Melt and Solid-state Polymerizations

Scimne 7. Synthesis of polymers containing the 2-phenylbenzimUazole-l,S4%l unit 

Numerous communications describe the use of hi -temperature solution polymerization techniques. Of the solvents used, poly(phosphoric acid), introduced in 1964 into polybenzimidazole chemistry by Iwakura et al. doubtlessly occupies the first position, and a great many syntheses employing the poly(phosphoric add) solution technique have since been reported from various polymer laboratories Iwakura s technique offers an advantage insofar as the tetraamine monomers may be employed as the stable and easy-to-handle hydrochlorides in place of the extremely air-sendtive free bases. In addi- 

Schraw 11. Folybenamidazole synthesis from tetraaminobenzene and bisulfite adduct of isophthalaldehyde 

As D Alelio s patent embodies conditions of low omdensation temperatures and so, just like some of the aforementioned solution polymerizations , teaches a nditions beneficial to reactant and nx duct integrity, it permits the preparation of polybenzimidazoles from monomers too unstable thermally or chemically for use in the melt or poly(phosphoric acid) solution processes, examples of sudi monomers being te-trahaloterephthalaldehydes or the crosslinkable 3-vin)fisophthalaldehyde. It does not, however, provide for the isolaticm and application of prepolymers and, thus, for utilization of die two-stage approadi so beneficial from a processing and application standpoint. 

Spectroscopic information, suf orted by the results of a non-polymeric model reaction study indicates the prepolymers to possess a predominantly open-chain structure of 

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In most practical situations the reaction system is not chosen because of its influence on the reaction kinetics. Economic feasibility is most often the dominating factor in this choice. Where adequate process control can be obtained, melt or a combination of melt and solid-state polymerization may be preferred for obvious reasons. When solvents must be used, they are often selected primarily on a combination of polymer solubility and cost. 

PBI synthesis chemistry is shown in FIGURE 5. Tetraaminobiphenyl and diphenylisophthalate are melt and solid state polymerized in two stages under an inert atmosphere. As polymerization occurs, the melting point of the polymer drops at first and then increases as shown in TABLE II. In the... 

ROP in the melt and solid-state polymerization has been developed. 

Aromatic polycarbonates are produced technically from bisphenols via transesterification and interfacial phosgenation. In addition, ROP in the melt and solid-state polymerization has been developed, although not yet commercialized [83]. It has also been shown that the ROP of individual macrocycles based on bisphenol-A may progress without a catalyst [84] (Scheme 12.9). 

Polyethylene and polystyrene are two of the most commercially important and ubiquitous polymers, primarily because of their commercial value. Since the early days of polymer research there has been considerable interest to produce copolymers from ethylene (E) and styrene (S) because of both academic and business interests. Depending on the nature and type of polymerization chemistry, a variety of different molecular architectures can be produced. In addition to the different monomer distributions (random, alternating or blocky nature), there are possibilities for chain branching and tacticity in the chain microstructure. These molecular architectures have a profound influence on the melt and solid-state morphology and hence on the processability and material properties of the copolymers. 

Polycondensation can be carried out by various polymerization techniques including melt polymerization, solution polymerization, interfacial polymerization, emulsion polymerization and solid-state polymerization. These polymerization processes will be summarized briefly in the following paragraphs. 

Water and carbonated beverage bottles are made predominantly from poly(ethylene terephthalate) (PET). The polymer is made by condensation reaction of ethylene glycol with either terephthalic acid or its dimethyl ester. The process in practice includes three steps prepolymer formation, melt condensation to increase viscosity, and solid-state polymerization at 180-230°C to yield a resin with an average molecular weight that is high enough for use as bottle resins. Antimony trioxide is used as a catalyst in polymerization (Duh, 2002). 

Burke, A. L., Givens, R. D., Jikei, M. and de Simone, J. M. (1997) Use of C02 in step-growth polymerizations From plasticised melts to solid state polymerization,y4C5 Polymer Preprints, 38(2), 387-388. 

Radiation and solid-state polymerization are eombined in this seetion because it is common to induce polymerization of liquid monomers by radiation below their melting points. In a patent published in 1955 [517], a simple proeedure for UV-induced polymerization of the monomer is described. The resulting polymer has a k value elose to 110. A number of sensitizers are used in this field, sueh as ZnCl2 and air [518], anthrachinone [519], and various metal perfluoroalkane sulfonates [492,520]. The polymerization of liquid NVP initiated by y-radiation is mueh faster than that of the solid NVP. However, the polymerization of the solid NVP becomes faster during irradiation and can reach rates comparable with those observed for hquid NVP. 

Two main types of polymerization procedures were employed. The traditional methods (Figure 2, lower path) use high temperatures (>200°C) for conversion in the melt and solid-state (7) or in polyphosphoric acid (PPA) solution (2,6). The milder, two-step procedure (Figure 2) involves polyamide formation and isolation followed by thermal cyclization to the polybenzoxazole. 

The polymerization of liquid monomers that have been cooled below their melting point is usually induced by radiation, the discussion of radiation and solid-state polymerizations has been combined in this section. 

Melt Salts and Solid-State Polymerization. By using a chloroalu-minate melt obtained from a mixture of iV-acetylpyridinium chloride or Af-butylpyridinium chloride and AICI3 at room temperature, highly conducting poly(p-phenylene) films were obtained by electrooxidation of benzene on Ft [368-371]. 

The PLA can be synthesized by various synthesis routes such as direct polycondensation, azeotropicdehydrative condensation, ROP, melt polycondensation (MP), and solid state polymerization (SSP) of low molecular weight (MW) PLA (Lunt, 1998). 

PTT is polymerized at a much lower temperature between 250 and 275 °C. Because of its higher melt degradation rate and a faster crystallization rate, it requires special consideration in polymerization, pelletizing and solid-state treatment. 

The polymerization proceeds under photo- [49,50],X-ray [51], and y-ray [52] irradiation in the dark in vacuo, in air, or even in water or organic solvent as the dispersant (nonsolvent) for the crystals, similar to the solid-state polymerization of diacetylene compounds [ 12]. The process of topochemical polymerization of 1,3-diene monomers is also independent of the environment surrounding the crystals. Recently, the thermally induced topochemical polymerization of several monomers with a high decomposition and melting point was confirmed [53]. The polymer yield increases as the reaction temperature increases during the thermal polymerization. IR and NMR spectroscopies certified that the polymers obtained from the thermally induced polymerization in the dark have a stereoregular repeating structure identical to those of the photopolymers produced by UV or y-ray irradiation. 

Low molecular weight PET and PBT resins are made by melt processes. For higher molecular weight resins, both melt processes or solid-state polymerization are used. Although terephthalic acid can be directly esterified, the most common process involves transesterification of dimethyl terephthalate with ethylene glycol or 1,4-butanediol in the presence of trace amounts of metal ion catalysts (67,68). 

Andrews76 gave results of the work of Reed and Martin on cis-polyisoprene specimens crystallized from a strained cross linked melt and on solid state polymerized poly-oxymethylene respectively, explaining the results by simple two phase models. He also summarized the studies of Patel and Philips775 on spherulitic polyethylene which showed that the Young s modulus increased as a function of crystallite radius by a factor of 3 up to a radius of about 13 n and then decreased on further increasing spherulite size. 

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