Aromatic-heterocyclic polymers processing

Polyimidazopyrrolone (ladder pyrrone, polypyrrolone) n. An aromatic, heterocyclic polymer that results from the reaction of an aromatic dianhydride with a tetramine. Due to the double-chain or ladder-hke structure, these polymers have outstanding resistance to radiation, chemicals, and heat (no weight loss to 550°C). However, this structure also makes them difficult to process. To overcome this difficulty pyrrone pre-polymers in the form of solutions and salt-like powders have been made available. The powders can be molded under conditions that complete the cychzation or conversion of the ladder-hke molecular structure during the molding cycle. The cychzation reaction generates water, which must be removed from the part. 

Work in our laboratory in the past few years has been concerned with the use of acetylene chemistry, both to aid the processing of aromatic heterocyclic polymers and to pro-, vide such materials a method by which they could become tougher and more durable in structural applications. The most attractive feature of the acetylenic carbon carbon triple bond is its capability to undergo various ionic and free radical addition reactions, leading to highly fused thermally stable aromatic systems. This paper will review our work on acety-lene containing aromatic heterocyclic polymers with respect to synthesis and characterization, as well as some already determined mechanical properties as composites and adhesives. 

Polyimides were developed during the 1960s and early 1970s in response to the demands of the aerospace industry for high temperature performance polymers, as matrix materials for laminates and other composites. Most aromatic/heterocyclic polymer systems that have a small number of oxidisable C-H bonds per molecule exhibit excellent oxidative stability, but tend to be intractible and extremely difficult to process. In the case of polyimides, however, this limitation has been largely overcome. Some of the materials now in use for structural applications can withstand continuous exposures in air to temperatures above 300°C (approximately 600°F). 

In a wide definition, PBI refers to a large family of aromatic heterocyclic polymers containing benzimidazole units. PBI with different structures can be synthesized from hundreds of combinations of tetraamines and diacids. In a specific way, PBI refers to the commercial product under the trademark Celazole , poly(2,2 -m-(phenylene)-5,5 -bibenzimidazole) (Fig. 4.1). In the context of PBI with different structures, this specific PBI is also named as meia-PBI because phenylene ring is meto-coordinated. As an amorphous thermoplastic polymer, the aromatic nuclei of PBI provide the polymer high thermal stabihty (glass transition temperature, Fg = 425-436 °C), excellent chemical resistance, retention of stiffness and toughness, but poor process-abihty [56-58]. Primarily used in textile fibers, the selection of poly(2,2 -m-(phenyl-ene)-5,5 -bibenzimidazole) as the commercial product was made on the basis of its... 

Considerable research effort has been devoted in recent years to the use of chloral derivatives for the synthesis of linear heterocyclic polymers. Of these, the most common are aromatic polyimides [1-12], Many of these polymers have been synthesised from compounds like 4,4 -diaminobenzophenone, and other diamines, which, as demonstrated in the previous chapter, can be obtained from chloral. Polyimides prepared from these diamines were largely synthesised by the conventional two-step procedure [11, 12] involving mild reaction of the diamines with the bis(phthalic)anhydrides, isolation of poly(o-carboxy)amide (PCA) prepolymers, and then processing into products followed by thermal or chemical imidisation [13—16] (Scheme 3.1). Some properties of polyimides prepared from 4,4 -diaminobenzophenone are provided in Table 3.1. 

Unfortunately, wholly aromatic and/or heterocyclic polymers are notoriously difficult to process because they (1) exhibit low solubilities in common organic solvents and (2) typically start to decompose at a lower temperature than they melt. Attempts to improve the processing characteristics of... 

The homopol5unerization of diisocyanates is only useful for specialty diisocyanates, such as aliphatic 1,2- or 1,3-diisocyanates (3) and aromatic o-diisocyanates (4), which polymerize via cycloaddition processes. Anionic homopolymerization of monoisocyanates takes place by addition across the 0=N bond to form nylon-1 polymers. Polyamides are also obtained fi"om diisocyanates and enamines or ketenaminals. This reaction proceeds by a [2 -i- 2] cycloaddition reaction with subsequent ring opening to form polyamides. [2 - - 4] cycloaddition polymerization to form heterocyclic polymers is observed with carbonyl diisocyanate (5). Ring-opening polymerization occurs in the reaction of bis-epoxides... 

Intensive research has occurred in the last twenty years concerning the synthesis and processing of thermally stable polymers Early explorations in this area can be traced back to the late fifties impelled by the discovery of heterocyclic and aromatic amide polymers, which had the ability to withstand extreme temperatures. 

Although the majority of flame-resistant fibres are produced from fully aromatic and/or heterocyclic polymers, an alternative approach, based on utilization of crosslinked polymers, is also feasible. Phenol-formaldehyde fibres were produced by a process involving spinning of a fusible novolac resin followed by crosslinking of the precursor fibre. A recently developed nonflammable fibre with a low smoke-evolution is based on a copolymer of acrylic acid and acrylamide the carboxylic groups are crosslinked by zinc ions. ... 

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