Achievements and prospects for the synthesis of poly(meth)acrylimide foams. Stage of the thermal imidisation of polymer precursors

Abstract

The review summarises the trends in the development of research on the synthesis of polymethacrylimides (PMIs) and polyacrylimides by the method of intramolecular thermal imidisation of (meth)acrylic polymers. Along with the widely used industry variant of PMI foam of the ‘Rohacell’ series based on bulk copolymers of methacrylonitrile and methacrylic acid, intensive research on alternative variants began after 2005. This review describes the main and side reactions when using polymer precursors of different structures as well as the effect of precursor composition and structure on the properties of the resulting poly(meth)acrylimides. It has been shown that the achievements of the last 15 years provide a basis for reducing the cost of poly(meth)acrylimides which will significantly expand the areas and scale of their application.

Keywords

Polymethacrylimides polyacrylimides foamed polymers Rohacell thermal imidisation

Introduction

Polymethacrylimide (PMI) foams are of particular importance because they are materials with a unique combination of low density, a closed porous cell structure, high strength and heat resistance.^1

-7 Since the 1970s, PMI foams of the ‘Rohacell’ brand (Evonik Company) have been successfully used as structural sandwich core in the manufacture of elements for space rockets, aeroplanes and helicopters as well as in a number of other areas such as the manufacturing of sports equipment and car parts.^8

-14 In terms of technical characteristics (primarily heat resistance), PMI foams surpass such alternative materials of similar purpose as rigid foams based on polystyrene, polyvinyl chloride and polyurethanes. At the same time, PMI foams of the ‘Rohacell’ brand are more expensive materials, but they have a lower price than alternative foams based on heteroaromatic polyimides obtained by polycondensation. Non-foamed thermoplastics, for example, ‘Pleximid’ (Evonik), or similar products from other companies are another type of PMI used in industry.^15

-19 These products are produced directly in the form of organic glass sheets by extrusion or in the form of granules for the subsequent production of organic glasses. Such glasses have high heat resistance and mechanical strength while maintaining high transparency, light transmission and weather resistance. They are more expensive, so they have limited application in light-transmitting products with increased requirements for heat resistance.

In conventional terminology, PMIs are methacrylic or acrylic–methacrylic polymers containing a significant amount of glutarimide fragments (Figure 1).^20

-23 Industrial PMI foams are produced by a chemical transformation of polymers with the formation of imide fragments due to the interaction of nitrile (or amide) with carboxyl groups of different macromolecular units. For example, the PMIs of the ‘Rohacell’ brand can be obtained by the thermal imidisation of high molecular weight copolymers of methacrylonitrile (MAN) and methacrylic acid (MAA) in the presence of ‘external’ or ‘internal’ foaming agents.^24

-30 Polymer precursors are preliminarily obtained by bulk polymerisation.^29

-33 The first group includes, for example, tert-butanol or a mixture of alcohols (physical agents), while the second group includes tert-butyl methacrylates (chemical agents). The units of the latter are a part of the polymer precursor and eliminate isobutene at the thermal imidisation stage. Industrial non-foamed PMIs are obtained by high-temperature interaction of suspension polymethyl methacrylate with ammonia or an amine.^34

-37 Another known method of PMI synthesis is the polymerisation of dimethacrylimides,^21,38 but this method is not used because of the high cost and low environmental friendliness of the production of initial monomers.

Figure 1.

Structures of PMIs and PAIs containing the glutarimide ring as a main fragment of P(M)AI foams. PMIs: polymethacrylimides; PAIs: polyacrylimides; P(M)AI: poly(meth)acrylimide.

In recent years, along with PMIs, polyacrylimides (PAIs) have also begun to attract the interest of researchers after a long pause. They also contain the glutarimide ring as a main fragment (Figure 1). PAIs can be obtained by the polymerisation of diacrylimides (but these monomers, similar to dimethacrylimides, are not promising for practical use); intramolecular thermal imidisation of copolymers of acrylonitrile (AN) and acrylic acid (AA)^39,40 or intramolecular thermal imidisation of copolymers of α-cyanacrylic acid.⁴⁰ We therefore decided to use a common name ‘poly(meth)acrylimides’ (P(M)AIs) when discussing imide polymers.

