Temperature of polymerization

Propagation cannot be reduced to a simple chemical reaction between the active centre and the monomer. Even if no compounds other than these two partners were present, molecules of the monomer not reacting at the moment 

Radical propagations proceed smoothly in non-polar and also in highly polar media. This is understandable as radical solvation is weaker than ion solvation by one or two orders of magnitude. Propagation on coordination polymerization catalysts is only possible in non-polar media which do not interfere with monomer (usually hydrocarbon) coordination on the transition metal atom. Ionic polymerizations also proceed in non-polar media and they are accelerated with increasing medium polarity. 

This statement has its limitations. Ionic polymerization in hydrocarbons is always kinetically complicated, it often starts only after the addition of a polar compound (co-initiator), and it is affected by the aggregation of initiating and propagating particles. In strongly polar media, activation of initiator by dissociation of acids and bases is easy. Such solvent is simultaneously a reactive transfer agent. Propagation usually does not occur, and only low molecular products are formed. Exceptions can, of course, be found. During anionic polymerization of lactams in DMF, the solvent only increases the amount of dissociated initiator [27]. 

Media of moderate polarity, with permittivities in the range 5-15 are therefore the domain of anionic and cationic polymerizations. 

Another factor affecting addition is the tendency of monomers to form complexes with other components of the polymerizing system. This was discussed in Chap. 2, Sects. 4.2 and 5.2. The effect of the medium is, of course, also reflected in the mechanisms of termination and transfer. A somewhat unusual but interesting and instructive effect of the environment of an active centre on the course of propagation was observed in polymerization on a liquid—gas phase boundary, and during polymerization of liquid crystals. 

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In the case of anionic polymerization (with 2-isoprOpenylthiazole) there is a chain-monomer equilibrium. Furthermore, lowering the temperature of polymerization increases the conversion of monomer to polymer (314). 

The most common water-soluble initiators are ammonium persulfate, potassium persulfate, and hydrogen peroxide. These can be made to decompose by high temperature or through redox reactions. The latter method offers versatility in choosing the temperature of polymerization with —50 to 70°C possible. A typical redox system combines a persulfate with ferrous ion ... 

Thermal and thermomechanical analyses44 are very important for determining die upper and lower usage temperature of polymeric materials as well as showing how they behave between diose temperature extremes. An especially useful thermal technique for polyurethanes is dynamic mechanical analysis (DMA).45 Uiis is used to study dynamic viscoelastic properties and measures die ability to... 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

Increasing the temperature of polymerization does not always lead to higher rates of polymerzation. Higher temperature leads to faster dissociation of the initiator and complete depletion of the initiator resulting in a "dead end" polymerization (Bohme and Tobol-sky (1966)). Dead-end polymerization refers to one in which initiator concentration decreases to such a low value that the polymerization stops short of completion and a limiting conversion of monomer to polymer is observed (Odian (1970)). 

Increasing the temperature may also increase the rate of depropagation resulting in equilibrium monomer concentrations well above an acceptable residual monomer concentration (Sawada (1976)). It seems there exists an optimum temperature of polymerization that will reduce the time of polymerzation but will avoid the problems of depolymetization and of initiator depletion. 

Fig. 30.—Log of the mole percent (6X100) of head-to-head linkages in poly-(vinyl alcohol) plotted against the reciprocal of the absolute temperature of polymerization of the poly-(vinyl acetate) from which the former was prepared by hydrolysis. (Flory and Leutner.i2 ...

Fig. 31.—A plot of the log of the trans/cis ratio for polybutadiene against the reciprocal of the absolute temperature of polymerization by a free radical mechanism. (Results of Richardson and Sacher obtained by infrared analysis.)...

The molecular mass of the polyacid obtained lies between 10000 and 55000. Increasing the temperature of polymerization and the concentration of ammonium persulphate serves to decrease the molecular mass of the poly(alkenoic acid). 

Figure 3. Conversion in polymer of HEMA incorporated in PET films. Dependence on time and temperature of polymerization.

Why is the reaction temperature of polymerization of polyvinyl chloride so important to its manufacture ... 

Catalysts Temperature of polymerization (°C) Density of polymer Mn Number of methyls per 1000 C atoms Number of vinyls per 1000 C atoms Trans, internal double bonds per 1000 C atoms Number of methyls per chain Number of terminal double bonds per chain... 

The rate of decomposition of initiators usually follows first-order kinetics and is dependent on the solvent present and the temperature of polymerization. The rate is usually expressed as a half-life time (h/2), where ti/2 = In 2/A d = 0.693/fcd- The rate constant ( d) changes with temperature in accordance with the Arrhenius equation as shown below ... 

