Monomers, polymerization heat

In estimating the enthalpy of polymerization, the physical state of both starting monomer and polymer must be specified. Changes in state are accompanied by ethalpy changes. Therefore, they also affect the level of the polymerization enthalpy. The AfT forN ylylene previously mentioned is apphcable to the monomer as an ideal gas. To make comparisons with other polymerization processes, most of which start with condensed monomer, a heat of vaporization for N ylylene is needed. It is assumed herein that it is the same as that for N ylene, 42.4 kJ /mol (10.1 kcal/mol). Thus the AfT of the hquid monomer -xylylene is 192.3 kJ/mol (46.0 kcal /mol). 

Cycloaliphatic Diene CPD—DCPD. Cycloatiphatic diene-based hydrocarbon resias are typically produced from the thermal or catalytic polymerization of cyclopeatadieae (CPD) and dicyclopentadiene (DCPD). Upon controlled heating, CPD may be dimerized to DCPD or cracked back to the monomer. The heat of cracking for DCPD is 24.6 kJ / mol (5.88 kcal/mol). In steam cracking processes, CPD is removed from C-5 and... 

Hall and his co-workers43 synthesized several 2-oxabicyclo[2.1.1 ]hexan-3-ones from the corresponding 3-chlorocyclobutanecarboxylic acids. These monomers polymerized readily when heated with a variety of basic or acidic initiators. Some of the results of the polymerization are listed in Table 2. 

These simulations clearly reveal the importance of considering M, in calculating the optimal temperature. is dependent on the heat of polymerization (-AH) as given by Eq. (13). Most monomers have heats of polymerization in the range of 50 to 80 KJ/mol. We thus decided to study the effect of (-AH) on optimal temperature and time for various half-life values of the initiator. The results are shown in Figme 5. 

Polymers may be made by four different experimental techniques bulk, solution, suspension, and emulsion processes. They are somewhat self-explanatory. In bulk polymerization only the monomers and a small amount of catalyst is present. No separation processes are necessary and the only impurity in the final product is monomer. But heat transfer is a problem as the polymer becomes viscous. In solution polymerization the solvent dissipates the heat better, but it must be removed later and care must be used in choosing the proper solvent so it does not act as a chain transfer agent. In suspension polymerization the monomer and catalyst are suspended as droplets in a continuous phase such as water by continuous agitation. Finally, emulsion polymerization uses an emulsifying agent such as soap, which forms micelles where the polymerization takes place. 

Simplest of the techniques requiring only monomer and monomer-soluble initiator, and perhaps a chain-transfer agent for molecular weight control. Characterized, on the positive side, by high polymer yield per volume of reaction, easy polymer recovery. Difficulty of removing unreacted monomer and heat control are negative features. Examples of polymers produced by bulk polymerization include poly(methyl methacrylate), polystyrene, and low-density (high pressure) polyethylene. 

These monomers polymerize readily in presence of light, heat or catalysts (such as benzoyl peroxide) and must always be stored or shipped with inhibitor present to avoid spontaneous and explosive polymerization. 

Monomer Specific Polymerization Heat, kcal/kg State... 

It would be useful to have liquid present in the polymerization reactor that provided the advantages of a solvent but without any of the disadvantages. Sound unlikely How about water One technique, called suspension polymerization, involves adding monomer to water in a reactor, agitating the mixture rapidly so that the monomer breaks apart into very small droplets, adding an initiator that is soluble in the monomer, and heating. Each droplet acts as a microbulk polymerization, the water very effectively removes the heat of polymerization, and the resulting polymer spheres are easily separated and filtered. This process, also known as bead polymeriza-... 

An alternative to solution polymerization is the whole realm of dispersed-phase polymerization. In this class of processes, the liquid monomer is dispersed in a second, continuous phase, usually water. As the monomer polymerizes, the viscosity of the dispersion remains low, aiding the removal of the heat of polymerization. If the dispersed phase is water, the high thermal conductivity provides a very effective heat transfer mediirm. The high specific heat and large latent heat of vaporization provide a large safety margin in the event of a runaway polymerization. In addition, water is plentiful, nontoxic, environmentally friendly, and inexpensive. 

The process of parylene polymerization is presented schematically in Figure 5.2 using parylene N, unsubstituted poly(para-xylylene). Parylene dimer is heated until it sublimes. The dimer vapor passes through a high temperature pyrolysis zone where it cracks and becomes monomer vapor, i.e., monomer is created in vacuum. The monomer polymerizes and deposits in the deposition chamber, which is usually at room temperature. Parylene polymerization completed in a vacuum is a process involving no solvents, no curing, and no liquid phase. Its use essentially eliminates concern about the operator s health and safety, air pollution, and waste disposal. 

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