This review is devoted to the analysis of the literature data on the synthesis of P(M)AIs only by the method of intramolecular thermal imidisation of (meth)acrylic polymer precursors of different compositions as well as to the analysis of variants of obtaining such precursors. Studies of the intramolecular thermal imidisation of (meth)acrylic polymers started in the 1950s to 1960s of the last century,^41

-44 and the development of these studies can be divided into three stages:

The stage of prospecting research which ended when the technology of production of PMI ‘Rohacell’ by the method of thermal imidisation of the bulk copolymer of MAN and MAA was implemented in 1972. The state of research carried out at this stage is well described in the work of Gänzler et al.²⁰

The stage of improvement of PMI ‘Rohacell’ production technology starting in the 1970s and continuing up to the present day: improvement of the properties of this material, development of new brands and expansion of the range of applications.

In the 1990s, the search for variants changing the composition of PMI polymer precursors, studies of the patterns of their thermal imidisation and investigation of the properties of obtained materials began. It intensified sharply in 2005.

Thus, for many years after the beginning of industrial development of PMI foams, the main directions of research were improving the technology of the synthesis of bulk MAN-MAA copolymers, their thermal imidisation and foaming,^{27,29,31,33,45

-49} studies of the various properties of PMI foams^{1

-7,50

-53} and improvement of the physico-mechanical, fire-resistant and other properties of PMI foams due to the introduction of various additives.^4,52,54
-56 Many works are devoted to obtaining and studying the characteristics of sandwich materials and products obtained using PMI.^8,57

-62 All these issues require separate consideration and are not the subject of this review.

However, despite the great progress made in the development of PMI technologies, their commercial potential is largely restrained by the high cost. This is due to the use of expensive initial monomers (MAN, MAA) and low productivity of the polymer precursor production stage. The implemented technology of the production of ‘Rohacell’ PMI does not allow the use of cheaper basic monomer raw materials, in comparison with MAN and MAA.²⁰

The situation has changed in the last 15 years. While many studies are still being published on improving the technology of production of ‘Rohacell’ PMI foams,^24,28,63 other foams (of different composition) are also being actively developed. Many studies are aimed at finding cheaper monomer raw materials to produce polymer precursors, from which PMI foams of good quality can be obtained. In this connection, chemical aspects of the reactions of imidisation of new poly(meth)acrylic precursors and properties of P(M)AI obtained are investigated. Another important direction of research is the attempt to find alternatives to the synthesis of polymer precursors by the bulk periodic polymerisation in moulds.

Previously, several brief reviews on the production and properties of PMIs were published,^64,65 but they focused on applied aspects. In addition, those reviews could not consider the numerous works published later, which have yielded new and interesting results on the thermal imidisation of nitrogen-containing precursors. The purpose of this review is:

to consider the chemical patterns of intramolecular thermal imidisation of nitrogen-containing poly(meth)acrylic precursors of different composition;

to analyse the dependence of P(M)AI properties on the characteristics of initial polymer precursors;

to analyse the variants of synthesis of polymer precursors with the required properties; and

to evaluate the possibility of reducing the cost of obtaining P(M)AI with different properties and the prospects for expanding the areas and scale of P(M)AI application.

Section 1: Thermal imidisation of copolymers of (meth)acrylonitrile and (meth)acrylic acid

The technology of the production of ‘Rohacell’ PMI foam by the method of thermal treatment of methacrylic copolymers in the presence of foaming agents was developed and patented by the specialists of the ‘Rohm’ company in the late 1960s to early 1970s.^{41

-44,66

-69} In 1970, ‘Rohm’ company’s researchers published the policy article.²⁰ In this article, the results of research on the synthesis of PMI foams based on copolymers of (meth)acrylonitrile and (meth)acrylic acid were systematised for the first time. Intramolecular imidisation involving nitrile and acid groups of a polymer precursor to form glutarimide fragments was presented as the main reaction (Figure 2). The conditions for this reaction were indicated: high temperature (from 170°C to 240°C) and sufficiently long thermolysis time (2–4 h).

