Polycarbonate melt transesterification

Polycarbonates of numerous bisphenols have been extensively studied. However, most commercial polycarbonates are derived from bisphenol A. At first, both direct-reaction and melt-transesterification processes were employed (Figure 4). In direct-reaction processes, phosgene reacts directly with bisphenol A to produce a polymer in a solution. In transesterification, phosgene is first reacted with phenol to produce diphenyl carbonate, which in turn reacts with bisphenol A to regenerate phenol for recycle and molten, solvent-free polymer. Transesterification is reported to be the least expensive route. It was phased out, however, because of its unsuitability to produce a wide range of products. 

Based on this work on aromatic polyesters, Schnell et al. [30] and Fox [31] independently prepared hnear, bigh-melting, bigh-molecular-weight aromatic polycarbonates in 1953—1954 that were derived from 4,4 -dihydroxy-diphenylalkane monomers. These aromatic polycarbonates could be prepared either (1) by a two-phase interfacial method (a modified Schotten-Baumann reaction) or (2) by a melt transesterification (monomers-as-solvent) process using diphenyl carbonate [32]. Versus earher aUphatic polycarbonates, the aromatic polycarbonates were unique in tbat they could be made into water-clear (colorless) transparent structures tbat possessed excellent long-term mechanical properties. 

It is not surprising, given the concurrent research and commercialization efforts that were focused on bringing BPA-based polycarbonates to the market in the late 1950s, that a patent interference arose. Bayer was eventually awarded the first U.S. patents on BPA-based polycarbonates [35] and for the interfacial method of production. GE was issued a patent covering the melt transesterification production process [36]. 

Although there is an extensive volume of work on the polymerization of polycarbonate and polycarbonate copolymers, today there are only two basic commercial condensation processes used to produce the majority of commercial product interfacial and melt transesterification. For a more in-depth review of these two processes as well as others that are not commercially significant, the following resource is recommended [49]. 

Polycarbonates are polyesters of carbonic acid formed by reaction of diols (aromatic, aliphatic or a mixture of both) with a derivative of carbonic acid. The first preparations of polycarbonates were reported by Einhorn in 1898 [155], by reaction of phosgene with resorcinol or hydroquinone in a pyridine solution. Bischoff and van Hedenstrom in 1902 [156] obtained the same aromatic polycarbonates via transesterification with diphenyl carbonate (DPC). Thus the main routes to polycarbonates were established early, but the properties of the products seemed uninteresting. Around 1930 aliphatic polycarbonates were studied by Carothers and van Natta [157]. These carbonates have low melting points and thermal resistance and are not commercially interesting as stand-alone thermoplastics. Low molecular... 

PCs of numerous bisphenols have been extensively studied. However, most commercial PCs are derived from bisphenol A (BPA) and is depicted in Fig. 1.9. Both solution and solvent free, melt-transesterification processes are used to manufacturer polycarbonates. 

Transesterification. There has been renewed interest in the transesterification process for preparation of polycarbonate because of the desire to transition technology to environmentally friendly processes. The transesterification process utilizes no solvent during polymerization, producing neat polymer direcdy and thus chlorinated solvents may be entirely eliminated. General Electric operates a polycarbonate plant in Chiba, Japan which produces BPA polycarbonate via this melt process. 

PBT is produced by the transesterification of dimethyl terephthalate with 1,4-butanediol by means of a catalyzed melt polycondensation (19). PBT is also semicrystalline and is an extremely tough resin. Several commercial resins use a blend of PBT with another resin, such as PET, polycarbonate, or nylon. Typically, composites of PBT contain 20—30 vol % fiber glass. 

Much the most important polycarbonate in commercial terms is made from 2,2-di(4-hydroxyphenyl)propane, commonly known as bisphenol A. This polymer was discovered and developed by Farbenfabriken Bayer [92], The synthesis and properties of this and many other polycarbonates were described by Schnell in 1956 [93], The polymer became available in Germany in 1959, and was given the trade name Makrolon by Bayer (in the USA, Merlon from Mobay). General Electric (GE) independently developed a melt polymerisation route based on transesterification of a bisphenol with DPC [94], Their product, Lexan, entered the US market in 1960. The solution polymerisation route using phosgene has since been displaced by an interfacial polymerisation. 

Preparation of a Polycarbonate from 4,4-lsopropylidenediphenol (Bisphenol A) and Diphenyl Carbonate by Transesterification in the Melt... 

The most practical nonphosgene process for manufacturing polycarbonates is the transesterification of diphenylcarbonate (DPC) with bisphenol-A. A nonphosgene process for the melt polymerization production of aromatic polycarbonates [reaction (14)], making use of EniChem technologies for the production of DMC and DPC as intermediates, has been commercially established and will account in a short time for about 300,000 ton/yr polycarbonates. 

