Exothermic addition polymerization

POLYMERIZATION (Emulsion). Since an aqueous system provides a medium for dissipation of the heat from exothermic addition polymerization processes, many commercial elastomers and vinyl polymers are produced by the emulsion process. This two-phase (warer-hydrophobic monomer) system employs soap or other emulsifiers to reduce the interfacial tension and disperse the monomers in the water phase. Aliphatic alcohols may be used as surface tension regulators,... 

Poly(2-methyl-1-pentene sulfone) (PMPS) is an alternating copolymer of 2-methyl-l-pentene (2MP) and sulfur dioxide. The formation of PMPS occurs only by a free radical polymerization mechanism and is complicated to a degree by ceiling temperature considerations. For all exothermic addition polymerization reactions there is a critical temperature called the ceiling temperature (Tc) above which no reaction occurs. The precise Tc depends upon the monomer concentration according to the expression (i)... 

Polymerization thermodynamics has been reviewed by Allen and Patrick,323 lvin,JM [vin and Busfield,325 Sawada326 and Busfield/27 In most radical polymerizations, the propagation steps are facile (kp typically > 102 M 1 s l -Section 4.5.2) and highly exothermic. Heats of polymerization (A//,) for addition polymerizations may be measured by analyzing the equilibrium between monomer and polymer or from calorimetric data using standard thermochemical techniques. Data for polymerization of some common monomers are collected in Table 4.10. Entropy of polymerization ( SP) data are more scarce. The scatter in experimental numbers for AHp obtained by different methods appears quite large and direct comparisons are often complicated by effects of the physical state of the monomei-and polymers (i.e whether for solid, liquid or solution, degree of crystallinity of the polymer). 

Polymerization of methyl methacrylate to Plexiglas is done in the bulk process. High pressure polymerization of ethylene is done this way also. But other addition polymerizations frequently become too exothermic and without adequate heat removal system, the reaction tends to run away from optimum conditions. 

A very important characteristic of polymerization reactors is their thermal stability as discussed by Sebastian (6). Chain addition polymerizations are thermally simple reactions, in that the polymerization exotherm is attributable almost in its entirety to the chain propagation step. For chain addition polymerization reactors the rate of reaction r is proportional to the product of the square root of initiator concentration, cu and to monomer concentration, cm... 

Other variables can also be used to influence the thermodynamics of a polymerization. For example, most step polymerizations involve equilibrium reactions, which may be driven to completion by removing the small molecule by-product in an open system. Addition polymerizations are influenced by the solvent used. That is, [ML depends on both the nature of the solvent and on [M]o- For example, the equilibrium monomer concentration of THF increases as the acidity of the solvent increases due to complex formation. In other cases, solvation of a polymer segment may be more exothermic than monomer solvation, resulting in a more exothermic AHP compared to the bulk polymerization. 

Polyurethanes are formed when a diisocyanate (or polyisocyanate) is reacted with hydroxyl groups at a molar ratio of 2 or higher (isocyanate hydroxyl). When the polyol and polyisocyanate are combined in the presence of a suitable catalyst, the exothermic polymerization reaction begins spontaneously. This type of synthesis is an addition polymerization. Most polyols and polyisocyanates used for manufacturing PUs are liquid at standard room temperature. Industrially, the PU synthesis reaction is rapid, and the product is a solid polymer. The reaction rate can be varied significantly by changing the catalyst type and concentration, facilitating the use of PUs in a variety of applications. ... 

Addition polymerization of vinyl monomers is one of the most popular classes of chain-growth polymerization. Depending on the nature of the active species, the polymerizations are categorized as cationic, radical, or anionic. These polymerizations are usually very fast and highly exothermic. 

In contrast to methane, acetylene (C2H2) is anisotropic, highly unsaturated, and has a much lower 1 1 hydrogen carbon ratio. The it bonding system allows for the possibility of facile addition polymerization reactions, and therefore acetylene is predicted to be much more reactive than methane upon shock compression. This is reflected in a significantly lower flyer plate velocity threshold for the onset of chemical reactions. Polymerization of acetylene is also exothermic a simple bond energy calculation shows that the reaction... 

Initiation by pyridine is much slower and its consumption is very low. The rate of the ensuing polymerization increases with decreasing temperature implying a reversible, exothermic addition of pyridine to the monomer. Plots of [monomer],/[monomer]o versus time are convex the slopes at t = 0 increase with increasing initial monomer concentration - evidence of a slow second step of initiation ... 

The simplest way to cany out an addition polymerization reaction of a vinyl-type monomer, CH2=CHX, is to let it stand at room terpperature over prolonged periods of time. However, not only is this pr dure economically unfavorable if attempted on an industrial scale, but the reaction itself is exothermic and may get out of hand if spontaneous polymerization starts in large masses. Technical polymerizations are, therefore, always carried out uuj er controlled heat and catalyst conditions which are closely followed throughout the course of the reaction. 