Figure 2.

Intramolecular thermal imidisation of copolymers of (meth)acrylic acid and (meth)acrylonitrile. R=H (acrylic acid, AA), CH₃ (methacrylic acid, MAA); R¹=H (acrylonitrile, AN), CH₃ (methacrylonitrile, MAN).

When reacting nitrile and carboxyl groups of different macromolecules, intermolecular imidisation occurs (Figure 3). However, this reaction should not be considered as a side reaction since cross linking of macromolecules in the production of PMI foams is also one of the tasks to be achieved. Partial cross linking of macromolecules is usually carried out at the stage of obtaining polymer precursors using cross-linking agents. Among them are triallyl cyanurate,^26,67 allyl methacrylate,^24,26,70 ethylene glycol dimethacrylate,⁶⁷ methacrylic diester of polyethylene glycol 400⁶³ and others. As shown subsequently, the ratio of intramolecular and intermolecular imidisation depends on the temperature and structure of a polymer precursor.

Figure 3.

Intermolecular thermal imidisation of (M)AN-(M)AA copolymers to form cross-linked macromolecules (designations are the same as in Figure 2).

During the high-temperature treatment of (M)AN-(M)AA copolymers, the interaction of neighbouring acid units proceeds to form anhydride fragments (Figure 4).^20,22,23 To reduce moisture absorption and increase the resistance of PMI foams to other influences, it is necessary to reduce the content of anhydride groups in the polymer.^66,68,69,71 Obviously, the contribution of the reaction (4) should decrease with a decrease in the content of carboxyl dyads and triads in polymer precursors. When the ratio of (M)AN and (M)AA units in the polymer is close to equimolar, the contribution of reaction (4) can be decreased by increasing the degree of alternation of nitrile and carboxyl units. A technique that allows the achievement of such a polymer microstructure was recently proposed.⁷² It is well known that at 180°C and above, a cyclisation reaction involving nitrile groups takes place in homo- and copolymers of AN (Figure 5).^73,74 Catalysis by neighbouring carbonyl groups accelerates this reaction,⁷⁵ so the thermal imidisation of nitrile-containing polymers is almost always accompanied by the formation of polyene nitrogen-containing structures.²² It is the formation of polyene structures that is the main cause of the appearance of the yellow–brown colour of PMI foams. Many researchers note that this colour is increased at imidisation temperatures above 200°C and especially with an excess of nitrile units in a polymer precursor.^23,76 It is also clear that the contribution of reaction (5) should decrease when the content of (meth)acrylonitrile microblocks in the polymer precursor decreases, that is, for precursors with alternating nitrile and carboxylic units.

Figure 4.

Interaction of carboxyl groups of the polymer precursor to form anhydride fragments in macromolecules.

Figure 5.

Interaction of nitrile groups of the polymer precursor to form polyconjugated nitrogen-containing heterocycles.

Foaming agents are often used when obtaining PMI foams, for example, urea, methyl urea, formamide, N-methylformamide and others.^{30,33,45,66,72,77
-79} They decompose with the release of ammonia or methylamine. In this case, the ammonia or methylamine formed reacts with anhydride units at high temperatures (Figure 6) which leads to the additional formation of imide fragments. Another option for the formation of imide units is the intermediate formation of amide groups due to the interaction of carboxyl groups with ammonia or methylamine (Figure 7) and subsequent condensation of amide and carboxyl groups.²² Such condensation occurs even more easily in those cases when amide units are pre-introduced into the polymer precursor (see Section 2).

Figure 6.

Formation of imide fragments due to the interaction of anhydride groups with ammonia or amines.

Figure 7.

Amidation of carboxyl groups and the reaction of carboxyl and amide groups to form imide units.

