Step-ladder polymers

Polyimides are step-ladder polymers, i.e. they have single-strand and double-strand linkages. Hie polymer chains are only as strong as their weakest link, and hence it is the single strand, i.e. the diamine, which limits the performance of the pyromellitimides. Hie softening point of the polyimide depends on chain flexibility which is inversely dependent on chain length. At least nine carbon atoms appear to be required for the resultant polyimide to be mouldable belew its decomposition temperature. Hie resultant polymers retain toughness for up to 25 hours in air at 175°C. 

In principle, the aminomethyl derivatives could be made, and these can be cyclised to the isoindolines. Apart from being chemical intermediates, little value can be placed on them, although the diisoindolines can be used to make step-ladder polymers unfortunately the current literature suggests that the resultant polymers have no advantage over polyimides, and, in addition, the starting material is far more difficult to synthesise(52). 

Ladder polymers. A type of high-temperature polymer. Double linear chains of the macromolecules are periodically linked together (Fig. 1.1). They are insoluble and infusible, being unsuitable for thermoplastic processing and thus are limited in applications. In the macromolecules of step-ladder polymers shorter units of cross-linked double linear chains (ladder structures) are joined by single bonds (Fig. 1.2). An example of a step-ladder polymer is polyimide. 

Polyimides, Common name of polymers characterized by repeated imide groups in the macromolecule (Fig. 1.4). Their structure corresponds to that of the step-ladder polymers. Thermally stable infusible insoluble thermosets. Their intermediate prepolymers can be shaped. Modified polyimides have repeated, mainly aliphatic, ester or amide groups in the main chain besides the imide units, in order to facilitate processing. Examples of applications high-temperature electric insulating materials... 

Step-ladder polymers. Related to the ladder polymers but different from them in molecular construction, and consequently, in processability. A characteristic representative of this group is polyimide. 

Sivy, Gordon and Coleman (1983) [205], studied the IR spectra of the cyclization and oxidation of an acrylamide copolymer in air at 200°C and showed that there were two competing reactions—the initial step being an intramolecular cyclization and crossover, producing the step-ladder polymer initiated by the acrylamide group (Scheme XXXI) ... 

Discussion of ladder polymers also enables us to introduce a step-growth polymerization that deviates from the simple condensation reactions which we have described almost exclusively in this chapter. The Diels-Alder reaction is widely used in the synthesis of both ladder and semiladder polymers. In general, the Diels-Alder reaction occurs between a diene [XVI] and a dienophile [XVll] and yields an adduct with a ring structure [XVlll] ... 

This idea was realized impressively in 1991 with the first synthesis of a soluble, conjugated ladder polymer of the PPP-type [41]. This PPP ladder polymer, LPPP 26, was prepared according to a so-called classical route, in which an open-chain, single-stranded precursor polymer was closed to give a double-stranded ladder polymer. The synthetic potential of the so-called classical multi-step sequence has been in doubt for a long time in the 1980s synchronous routes were strongly favoured as preparative method for ladder polymers. 

The synthetic route represents a classical ladder polymer synthesis a suitably substituted, open-chain precursor polymer is cyclized to a band structure in a polymer-analogous fashion. The first step here, formation of the polymeric, open-chain precursor structure, is AA-type coupling of a 2,5-dibromo-1,4-dibenzoyl-benzene derivative, by a Yamamoto-type aryl-aryl coupling. The reagent employed for dehalogenation, the nickel(0)/l,5-cyclooctadiene complex (Ni(COD)2), was used in stoichiometric amounts with co-reagents (2,2 -bipyridine and 1,5-cyclooctadiene), in dimethylacetamide or dimethylformamide as solvent. 

In the field of polymer chemistry the regio- and stereoselectivity of the Diels-Alder reaction is used for the concerted synthesis of structurally homogeneous double-stranded ladder polymers [39], which are useful materials with nonlinear optical properties and high electrical conductivity. It has turned out that the repeated Diels-Alder method is superior to an alternative two-step process, in which first an open chain precursor is formed followed by polymer ring closure as structural defects can occur [40]. 

A polyhedron silsesquioxane ladder polymer containing polymerizable components was prepared in a three-step process to address this concern. The process initially entailed preparing the reversible addition-fragmentation transfer (RAFT) ladder iniferter, polysilsesquioxane dithiocarbamate. This intermediate was then polymerized with methyl methacrylate at ambient temperature by irradiating with ultraviolet (UV) light and poly(si Isesquioxane-g-methyl methacrylate) was obtained. 

Perhaps one of the best known syntheses of a heterocyclic polymer via the modification method is the generation of nitrogen-containing ladder polymers by pyrolysis of polyacrylonitrile) (77MI11109). The thermolysis is known to take place in discrete steps. The first step in the sequence, which can take place with explosive violence if the heating rate is not sufficiently slow, occurs at about 150 °C and can be detected by the onset of intense color formation. The product of this reaction (Scheme 101) is the cyclic tetrahydropyridine ladder structure (209). The next step, which is conducted in the presence of air at ca. 250 °C, involves the thermooxidation of polymer (209) to form what is best described as terpolymer (210) containing dihydropyridine, pyridone and pyridine units. 

Polyacrylonitrile (PAN) is the most common precursor used to make carbon fibers. A flow diagram showing the steps involved in making PAN-based carbon fiber is shown in Fig. 8.3. The PAN precursor has a flexible polymer chain structure like any other polymer, but it has an all carbon backbone chain that contains polar nitrile groups, as shown in Fig. 8.4. During the stabilization treatment, the PAN precursor fiber is heated to 200-220 C, under tension. When this is done oxygen is absorbed, and it serves to cross-link the chains the fibers turn black, and a stable ladder structure is formed. A ladder polymer is a rigid... 

Experimental approaches have been reported by two groups. The group at Gunma University (25) synthesized a prototype of ladder polymers, poly(bicyclosilane)s. Polymers with three, four, and five ladder steps were synthesized, and the bathochromic shifts were measured, Ikehata et al, (26) attempted substitutional doping by thermal neutron irradiation of (SiH) ladder polymers synthesized through the reduction of trichlorosilane with lithium in THE (tetrahydrofuran). Thermal neutron capture by Si will yield P in this process for total flux amounts of up to 10 /cm. The concentration of phosphorus impurity is 0.1 ppm. No serious deterioration was caused by strong neutron irradiation. However, in this study, the detailed properties... 

The difficulties of the second, stepwise, synthesis of ladder polymers involve, in particular, the final step, namely the polymer-analogous formation of the double-stranded structure. Here, an optimal design of the open-chain precursor polymer is mandatory to arrive at a quantitative conversion with hi chemo-and regioselectivity in order to minimize structural defects. These problems have raised serious doubts in the literature [3] about the feasibility of this concept. 

Generally, two ways have been pursued to overcome these drawbacks. The first is to suppress the strong interchain interactions which cause the poor solubility and processibility [16]. These interchain interactions are drastically reduced when the distance between the polymeric backbones is increased, for instance via the introduction of bulky side-groups. The second possibility is to separate the formation of the first and second chain of the double-stranded polymer, and to process the materials at the stage of the soluble single-stranded intermediates. These precursors can be converted into the (insoluble) ladder polymers during the final step that is a favored thermal process carried out in the solid state. 

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