The polycarbonates 62a-f and 63a-f were prepared by transesterification of the corresponding diols with the biscarbonate 55 in the molten state. At least after annealing all members of both series were semicrystalline showing a melting endotherm in the DSC heating traces. Unfortunately, DSC curves of 62a-f were not published. An unidentified birefringent mesophase was mentioned for 62a-e, whereas a nematic melt was reported for 62f. However, the published texture of this melt is not a typical nematic Schlieren texture, but looks like a suspension of solid particles in an isotropic melt. For all members of series 63a-f a nematic... 

Brief reviews covering redistribution reactions in polyester and in polycarbonate binary blends have been prepared by Porter et al. [1989] and Porter and Wang [1992]. Selected references for redistribution processes in PEST/PEST blends are listed in Table 5.7. Early studies of these processes focused on measuring the extent of redistribution under specific processing conditions rather than on producing compatibilized polymer blends with an attractive balance of properties. A number of more recent studies have reported the limits of miscibility for certain melt-mixed polyester pairs in the absence of transesterification — see for example the NMR study of PC/PET blends [Abis et al., 1994]. 

Transesterification of diphenyl carbonate and bisphenol A. The final step in the nonphosgenation process for polycarbonates is the reaction of bisphenol A (BPA) and the carbonate ester, diphenyl carbonate (DPC). Research has focused on the transesterification melt process because it has the advantage over the conventional interfacial process of allowing the reaction of the diphenyl carbonate and bisphenol A to take place completely in the liquid phase. The disadvantage of this approach is that elevated temperatures are needed to ensure that unreacted DPC and BPA are completely volatilized from the product. Only a lower molecular weight (30,000-50,000) polymer can be made in this way. Typical molecular weights for polycarbonate produced by phosgenation in the interfacial pro-... 

The polycondensation of a diol and the diester of a dicarboxylic acid (e.g., the dimethyl ester) can be carried out in the melt at a considerably lower temperature than for the corresponding reaction of the free acid. Under the influence of acidic or basic catalysts a transesterification occurs with the elimination of the readily volatile alcohol (see Example 4.3). Instead of diesters of carboxylic acids one can also use their dicarboxylic acid chlorides, for example, in the synthesis of high-melting aromatic polyesters from terephthaloyl dichloride and bisphenols. The commercially very important polycarbonates are obtained from bisphenols and phosgene, although the use of diphenyl carbonate as an alternative component is of increasing interest (see Example 4.4). Instead of free acids, cyclic carboxylic... 

The binary blends of polycarbonate with poly(butylene terephthalate) (PBT/PC) or poly(ethylene terephthalate) (PET/PC) are now known to be essentially phase-separated blend systems exhibiting two glass transition temperatures in each case, one for the polycarbonate-rich phase and another for the polyester-rich phase (Murff et al. 1984 Huang and Wang 1986 Wahrmund et al. 1978). The evaluation of the amorphous phase miscibility in these blends was often complicated by the potential of a transesterification reaction between the two polymers during the melt blending, which may in principle lead to a block copolymer and eventually to... 

In a recent study supercritical carbon dioxide has been explored as a medium for conducting melt phase transesterification of bisphenols with diphenyl carbonate to produce polycarbonate (at yields approaching 95 %) at temperatures around 270 and pressures of about 200- 275 bar [33-35]. Supercritical carbon dioxide provides a means of efficient extraction of phenol, which is the byproduct of reaction, and furthermore lowers the viscosity of the molten polycarbonate for easier handling. Swelling improves also the rate of polymerization by increasing the surface area available for condensate removal. 

Several decades later, in the early 1930s, work (and interest) once more revived on aliphatic polycarbonates when Carothers and van Natta were able to produce low-melting, low-molecular-weight, microcrystalline aliphatic polycarbonates via two different synthetic pathways [27]. The first method was the transesterification of aliphatic dihydroxylic compounds with diethyl carbonate. The second approach was via ring-opening polymerization of cyclic carbonates of aliphatic dihydroxylic... 

The polymer is exposed to an extensive heat history in the melt process. Early work on transesterification technology was troubled by thermal-oxidative reactions of the polymer, especially in the presence of basic catalysts (8-11). Early polycarbonates prepared by Fox and others via the melt process had noticeable brown colors. More recent work on catalyst systems, more reactive carbonates, and modified processes have improved the process to the point where formation of color and product decomposition can be effectively suppressed. Polymers with color at least as good as interfacially prepared materials can now be prepared commercially. One of the key requirements for the transesterification process is the use of clean starting materials. Methods for the purification of both BPA and diphenyl carbonate have been developed and patented. Activated carbonates that form high molecular weight polycarbonate at equilibrium in solution at or below room temperature have also been reported, although they are chiefly only of academic interest (66,67). 

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