Polymer precursors may contain internal foaming agents – tert-butyl methacrylate units which eliminate isobutene and turn into MAA units when heated (Figure 8).^28,63,79,80 As a result, the fraction of carboxyl groups capable of participating in the imidisation reaction increases in the polymer chain. For example, in the work of Takashi et al.,⁷⁹ the initial polymer precursors were the copolymers of tert-butyl methacrylate and MAN (with molar ratio from 2:3 to 3:2), that is, all carboxylic units were formed as a result of thermal decomposition of ester groups. However, in this case, the resulting PMI foam had a non-uniform cell size and, as a result, non-uniform density and low physico-mechanical properties. The use of MAN-MAA-tert-butyl methacrylate terpolymers as polymer precursors allows obtaining PMI foam with small uniform cells (which leads to good physico-mechanical properties and low moisture absorption).⁸⁰ The authors explain this by the fact that the optimal content and uniform distribution of the ester units along the macromolecular chain leads to the formation of isobutene in the quantities necessary for optimal foaming and the content of carboxylic units is optimal for the imidisation reaction.

Figure 8.

Intermediate formation of MAA-MAN copolymer during thermolysis of tert-butyl methacrylate-MAN copolymer. MAA: methacrylic acid; MAN: methacrylonitrile.

Infrared (IR) spectroscopy is the primary method used to estimate the degree of completion of most reactions presented due to the insolubility of the cross-linked PMI foam in water and organic solvents. Spectra of model low-molecular-weight compounds, polymers and products of their thermal transformations are used for peak identification. For example, peaks of carbonyl groups of anhydride cycles (1820–1850 cm⁻¹ and 1750–1770 cm⁻¹) and the peak of the anhydride fragment C–O–C (1020 cm⁻¹) appear in the IR spectra of polymethacrylic acid after its thermolysis (reaction (4)).^22,23,81 Moreover, the high intensity of the latter indicates a significant fraction of intermolecular linear anhydride cross links, since for linear model anhydrides, this band is more pronounced, while for cyclic anhydrides, it is weaker.⁸¹

In the IR spectra of polyacrylonitrile and polymethacrylonitrile, multiplet broad bands appear in the region of 1490–1690 cm⁻¹ after heat treatment which correspond to the C=N–C fragments of the conjugated polycyclic structure (reaction (5)).^76,82,83 The appearance of the absorption band of the C–N–C fragment (1210–1220 cm⁻¹) after heat treatment of MAN-MAA, AN-MAA and AN-AA copolymers indicates the imidisation process while the appearance of weak bands at the frequencies corresponding to the vibrations of anhydride groups indicates that the side reaction (4) or formation of intermolecular anhydride cross links is proceeding.^22,23,39 In the case of thermal imidisation of AN-MAA copolymers which have an excess of nitrile units the intensity of the C=N–C band of the conjugated polycyclic structure increases in PMI foam spectra.⁸⁴ Based on the comparison of the IR spectra of the heat-treated AN-MAA copolymer with the spectra of polymethacrylic acid and low-molecular models, it is noted⁸¹ that the band of anhydride groups can overlap with the bands of ketone groups. Such groups can be formed during the intramolecular or intermolecular decarboxylation of MAA units, for example, in cases when MAA units are arranged in a ‘tail-to-tail’ manner or when non-neighbour carboxylic groups interact (Figure 9). In these cases, dehydration with the formation of seven- or eight-membered anhydride cycles is disadvantageous. It is known⁸⁵ that decarboxylation during the thermolysis of binary acids is more likely than dehydration if the formation of five- or six-membered anhydride cycles through dehydration is impossible.

Figure 9.

Formation of ketone structures as a result of decarboxylation involving two carboxylic groups.

Thus, when heated the (M)AN-(M)AA copolymers can undergo a number of different transformations depending on various factors. The conditions suggested in the literature for the heat treatment of polymer precursors are determined by the requirements for achieving the maximum degree of imidisation while minimising undesirable side transformations. IR spectroscopy has made it possible to establish that the imidisation reaction during heat treatment of bulk copolymers of (M)AN and (M)AA proceeds intensively in the temperature range from 170°C to 220°C.^22,23 At lower temperatures, the reaction proceeds too slowly.⁷⁶ For example, the formation of imide cycles in MAN-MAA copolymers was not observed even after a long heating time (21 h) at the temperature of 140°C. The use of temperatures above 220°C for imidisation is limited by the noticeable cyclisation of neighbouring nitrile groups of polymers (reaction (5)).

