.ph, duration of the constant rate period and polymerization period up to disappearance of tbe monomer phase i, polymerization period S, overall particle surface. 

Figure 2. Thickness (d) of styrene polymerized film as a function of polymerization period (t)...

Figure 6. The I—V characteristic of the single probe without and with the styrene film cover t is the polymerization period the curve fort = 0 was obtained with no apparent contamination the probe measurements were performed in the dc glow discharge in nitrogen gas at 5 torr and 12 mA the frequency of sawtooth pulses applied to the probe was 47.6 Hz and their peak values were 26 V and...

We have recently shown that solid state polymerizations can also be carried out via plasma initiation ( ). In that work, 1, 3,5-trloxane and 1,3,5,7-tetraoxane were used as monomer crystals. Highly crystalline polyoxymethylene were obtained using either monomer. However, if the monomers were dissolved in an appropriate solvent, such as cyclohexane, then no polymer was formed with plasma initiation. These observations are the reverse of those for AM and MAM, where plasma initiated polymerizations in solution appear to proceed satisfactorily during homogeneous post-polymerization periods, but not in the bulk crystalline state. The unresolved question is then if water molecules may have dissociated in the plasma in highly active radical species, perhaps OH or H, to promote efficient initiation. 

We note in Table 1 that although methyl methacrylate polymerizes readily upon plasma initiation, ethyl methacrylate (EMA) and n-butyl methacrylate (BMP) gave only low yields (1- ) of polymer regardless of the length of post-polymerization period. Apparently a small amount of polymer was formed during the plasma initiation period (up to 60 seconds), and no further polymerization took place when plasma was turned off. Parallel experiments of dark polymerization (no plasma initiation) under the same conditions also resulted in no polymer formation. 

Figure 6. Mass flowmeter trace of run 2.b. in Table, Key A to B, solvent saturation with 60 psig CsHj, B to D, polymerization period affording the catalyst...

The overall rate equation, describing the steady-state polymerization period is thus given by... 

It is apparent from the above results that an asymmetric cation exchange membrane, of which one surface of the membrane has a polypyrrole layer, can be prepared by contacting one surface of the ferric ion form cation exchange membrane with an aqueous pyrrole solution. Figure 5.9 shows a cross-section of a cation exchange membrane having a polypyrrole layer (polymerization period 4 h).57 The dark part of the photograph is where polypyrrole exists. Furthermore, EPMA (electron probe micro analysis) of the cross-section of a similar membrane (polymerization period 1 h) also reveals a thin layer of polypyrrole on the membrane surface.57... 

To a 4-liter, stirred reactor at 8°C are added 2 liters of deaerated water, 8 ml of Triton X-100, 300 gm (5.6 mole) 1,3-butadiene, 67.0 gm (1.0 mole) of cyclopentadiene, and 80 gm (1.25 mole) of sulfiir dioxide. The reaction is polymerized at 80°C after adding 2 ml of 20% aqueous ammonium persulfate solution. Ehiring the polymerization additional initiator and reactants are cautiously metered into the reactor 0.18 gm/min (initiator), 1.0 gm/min (cyclopentadiene), and 1.7 gm/min (sulfur dioxide). The temperature of the polymerization is approximately 8°-12°C for the 50 min polymerization period. The reaction is stopped by discharging the reactor into 8 liters of methanol. The product is filtered, washed with water several times to remove emulsifier, and then washed with methanol to give 207 gm of a white polymer having 36 mole% cyclopentadiene derived units in the polymer and having an inherent viscosity of 0.75 dl/gm in 1 1 p-chlorophenol m-cresol at 25°C. 

Scheme 7 Plausible transformation of metathesis site into polymerization site from induction period to polymerization period on the Phillips catalyst...

During the first stage, primary radicals formed by the thermal fragmentation of the initiator molecules rapidly react with monomer to produce the first polymer chains. During this early polymerization period, the polymer concentration is below its solubihty hmit in... 

Control over the quantity and type of catalyzers, hardeners, accelerators, and inhibitors allows for matrices with very short polymerization periods (in the order of a few minutes) even at room temperature or, vice versa, very long periods (in the order of several hours) at high temperatures, depending on the needs of the composite. 

That is, the structure of isotactic active site is not always stable during polymerization period, because it is supposed that Lewis base makes isotactic active site by co-ordinating to Ti atoms in the catalyst, while, AlEt3 attacks Lewis base and reduces its electron donating ability. 

Specialist periodical report Royal Society of Chemistry) 1. Polymers and polymerization - Periodicals I. Royal Society of Chemistry 547,7 05 QD 380... 

As shown by McDaniel (1) experinentally by using catalysts of different particle size, this is not the case (except perhaps during the extreme initial polymerization period). Furthermore other types of silica-supported PE catalysts (Ziegler type) even show very high activity during the first minutes of polymerization under similar process conditions. 

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