TABLE 8-12 Effect of Counterion, Solvent, and Temperature of Polymerization of Methyl Methacrylate... 

B. Characteristic Temperatures of Polymerization and Glass Formation, Transition Broadening, and Fragility... 

It was reported that, in the contrast to acrylic acid, methacrylic acid does not exhibit any template effect under conditions described. However, template effect appears if a solvent such as water or methanol is added, and also at higher temperatures of polymerization. 

Fluorothioketones are more difficult to polymerize. There are two reasons. First, agents that promote polymerization also catalyze dimerization to dithi-etanes, which is a very fast reaction. Second, ceiling temperature of polymerization is low with the result that polymer decomposes back to monomer as it is being isolated. However, poly(hexafluorothioacetone) can be formed at very low temperatures by initiation with dimethylformamide or BF3 etherate, even though at — 78° C the only product isolated is 2,2,4,4-tetrakis(trifluoromethyl)-l,3-di-thietane. 

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

Apparently, the choice of a catalyst, the protecting groups, the temperature of polymerization, and the solvent all affect the stereoregularity of polymerization. It appears probable that the synthesis of a number of regular, (1— 4)-Iinked aldopyranans and, perhaps, (1— 5)-linked aldofuranans, should be possible in the near future. 

Miscibility of a natural lipid (DMPC) and the monomeric and polymeric lecithin analogue (26) was studied in large unilamellar vesicles using freeze-fracture electron microscopy and photobleaching by H. Gaub 100>. Before polymerization the two lipids appear miscible at all compositions in the fluid state and at DMPC concentrations at or below 50 mol/o in the solid state. After polymerization a two-dimensional solution of the polymer in DMPC is obtained at T > T (T phase transition temperature of polymeric 26) while lateral phase segregation into DMPC-rich domains and patches of the polymer is observed T < T. The diameter of the polymerized lipid domains was found to average 400 A. 

Once the initiation has occurred and polymerization has begun, the MW of the product can be controlled by several reactor variables. For example, the effect of reactor temperatrue is shown in Fig. 6. Raising the temperature of polymerization greatly enhances the rate of termination, i.e., it makes the metal-polymer bond less stable and more inclined to undergo -elimination. But its effect on the propagation rate is minor in comparison, so the result is shorter chains and increased MI. 

Grafting by chain transfer initiation has been carried out not only in homogenous medium but also by emulsion polymerization techniques, where the monomer and the catalyst are added to a latex containing the original backbone polymer (99). The efficiency of grafting increases with an increase of temperature of polymerization and with an increase of initiator concentration (generally potassium persulfate) these results indicate not only that the chain transfer reaction has a higher activation... 

An investigation of the solution properties of poly-acrylonitriles produced by anionic polymerization and by radical polymerization seem to indicate that the former is a branched polymer while the latter is a linear polymer. The amount of branching seems to increase with rising temperature of polymerization and this suggests a termination due to a proton transfer from the polymer followed by polymerization that starts on the resulting negative center of the previously formed polymer. This leads to the formation of a branch. 

The success of the preparations involving a-methyl styrene is due to two factors. The low temperature of polymerization (—70° C) prevents any side reactions and the slowness of the reaction permits a good mixing of the reagents. In a rapid polymerization of styrene the adequate mixing is a serious problem, and the samples obtained in the early stages of our... 

The crystalline state configuration of polymeric selenium is a helix in which all the rotational angles are of the same sign with a value of 78°. This is fairly close to the expected value of 90°.52 The barrier to rotation about the Se-Se bond is thought to be roughly the same as that about S-S bonds. No reliable experimental results are available on the statistical properties of these chains. Some values of the glass-transition temperature of polymeric selenium have been reported15 and these could at least provide a measure of the dynamic flexibility of the chains. However, these results are probably compromised by the presence of cyclic selenium molecules that act as plasticizer. 

The experimental factors that affect the stability of the latex during and after polymerization are the recipe used for the polymerization, the type and intensity of agitation during and after the polymerization, the temperature of polymerization and storage, and the age and storage conditions of the latex. The recipe used in the polymerization included the mode of polymerization, the monomer-water ratio, the solubility of the monomer in water, the emulsifier type and concentration, initiator type and concentration, total electrolyte concentration, and impurities present in the system. 

Polyvinyl chloride. Organic peroxides are used to catalyze the free radical polymerization of vinyl chloride monomer in water. The organic peroxide is selected to generate free radicals thermally at the temperature of polymerization. 

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