The physical process of polymer foaming which is parallel to chemical reactions during high-temperature treatment of polymer precursors complicates the process considerably. Many papers cover the study of the influence of temperature–time parameters of heat treatment on the properties of the resulting P(M)AI foams. Typically, both the temperature and the treatment time are varied (from 20 min to several hours), as well as various options are used to organise the process: single-stage foaming or the use of multiple heat treatment stages with different temperature modes.^{20,25,29,42,43,45,66

-69,86

-89} The choice of specific process conditions depends on the specified requirements (especially, the density of PMI foam and its strength characteristics), on the properties of the foaming agent and other factors. As mentioned above, consideration of these issues is beyond the scope of this review.

From the very beginning of research on the thermal imidisation of (M)AN-(M)AA copolymers, the most important practical question was to determine the optimal composition of a polymer precursor for the production of PMI foams. In the first stages of the development of PMI foams, along with nitrile and carboxyl units, the units of other monomers were often introduced in polymer precursors mainly with the aim of reducing the cost of a polymer and softening conditions of bulk polymerisation. For example, methyl methacrylate,^41,66,69,90 butyl methacrylate⁶⁷ and other alkyl (meth)acrylates,^21,22,25 styrene or α-methylstyrene,^21,26,30 maleic anhydride^30,91 and other comonomers were suggested to be introduced. However, by the beginning of the third stage of the development of studies on the thermal imidisation of (M)AN-(M)AA copolymers, the use of additional comonomers (with the exception of cross-linking monomeric agents) was reduced to almost two options: tert-butyl methacrylate as an internal foaming agent and acrylamide (AAm) additives (see Section 2). This is due to the fact that the use of other monomers reduces the degree of imidisation and quality of PMI foams.

The developers of ‘Rohacell’ technology in the early 1970s^20,90,92 presented the following considerations for the choice of a polymer precursor obtained by bulk polymerisation in silicate forms in water baths:

The polymerisation rate should not be too high as it happens during the synthesis of polymer precursors based on pairs of highly active monomers – MAA, AA and AN. In particular, when using AN, the polymerisation proceeds so vigorously that it is difficult to carry out while obtaining a high-quality product and good reproducibility.

The copolymerisation of MAN and AA proceeds too slowly (5–10 days) to obtain a polymer precursor with the desired properties.

The bulk copolymerisation of MAN and MAA proceeds at a rate sufficient for efficient heat removal and the maintaining of the isothermal mode.

The use of AA causes a strong adhesion of the obtained polymer sheets to the silicate form (this leads to an increase in rejects when removing polymer glass).

The presence of AN units in a polymer precursor leads to a stronger colour of PMI foam (as compared to MAN-based polymers).

PMI foams containing only imide units have the lowest water absorption.

For the other physicochemical properties (e.g. heat resistance and resistance to solvents), P(M)AIs synthesised on the basis of different pairs of (meth)acrylonitrile and (meth)acrylic acid do not have significant differences. However, it has been noted²⁰ that precursors based on AA form less heat-resistant PMI compared to MAA-based precursors. Heat resistance is also noticeably reduced when methyl methacrylate or styrene units are introduced into precursors.

Based on these arguments, the authors concluded^20,90,92 that the use of the MAN-MAA pair is optimal. Therefore, for 50 years, PMI of ‘Rohacell’ brand has been produced on the basis of the polymer precursor MAN-MAA despite the much higher cost of MAN and MAA compared to AN and AA, respectively. In many patents, the copolymerisation of MAN and MAA is still suggested to obtain polymer precursors.^31,46
-48

Later on, in several studies, the peculiarities of thermal imidisation of precursors containing acrylic or methacrylic nitrile and carboxylic units were compared. Thus, it was shown⁹³ that for AN-AA copolymers the activation energy of cyclisation reactions is 1.5 times higher than for the AN-MAA pair. Due to the shortage of carboxylic groups in the polymers, the main direction of cyclisation is the reaction (5), and the authors even consider the formation of imide structures as the initiating stage of nitrile oligomerisation to form polyconjugated systems. A direct correlation was found between the glass transition temperature and the temperature of the beginning of the foaming of AN-MAN-MAA terpolymers⁹⁴; the increase in the content of MAA units increases the required foaming temperature; and the replacement of MAA units with AA units does not have a noticeable effect.

