Imide prepolymers

The idea of synthesizing imide oligomers which carry acetylenic terminations appeared attractive because homopolymerization through acetylenic endgroups occurs without any volatile evolution and provides materials with good properties. Landis et. al (8,9) published the synthesis of such acetylene terminated imide oligomers from benzophenone tetracarboxylic anhydride, aromatic diamine and 3-ethynylaniline via the classical route. As usual, the amide acid is formed as an intermediate which, after chemical cyclodehydration, provides the polymide. Since ethynyl-terminated polyimide is used as a matrix resin for fiber composites, processing is possible via the amide acid, which is soluble in acetone, or via the fully imidized prepolymer, which is soluble in NMP. The chemical structure of the fully imidized ethynyl-terminated polyimide is provided in Fig. 44. 

The fiber reinforcement was prepregged with a solution of the monomers in alcohol, followed by removal of the solvent. The prepared prepreg was then heated at 150-200°C involving an in situ condensation reaction to form the norbornene-endcapped imide prepolymer. Crosslinking was subsequently achieved by applying pressure and heating at 250-300° C. 

The first commercial product was introduced in 1975 as Thermid 600 by Gulf Oil Chemicals and is now sold by National Starch and Chemical Corporation. The fully imidized prepolymer has to be processed in NMP, with all its attendant problems. However, the molecular weight can be adjusted to provide a resin which melts at about 200° C and immediately starts to polymerize when molten and so, has a narrow processing window. 

The concept of low-molecular-weight imide prepolymers can be viewed as an alternative route to enhanced processability. The development of such systems has been conducted on the basis of three fundamental requirements. First, the prepolymers should be of low molecular weight, allowing for the possibility of a low melting point and low viscosity. Second, imide groups should be present in the prepolymer so as to remove the particularly troublesome polyamic acid to imide conversion process mentioned previously. Third, the prepolymers should have reactive terminal groups capable of reaction by an addition mechanism so as to convert the molten prepolymer to a cross-linked polymer without the harmful evolution of volatiles. 

A wide range of imide prepolymers have been developed on the basis of this approach. The various forms differ primarily in the type of terminal reactive group employed so as to convert the prepolymer to a cross-hnked product Three main types have achieved prominence and shown potential as High-temperature adhesives, these being prepolymers based upon norbomene, acetylene and maleimide (bismaleimide) functionality. 

Potyimide adhesives S J SHAW Condensation and thermoplastic polyimides, imide prepolymers high-temperature stability... 

Polydithiazoles Polyoxadiazoles Polyamidines Pyrolyzed polyacrylonitrile Polyvinyl isocyanate ladder polymer Polyamide-imide Polysulfone Decompose at 525°C (977°F) soluble in concentrated sulfuric acid. Decompose at 450-500°C (842-932°F) can be made into fiber or film. Stable to oxidation up to 500°C (932°F) can make flexible elastomer. Stable above 900°C (1625°F) fiber resists abrasion with low tenacity. Soluble polymer that decomposes at 385°C (725°F) prepolymer melts above 405° C (76l.°F). Service temperatures up to 288° C (550°F) amenable to fabrication. Thermoplastic use temperature —102°C (—152°F) to greater than 150° C (302°F) acid and base resistant. 

Polyvinyl isocyanate ladder polymer Polyamide-imide Soluble polymer that decomposes at 385°C (725°F) prepolymer melts above 405°C (761°F). Service temperatures up to 288°C (550°F) amenable to fabrication. 

The classical synthetic pathway to prepare polyimides consists of a two-step scheme in which the first step involves polymerization of a soluble and thus processable poly(amic acid) intermediate, followed by a second dehydration step of this prepolymer to yield the final polyimide. This preparative pathway is representative of most of the early aromatic polyimide work and remains the most practical and widely utilized method of polyimide preparation to date. As illustrated in Scheme 4, this approach is based on the reaction of a suitable diamine with a dianhydride in a polar, aprotic solvent such as dimethyl sulfoxide (DMSO), dimethylacetamide (DMAc), dimethylformamide (DMF), or AT-methylpyrrolidone (NMP), generally at ambient temperature, to yield a poly(amic acid). The poly(amic acid) is then cyclized either thermally or chemically in a subsequent step to produce the desired polyimide. This second step will be discussed in more detail in the imidization characteristics section. More specifically, step 1 in the classical two-step synthesis of polyimides... 

The PMR-15 chemistry looks very straightforward and ideal for the application as a composite matrix resin. In addition, the starting monomers are readily available and cheap. The fact that the imidization reaction, which forms the prepolymer at moderately low temperatures, could be separated from the crosslinking reaction was thought to be the key to easy laminate processing and void free laminates. However, after more than twenty years of research and development, it is known that both reactions are very complex and dependent... 

Considerable interest exists for the development of powder impregnation methods. In this technique, the resin is applied to the fiber as a dry powder or as a powder carried in a liquid slurry. However, here it is necessary to synthesize a fully imidized PMR prepolymer as a fine powder and thus the PMR concept as such is lost. 

