Polyamic Acids and Polyimides

Synthesis, Transformations, and Structure

Michael I. Bessonov, Vladimir A. Zubkov

• Publisher: CRC Press
• Year: 2020
• Print ISBN: 9780849367045
• eBook ISBN: 9781000141917
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Polyamic Acids and Polyimides surveys significant developments in basic research in the chemistry and physics of polyamic acids and polyimides over the last several years. Traditional and new topics are discussed, including catalytical imidization, chemical reactions at thermal treatment, quantum-chemical study of synthesis and structure, properties of isolated molecules, and supermolecular and crystalline structures. The book will be an excellent reference for researchers, practitioners, and graduate students working with polyimides and related heat-resistant polymers and materials.

Chapter 1

POLYIMIDE SYNTHESIS BY CHEMICAL CYCLIZATION OF POLYAMIC ACIDS

V. V. Kudryavtsev

TABLE OF CONTENTS

I.	Introduction
II.	Kinetics of Catalytic Cyclization in an Acetic Anhydride/Pyridine (Substituted Pyridine) System
III.	Kinetics of Catalytic Cyclization in an Acetic Anhydride/Triethylamine System
IV.	Effect of Selected Dehydrating Agents on Catalytic Cyclization Kinetics
V.	Kinetics of Polyisoimide Catalytic Isomerization
VI.	Kinetics of Polyisoimide Thermal Isomerization
VII.	Effect of Cyclization Conditions on Polyimide Properties
VIII.	Conclusion
Acknowledgments
References

I. INTRODUCTION

Data on polyimide formation from their prepolymers — polyamic acids — with the aid of chemical dehydrating agents first appeared in the patent literature in the mid-1960s.^{1, 2, 3} This process of polyimide formation is referred to as polyamic acid chemical (catalytic) cyclization (imidization).

Numerous patent publications suggest that chemical imidization can be used for obtaining polyimide films, fibers, press-powders, and other materials. This paper does not include a full patent literature analysis, but shall focus on the two principal ways of carrying out chemical imidization. One method discussed in detail in patents is that of keeping polyamic acid films or powders in baths containing catalytically active reagents in organic solvents; the other way is by adding the same reagents to polyamic acid solutions which are

The most suitable dehydrating agents have also been specified in earlier publications dealing with chemical imidization. The use of dehydrating agents such as the carboxylic acid anhydrides in the presence of tertiary amines having a catalytic effect,^{1, 2, 3, 4, 5} acetyl chloride,^4,6 thionyl chloride,⁴ phosphorus halides,⁷ and N,N-dicyclohexyl carbodiimide^4,8 has been reported. Data on chemical imidization occurring in the presence of organosilicon compounds are available.⁹

An acetic anhydride/pyridine (or 3-picoline) catalytic system is most frequently used for chemical imidization in production processes. For polyimide formation in solution, metal carboxylates (mainly acetates) are added along with acetic anhydride and pyridine.^{10, 11, 12} Acetic anhydride/triethylamine (quinuclidine) catalytic systems are rarely used.

The principal advantage in using the acetic anhydride/pyridine catalytic system is that it does not cause the degradation of polyamic acids in amide solvents, even at temperatures of 140 to 150°C,^13,14 and the reaction proceeds at rather high rates at substantially lower temperatures, beginning with room temperature.

Either imide or isoimide (γ-iminolactone) rings are observed in polymers resulting from chemical imidization, but most frequently rings of both types are present in these polymers in different ratios. Controlling the ratio of imide to isoimide rings in cyclization products is the main problem in the polyamic acid chemical imidization process. With regard to this problem, studies of the cyclodehydration reactions of the monoamides of dicarboxylic (phthalic, maleic, camphoric, etc.) acids are of particular importance. The reason is that the composition of the cyclic products of these reactions can be studied more thoroughly than the products of polyamic acid cyclizations.

It has been found that isoimides are mostly formed when treating monoamic acids with trifluoroacetic anhydride,^{15, 16, 17, 18, 19} ethylchloroformate,²⁰ or thionyl chloride.¹⁶

Imides have been mostly formed when treating monoamic acids with phosphorus anhydride and acetyl chloride.^16,21 Acetic anhydride can lead to amic acid cyclodehydration only in the presence of nucleophilic agents, which should be considered as the reaction catalysts. Both imides and isoimides and their mixtures have been obtained by using acetic anhydride mixed with triethylamine, sodium acetate, or acetic acid. Higher temperatures (60 to 100°C) as well as the presence of triethylamine and sodium acetate contributed to the imide formation.^{16,20, 21, 22, 23, 24, 25, 26}

Reaction mechanisms of amic acid cyclodehydration, including those in media containing acetic anhydride and tertiary amines, are discussed in Chapter 3 of this book.

Data on isomerization of the isoimides of dicarboxylic acids into imides are of great importance in investigating the isoimide-imide rearrangement in the products of polyamic acid cyclization. Results of isomerizations in which isoimides have served as the initial reactants are of particular interest.^24,26,...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface
• The Editors
• Contributors
• Table of Contents
• Chapter 1 Polyimide Synthesis by Chemical Cyclization of Polyamic Acids
• Chapter 2 Transformations of Solid Polyamic Acids at Thermal Treatment
• Chapter 3 Quantum Chemical Analysis of Polyamic Acids and Polyimides
• Chapter 4 Supermolecular Structure of Polyamic Acids and Polyimides
• Chapter 5 Macromolecules of Polyamic Acids and Polyimides..
• Index