An important issue is the effect of the ratio of nitrile and carboxyl units and microstructure of polymer precursors on the P(M)AI properties. To increase the contribution of reaction (1), both the overall ratio of nitrile and acid units close to equimolar and the highest degree of their alternation should be provided. However, in the first decades of research on the development of PMI foams, a variety of ratios of (M)AN and (M)AA units in polymer precursors were used: from a 1.5-fold (or even more) excess of nitrile groups^20,30,41,67 to a no less significant excess of carboxyl units.^{29,43
-45,49,66} Sometimes the expediency of using an excess of nitrile units in a precursor is attributed to the importance of minimising the content of anhydride units in PMI.²⁰ At the same time, the non-equimolar ratio of nitrile and carboxyl units leads to an increase in the content of microblocks in the polymer precursor. As a result, the contribution of the side reactions may increase.

As the PMI production process improved and many technical problems were being solved, in practice, a ratio of (M)AN and (M)AA units close to equimolar became frequently used,^{24,27,69,70,72,95,96} although to obtain PMI with the required properties, the ratio of nitrile and carboxyl units was sometimes significantly different from equimolar according to recent publications.^28,63 In a few recent papers, the effect of the structure and ratio of (meth)acrylonitrile and (meth)acrylic acid units in polymer precursors on thermal imidisation and the properties of PMI foams was also considered.^39,72,84,97 A comparison of the IR spectra of heat-treated copolymers of MAN-MAA (1:1) and AN-AA (1:1) showed⁸⁴ that in both cases a similar degree of imidisation was achieved. At the same time, the acrylic precursor, unlike the methacrylic one, is poorly foamed during heat treatment. The reason is that the macromolecules are cross linked too much due to the chain transfer reaction to the monomer which is typical for the radical polymerisation of acrylic monomers in bulk. The authors noted that the preparation of non-foamed heat-resistant PAIs based on AN-AA copolymers is an achievable task and may have practical prospects.

Copolymers of AN-MAA have a high degree of microblockiness due to the strong differences in the reactivity of the comonomers. The low content of nitrile-acid dyads limits the maximum attainable degree of intramolecular imidisation. Comparison of IR spectroscopy data, thermomechanical studies and data on the imidisation at different temperatures allowed authors to distinguish between the processes of intramolecular (reaction (2)) and intermolecular imidisation (reaction (3)).³⁹ The glass transition temperature of the non-imidised copolymer of AN-MAK (1:1) is 55°C; for the copolymer with a statistical distribution of units and degree of intramolecular imidisation of 25%, the glass transition temperature is 190°C; for polymers cross linked by intermolecular imide bonds, the flow temperature coincides with the temperature of the beginning of decomposition: 320°C. The maximum degree of imidisation (or maximum mole fraction of imide groups in a copolymer) can be determined based on the results of NMR studies of the microstructure of a polymer precursor by the formula⁹⁷

X_{max} = X_{ANA} + 0.5 X_{ANN}

where X _ANA and X _ANN are the mole fractions of MAA-AN-MAA and MAA-AN-AN triads in the copolymer, respectively.

For example, for the AN-MAA copolymer (1:1) obtained by the bulk copolymerisation, the maximum degree of intramolecular imidisation was calculated to be 32% and according to the experimental data (IR spectroscopy) was equal to 30%; for the copolymer containing 31 mol% of nitrile units, the calculated and experimental maximum degrees of imidisation were 13%. According to Dyatlov et al.,⁹⁷ for AN-MAA copolymers, the intramolecular imidisation proceeds at 120°C and the imidisation degree reaches the maximum values in 0.5 h. Further, imidisation has an interchain nature; it increases the degree of cross linking of polymers and requires a higher temperature (the same as in the preparation of PMI foam). However, from the point of view of optimising the properties of PMI foams, it is necessary to optimise the ratio of intramolecular and interchain imide bonds.

