Step polymerization polyimides

Chemical vapor deposition (CVD) is a process whereby a thin solid film is synthesized from the gaseous phase by a chemical reaction. It is this reactive process that distinguishes CVD from physical deposition processes, such as evaporation, sputtering, and sublimation.8 This process is well known and is used to generate inorganic thin films of high purity and quality as well as form polyimides by a step-polymerization process.9-11 Vapor deposition polymerization (VDP) is the method in which the chemical reaction in question is the polymerization of a reactive species generated in the gas phase by thermal (or radiative) activation. 

Polyimides are class of polymers generally prepared from organic diamines and organic tet-racarboxylic acid dianhydrides by condensation polymerization. There are mainly two synthetic routes for polyimides one-step and two-step polymerization. Scheme 3.1 shows a generalized route of polyimide synthesis from an aromatic dianhydride and an aromatic diamine. 

In the second step, polyamic acid is cyclo-dehy-drated at elevated temperatures (thermal imidization) or in the presence of a cyclizing agent (chemical imidization). Advantages of this method over one-step polymerization are the use of less toxic solvents and direct processing of soluble polyamic acids to form the final polyimide products in the form of films or fibers by thermal imidization. However, the storage instability of polyamic acid intermediates and the control of thermal imidization are still important issues [28]. A detail description of the thermal and chemical imidization of poly(amic acid) is given below. 

As discussed above, numerous polyimides have been synthesized in many different methods and characterized. The direct production of high molecular weight aromatic polyimides in a one-step polymerization could not be accomplished because the polyimides are usually insoluble and infusible. The polymer chains precipitate from the reaction media (whether soluble or melt) before high molecular weights are obtained. Commercial polyimides are, therefore, classified by the several different processes they were prepared to afford reasonable processibility in the final product. 

Polyimides (PI) exhibit very good chemical, mechanical and dielectric stability at temperatures from -150 to 250 C. These rigid polymers with a high glass transition temperature are mostly used in (micro)electronics, aircraft industry, space exploration and as polymeric separation membranes. Linear polyimides (LPI) are traditionally prepared by the two-step polymerization. The polyimideprecursor, polyamic acid (PAA) (the most often a solution in N-methyl-2-pyrrolidone), is prepared from an aromatic dianhydride and an aromatic diamine. This precursor is transformed into a polyimide using thermal or chemical imidization (Figure 2) [5]. 

Polymerization by Transimidization Reaction. Exchange polymerization via equihbrium reactions is commonly practiced for the preparation of polyesters and polycarbonates. The two-step transimidization polymerization of polyimides was described in an early patent (65). The reaction of pyromellitic diimide with diamines in dipolar solvents resulted in poly(amic amide)s that were thermally converted to the polyimides. High molecular weight polyimides were obtained by employing a more reactive bisimide system (66). The intermediate poly(amic ethylcarboamide) was converted to the polyimide at 240°C. 

The thermal polymerization of reactive polyimide oligomers is a critical part of a number of currently important polymers. Both the system in which we are interested, PMR-15, and others like it (LARC-13, HR-600), are useful high temperature resins. They also share the feature that, while the basic structure and chemistry of their imide portions is well defined, the mode of reaction and ultimately the structures that result from their thermally activated end-groups is not clear. Since an understanding of this thermal cure would be an important step towards the improvement of both the cure process and the properties of such systems, we have approached our study of PMR-15 with a focus only on this higher temperature thermal curing process. To this end, we have used small molecule model compounds with pre-formed imide moieties and have concentrated on the chemistry of the norbornenyl end-cap (1). 

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 rate is slower in basic aprotic amide solvents, and faster in acidic solvents such as / -cresol. In general, the imidization reaction has been shown to be catalyzed by acid (14,32,33). Thermal imidization of poly(amic acid)s is catalyzed by tertiary amines (34). High temperature solution polymerization in -cresol is often performed in the presence of high boiling tertiary amines such as quinoline as catalyst. Dialkylaminopyridines and other tertiary amines are effective catalysts in neutral solvents such as dichlorobenzene (35). Alkali metal and zinc salts of carboxylic acids (36) and salts of certain organophosphorus compounds (37) are also very efficient catalysts in one-step polycondensation of polyimides. 

When use of one-step solution polymerization is an available option, generally it is the superior method to synthesize structurally pure polyimides because complete imidization and quantitative capping of end groups can be readily achieved in solution. 

Figure 5.21. Reaction schemes for the most common types of step-growth polymerization. Shown are (a/c) polyester formation, (b/d) polyamide formation, (e) polyamide formation through reaction of an acid chloride with a diamine, (f) transesterification involving a carboxylic acid ester and an alcohol, (g) polybenzimidazole formation through condensation of a dicarboxyhc add and aromatic tetramines, and (h) polyimide formation from the reaction of dianhydrides and diamines.

Chemical vapor polymerization of polyimides follows a different route from that of parylenes described above, in that, it is usually a two step process. First the monomers are adsorbed on the surface of the substrate resulting in the formation of a short-chained oligomer intermediate, and then the films are cured at a higher temperature ( 300°C) to form the desired... 

The differing behavior can be explained on the basis of their respective chemistries and their initial physical states. The polyimide polymerization occurs via a two step mechanism (Figure 4). First, the PMDA reacts with the ODA to yield a polyamic acid polymer. The polyimide is then formed by a ring closure reaction and yields water as a condensation by-product. Both reactions take place fairly rapidly at the temperatures at which the PMDA-ODA is manufactured. 

Step-growth polymerization is a very important method for the preparation of some of the most important engineering and specialty polymers. Polymers such as polyamides [7], poly(ethylene terephthalate) [8], polycarbonates [9], polyurethanes [10], polysiloxanes [11], polyimides [12], phenol polymers and resins, urea, and melamine-formaldehyde polymers can be obtained by step-growth polymerization through different types of reactions such as esterification, polyamidation, formylation, substitution, and hydrolysis. A detailed list of reaction types is shown in Table 3.2. 

A limited number of reports have appeared in the literature showing the use of water as a solvent for the microwave-promoted synthesis of thermoplastics. An example is the synthesis of water-borne polyimides using the standard polycondensation reaction of a dianhydride with a diamine (Seheme 3.1). In a scientific microwave unit, polymers with high molecular weights (Af up to 35.460 g/mol) were obtained within 22 min of heating using a one-pot two-step procedure. The dianhydride was first hydrolyzed in water to obtain the corresponding tetracarboxylic acid. This was then condensed with the diamine. The obtained polymers were completely comparable in their chemical and thermal properties to those obtained by conventional polymerization in m-cresol as solvent. 

Sequenced naphthalenic polyimides have been synthesized via a two-step condensation polymerization of aromatic diamines and dianhydrides as depicted in Scheme 6. The average length of the ionic sequence is controlled by... 

The synthesis of the polyimides (PI) is illustrated in Scheme 2. The polymerizations were conducted in a single "one pot" reactor, which minimizes solution transfer steps. Number average molecular weights, , of the oligomers were evaluated by end group analysis using from the integral ratio of the proton for 2AP, at... 

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