Commercial polymer step polymerization

In this section we examine some examples of cross-linked step-growth polymers. The systems we shall describe are thermosetting polymers of considerable industrial importance. The chemistry of these polymerization reactions is more complex than the hypothetical AB reactions of our models. We choose to describe these commercial polymers rather than model systems which might conform better to the theoretical developments of the last section both because of the importance of these materials and because the theoretical concepts provide a framework for understanding more complex systems, even if they are not quantitatively successful. 

The nylon 66 molecule shown in Fig, 1,11 is a thermoplastic polymer, created by the step growth polymerization of hexamine and adipic acid. The majority of commercial polymers are thermoplastics, which permits us to readily mold them to many useful shapes. 

Many reactions familiar to organic chemists may be utilized to carry out step polymerizations. Some examples are given in Table 2.2 for polycondensation and in Table 2.3 for polyaddition reactions. These reactions can proceed reversibly or irreversibly. Those involving carbonyls are the most commonly employed for the synthesis of a large number of commercial linear polymers. Chemistries used for polymer network synthesis will be presented in a different way, based on the type of polymer formed (Tables 2.2 and 2.3). Several different conditions may be chosen for the polymerization in solution, in a dispersed phase, or in bulk. For thermosetting polymers the last is generally preferred. 

Figure 3.13 shows the variation of the gel conversion of the limiting reactant as a function of the stoichiometric ratio. For r > 3, no gel is formed and the polymer remains in the liquid state after complete reaction of epoxy groups. If the amount of epoxy monomer necessary to obtain a stoichiometric system is added in a second step, polymerization restarts, leading to gelation and the formation of a network. The two-step polymerization is the basis of several commercial thermosetting polymers. 

Aromatic polysulfones are a commercially important class of thermoplastic polymers [127]. They have highly desirable qualities such as chemical inertness, thermal stability, and flame retardency [128,129]. Although a number of methods are available for the synthesis of polysulfones [130,131,132], step polymerization methods are the most widely used industrially [127]. Polysulfones have been synthesized with the involvement of sulfonylium cations as propagating species. 

Polyesters are produced commercially by melt polymerization, ester interchanges, and interfacial polymerization. Commercial poly(ethylene terephthalate) is produced traditionally by two successive ester interchange reactions. In the first step, dimethyl terephthalate is heated with ethylene glycol at temperatures near 200°C. This yields an oligomeric dihydroxyethyl terephthalate (x = 1 to 4) and methanol, which is removed. In the second step, the temperature is increased, leading to polymer formation, while ethylene glycol is distilled off. 

These two characteristics are not always encountered, especially in the cases of addition polymerization of monomers with double bonds. Isomerization of the monomers may occur, or false bonding of monomeric units into the chain may take place during the actual step of adding monomer onto the polymer chain. Consequently the assumed chemical structure of the polymer must always be carefully verified by analytical methods. Analysis is especially important with industrial polymer production, since the preparation history is not often known exactly. The chemical names of industrial or commercial polymers are often nothing more than a kind of generic name. Commercial poly(ethylenes), for example, despite the ascribed name, are often not homopolymers, but copolymers of ethylene and propylene. As well as that, commercial polymers practically always contain additives such as antioxidants, uv absorbers, fillers, etc. In the addition polymerization of monomers with multiple bonds, head-to-head and tail-to-tail structures are always to be expected together with the normal head-to-tail bonding, as can be seen, for example, with vinyl compounds such as CH2=CHR ... 

Union Carbide Corporation developed a gas-phase process to make HOPE. In the gas-phase process, ethylene is catalyzed into HOPE in a fluidized-bed reactor consisting of catalyst-polymer particles. The polymerization occurs at the interface between the solid catalyst and the polymer matrix, which is swollen with monomers during polymerization. Ethylene is then easily separated from the HOPE particles which are then converted into pellets using an extrusion step. The first commercial gas-phase polymerization plant for making HOPE using a fluidized-bed reactor was constructed by Union Carbide in 1968 at Seadrift, Texas. Union Carbide also developed the dual reactor UNIPOL-II gas phase process in the 1980s to make bi-modal HOPE resins with superior mechanical properties compared to UNIPOL single-reactor HOPE resins. A world scale UNIPOL-II plant was constructed in 1996 to make differentiated bi-modal HOPE resins. 

Direct esterification of PDO with TPA is the preferred commercial route to polymerize PTT because it is more economical than using DMT. Figure 1 shows the reaction scheme. Because TPA has a melting point > 300°C and has poor solubility in PDO, esterification is carried out in the presence of a heel and imder a pressure of 70-150 kPa at 250-270°C for 100-140 min. Heel is an oligomeric PTT melt with a degree of polymerization (DP) of 3-7 purposely left in the reaction vessel from a previous reaction to act as a reaction medium and to increase TPA solubility. The esterification step is self-catalyzed by TPA. After reaching a DP of about 3-7, 40-50% of the oligomers is transferred to the polymerization vessel. Titanium butoxide or dibutyl tin oxide catalyst (50-150 ppm) is added to initiate polymerization at 260-275°C. Vacuum (<0.15 kPa) is applied to remove the condensed water until the polymer reaches an intrinsic viscosity (IV) of 0.7-0.9 dL/g. 

As discussed above, numerous polyimides have been synthesized in many different methods and characterized. The direct production of high molecular weight aromatic polyimides in a one-step polymerization could not be accomplished because the polyimides are usually insoluble and infusible. The polymer chains precipitate from the reaction media (whether soluble or melt) before high molecular weights are obtained. Commercial polyimides are, therefore, classified by the several different processes they were prepared to afford reasonable processibility in the final product. 

The insertion of coordinated alkenes into M-H bonds leads to metal alkyls and constitutes a key step in a variety of catalytic reactions (Chapter 9). For example, the commercially important alkene polymerization reaction (Chapter 12) involves repeated alkene insertion into the growing polymer chain. 

Formaldehyde-based resins were the first network polymers prepared by step polymerization to be successfully commercialized. They are prepared in two stages. The first involves the formation of a prepolymer of low molar mass which may either be liquid or solid. In the second stage the prepolymer is forced to flow under pressure to fill a heated mould in which further reaction takes place to yield a highly crosslinked, rigid polymer in the shape of the mould. Since formaldehyde is difunctional, the coreactants must have a functionality, /, eater than two and those most commonly employed are phenol (/= 3), urea (/= 4) and melamine (/= 6)... 

Free radical chain-growth addition polymerization of vinyl monomers is an important route to commercial polymers. The chain-growth mechanism involves three steps initiation of a chain, propagation of the growing chain, and termination of the reactive intermediates. 

The polymer described in the last problem is commercially called poly (phenylene oxide), which is not a proper name for a molecule with this structure. Propose a more correct name. Use the results of the last problem to criticize or defend the following proposition The experimental data for dimer polymerization can be understood if it is assumed that one molecule of water and one molecule of monomer may split out in the condensation step. Steps involving incorporation of the monomer itself (with only water split out) also occur. 

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Polymerization. Commercial productioa of PET polymer is a two-step process carried out through a series of coatiauous staged reactioa vessels. Eirst, monomer is formed by transesterificatioa of DMT or by direct esterificatioa of TA with 2G ... 

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