Addition polymerization may be carried out in bulk so that monomers react while in solution or the monomers may be first dispersed in suspension or emulsion. Polymerizing in bulk increases the risk of reactants to overheat and induce an uncontrolled reaction. Heat is generated because polymerization reactions tend to be exothermic and polymers are poor thermal conductors. Stirring of reactants acts to disperse the generated heat. Another challenge is to recover excess solvent following polymerization. This is necessary to reduce pollution of the surrounding area, to limit the risk of fire and for cost reasons. 

Addition polymerization is exothermic, and one of the major constraints to high production rates is the problems associated with heat removal. In processes using ethylene, the pressure of the gas determines solubility in the liquid phases (i.e., water and vinyl acetate monomer droplets) and in the polymer particles. This concentration of ethylene at the point of polymerization determines the ethylene content of the final polymer. Use of high pressures in such systems eliminates refluxing of the vinyl acetate, losing a very eflective heat removal mechanism available to simple batch-process PVA production. Refluxing,... 

Addition polymerization is an exothermic process, and the change in enthalpy is typically in the range 34 to 160 kJ molThe particnlar valnes differ for each monomer and are influenced by several factors, i.e., (1) the energy difference between monomer and polymer resulting from resonance stabilization of the double bond by the substituent or by conjugation, (2) steric strains in the polymer imposed on the new single bonds by substitnent interactions, and (3) polar or secondary bonding effects. 

There are many reaction mechanisms for vinyl addition polymerizations. In approximate order of importance they are free radical polymerization, coordination metal catalysis (Ziegler-Natta), anionic polymerization, cationic polymerization, and group transfer polymerization. Regardless of specific mechanism, these polymerizations tend to be fast, essentially irreversible, highly exothermic and approximately first order with respect to monomer concentration. 

Heat of polymerization (enthalpy of polymerization). The difference between the enthalpy of 1 mol of monomer and the enthalpy of the products of the polymerization reaction. Addition polymerizations are exothermic, values ranging from about 35 to lOOkJ/mol, and removal of this heat is an important aspect of reactor design. The reported value of an enthalpy of... 

Cast allylics. The ally lie ester resins possess excellent clarity, hardness, and color stability and thus are used to cast them into optical parts. These castings can be either homo or co-polymers. The free radical addition polymerization of the allylic ester presents some casting difficulties such as exotherm control, monomer shrinkage during curing and the interaction between the exotherm, the free radical source, and the environmental heat required to decompose the peroxide and initiate the reaction. 

Polyurethanes are generally synthesized by addition polymerization between a polyalcohol and a poly-isocyanate. This is an exothermic reaction caused by the release of a proton from the alcohol group followed by a general molecular rearrangement by the formation of the urethane bond [23], If both reagents are bi-functional linear polyurethanes are obtained, while if functionalities are increased some cross-linked chains are formed, with the formation of reticulated structures. In summary, one of the most common s mthesis routes for TPUs consists basically of the reaction of three main components. 

As in any exothermic reaction, the preference for the products will be diminished at higher temperature. In the case of most addition polymerization reactions, there will be a temperature above which no further polymerization can occur. This temperature is called the ceiling temperature and is always associated with a specific monomer concentration [M] through eqn [11], which relates the enthalpy and entropy of polymerization calculated for some standard state concentration of monomer. 

Now that we have looked at the kinetics of polymerizations, let s briefly examine the thermodynamics. Table 13.3 provides some quantitative data. Recall that the pathway (mechanism) does not affect the thermodynamics. Instead, the relative energies of the monomers and polymers set the thermodynamics. As we noted in discussing autoacceleration, the propagation step of a radical chain reaction is typically quite exothermic. This is also seen in the thermodynamics of many addition polymerizations (see Table 13.3), because in each step a tt bond is converted to a a bond (which is stronger). Note, however, that the entropies of polymerizations are quite negative. This is due to the fact that the free translation of individual monomers is lost for each additional propagation step. 

In order to simplify the kinetic scheme a steady-state approximation has to be made. It is assumed that under steady-state conditions the net rate of production of radicals is zero. This means that in unit time the number of radicals produced by the initiation process must equal the number destroyed during the termination process. If this were not so and the total number of radicals increased during the reaction, the temperature would rise rapidly and there could even be an explosion since the propagation reactions are normally exothermic. In practice it is found that the steady-state assumption is usually valid for all but the first few seconds of most free radical addition polymerization reactions. 

Most addition polymerization reactions are exothermic and exentropic. The free energy of polymerization per monomer unit therefore becomes less negative as the temperature is raised. At the ceiling tmperatm T, free energy of polymerization under the prevailing conditions is zero, and above... 

Both the hquid and cured 2-cyanoacryhc esters support combustion. These adhesives should not be used near sparks, heat, or open flame, or ia areas of acute fire ha2ard. Highly exothermic polymerization can occur from direct addition of catalytic substances such as water, alcohols, and bases such as amines, ammonia, or caustics, or from contamination with any of the available surface activator solutions. 

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

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