The key to acetylene terminated polyimides is the availability of the end-capper which carries the acetylene group. Hergenrother (130) published a series of ATI resins based on 4-ethynylphthalic anhydride as endcapping agent. This approach first requires the synthesis of an amine-terminated amide acid prepolymer, by reacting 1 mole of tetracarboxylic dianhydride with 2 moles of diamine, which subsequently is endcapped with 4-ethynylphthalic anhydride. The imide oligomer is finally obtained via chemical cyclodehydration. The properties of the ATI resin prepared via this route are not too different from those prepared from 3-ethynylaniline as an endcapper. When l,3-bis(3-aminophenox)benzene was used as diamine, the prepolymer is completely soluble in DMAc or NMP at room temperature, whereas 4,4 -methylene dianiline and 4,4 -oxydianiline based ATIs were only partially soluble. The chemical structure of ATIs based on 4-ethynylphthalic anhydride endcapper is shown in Fig. 45. 

Conditions for low temperature solution polymerizations of pyromellitic dianhydride (PMDA) have been developed for a wide variety of aromatic 1,4-phenylene [54, 55] and 4,4 -biphenylene [56-58] diamine monomers in a number of aprotic solvents to give high molecular weight prepolymers referred to as polyamic acids. Since the imidized structures are insoluble, they must be processed in the form of their polyamic acids which are subsequently imidized thermally or by chemical dehydrating agents. Although this procedure is acceptable for thin film or fibers, the fabrication of thick parts is complicated by the water of imidization. 

Hoback JT, Holub F (1971) Siloxane-containing prepolymers for making poly(amide imides). US Patent 3723385 5 pp... 

In order to control the pore texture in carbon materials, blending of two kinds of carbon precursors, the one giving a relatively high carbonization yield and the other having a very low yield, was proposed and called polymer blend method [112], This idea gave certain success to prepare macroporous carbons from poly(urethane-imide) films prepared by blending poly(amide acid) and phenol-terminated polyurethane prepolymers [113]. By coupling this polymer blend method with... 

Other linear polymers suitable for high-temperature structural laminates are obtained by the reaction of bis(furfuryl) imide via Diels-Adler reactions (56,57). Stability in air up to 500 °C has been reported for these polymers. The prepolymers have pendent phenyl substituents and are soluble in organic solvents, an important processing improvement for hetero-aromatic polymers. 

Core-shell polystyrene-polyimide high performance particles have been successfully prepared by the dispersion copolymerization of styrene with vinyl-benzyltrimethyl ammonium chloride (VBAC) in an ethanol-water medium using an aromatic poly(amic acid) as stabilizer, followed by imidization with acetic anhydride [63]. Micron-sized monodisperse polystyrene spheres impregnated with polyimide prepolymer have also been prepared by the conventional dispersion polymerization of styrene in a mixed solvent of isopropanol/2-methoxyethanol in the presence of L-ascorbic acid as an antioxidant [64]. 

Examples of homopolymers are given. Poly(4-vinylphenol) was prepared as a prepolymer for the subsequent alkylation [55]. Poly[2-(4-vinylbenzyl)hydroqui-none] 65 is an example of the unhindered phenolic antioxidant for rubbers. Many homopolymers bear a hindered phenolic moiety. Homopolymer 66 was proposed for blending with BR and IR [56]. Other examples are poly[vinyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] [57] (67), poly(3,5-di-/ert-butyl-4-hydroxy-benzyl methacrylate) [58] (68) or poly[iV-3,5-di-tert-butyl-4-hydroxybenzyl) male-imide] [59] (69). Numerous polymeric antioxidants are functionalized with aromatic amine groups. Poly(4-anilinophenyi methacrylate) [53] (70) serves as an example. 

The PMR range of resin materials utilize a two-step reaction, in which a norbornene-terminated prepolymer is formed in situ by the condensation reaction of low molecule weight diamines with norbornene-capped imides or acid esters [37]. The PMR-15 designation refers to polymerization of monomeric reactants, with a mixture of diaminodiphenylmethane and a dimethyl ester and a norbornene-containing monomethyl ester, to form a condensation prepolymer having molecular weight approximately equal to 1500. PMR resins are used as matrix materials in high performance composites in the aerospace industry. 

Wong and coworkers. [82, 83] used solution and solid-state NMR to study the cure of the norbornene end-capped poly(imide)s 2NE/DDM and PMR-15. At lower cure temperatures, exo-endo isomerization was observed to be a major product [84]. Loss of cyclopentadiene was suggested to result in initiation of the partner maleimide, as well as Diels-Alder addition of cyclopentadiene with norbornene. There was no evidence of internal double bonds formed by incorporation of the cyclopentadiene into the polymer backbone. The solid-state spectra were very poorly resolved, but did allow confirmation of the mechanism of reaction [83]. Spectra obtained at higher fields did not show improved resolution, indicating that the dominant mechanism for line-broadening in these materials is the dispersion of isotropic chemical shifts resulting from frozen conformations. In later work, Milhourat-Hammadi and coworkers [84, 85] reported solution-state and NMR studies of PMR-15 prepolymers. 

Through the synthesis of poly(urethane-imide) films and their carbonization, carbon films were obtained whose macropore structure could be controlled by changing the molecular structure of polyurethane prepolymer [164-166]. Poly(urethane-imide) films were prepared by blending poly(amide acid), which was synthesized from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA), and phenol-terminated polyurethane pjrejwlymers, which were synthesized through the reaction of polyester polyol with either hexamethylene diisocyanate (HDI), tolylene-2,4-diisocyanate (TDI) or 4,4 -diphenyknethane-diisocyanate (MDI). The reaction schemes of two components, poly(imide) (PI) and poly(urethane) (PU), are shown in Fig. 46a). 

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