In recent years, methods have also been proposed for increasing the degree of alternation of nitrile and acid units by introducing additives into monomer mixtures during the synthesis of AN-MAA polymer precursors.⁷² PMI foams based on the polymer precursors with a high degree of alternation of AN and MAA units are not inferior in their properties to PMI foams based on the MAN-MAA copolymers.^72,95,96 This opens up opportunities for obtaining high-quality PMI foams based on cheaper monomer raw materials using AN instead of MAN. Without ordering nitrile and carboxyl units, the strength characteristics of PMI foams based on AN-MAA copolymers are somewhat inferior to those of the ‘Rohacell’ brand, but their properties can be improved using stress-whitened polymer precursors,⁹⁸ by increasing the concentration of cross-linking agents when obtaining precursors or by additional heat treatment of foams (160°C, 6 h).⁹⁹ The introduction of small amounts of AAm units into the precursor polymer of AN-MAA is an even more effective technique (see Section 2).

Section 2: Thermal imidisation of amide-containing poly(meth)acrylic precursors

As shown in Section 1, the thermal imidisation of (M)AN-(M)AA copolymers in the presence of nitrogen-containing foaming agents can proceed through partial intermediate formation of unsubstituted amide or N-alkylamide fragments (Figure 7). If a polymer precursor initially contains amide units, they are also capable of participating in imidisation reactions. Thus, the introduction of amide units in the (M)AN-(M)AA copolymers should not lead to a decrease in the degree of imidisation of P(M)AI foams. Moreover, various authors have repeatedly published data on the improvement of the properties of PMI foams in cases of the use of amide-containing polymer precursors.^50,91 However, in spite of the sufficiently large amount of information, for many years, such precursors were only proposed in patents and there were no articles describing research on this topic. The situation has changed in the last 15 years. In this section, we will retrospectively review patent information on the thermal imidisation of amide-containing polymer precursors, but we will mainly focus on articles of recent years.

At the first stage of research on the development of PMI, the options of using amide-containing precursors that do not contain nitrile and carboxyl units were considered along with the use of (M)AN-(M)AA copolymers. In this case, imidisation can proceed in accordance with Figure 10 (intramolecular imidisation) and Figure 11 (intermolecular imidisation).^22,23 In particular, thermal imidisation of polyacrylamide or polymethacrylamide was proposed for the preparation of foams in patents.^100
-102 According to the patent,¹⁰¹ homopolymers of methacrylamide (MAAm), copolymers of MAAm with methyl methacrylate or butyl methacrylate, MAAm-methyl methacrylate-methyl acrylate terpolymers underwent imidisation (according to IR spectroscopy) and did not contain residual amide units when heated at 130–170°C. The copolymer of N-methylmethacrylamide and methyl methacrylate after heating for 5 h at 230°C formed the product containing 63% of imide units.¹⁰³

Figure 10.

Intramolecular imidisation involving two amide units.

Figure 11.

Intermolecular imidisation involving two amide units.

It is reported^102,104 that thermal imidisation (180–220°C, 0.5–6 h) of MAAm homopolymers, copolymers of MAAm (or N-methylmethacrylamide) with methyl methacrylate in the presence of methanol provides a high degree of imidisation of amide groups (more than 50%) and a low content of amide, carboxyl and anhydride units in the resulting non-foamed PMI. The basis for the formation of imide groups during the reaction is amide units while methanol esterifies carboxyl and anhydride groups resulting from side reactions. For copolymer precursors containing methyl methacrylate units, the degree of imidisation can even exceed 100% (taking into account the content of the initial amide units). The reason is the partial imidisation of the ester groups with ammonia, which is formed by reaction (10). The properties of the obtained non-foamed PMI depend on the ratio of imide and ester units; in particular, the glass transition temperature increases with decreasing content of ester units in the product.

PMI foams can also be obtained on the basis of other polymer precursors of amide–ester type. Such a precursor is, for example, the copolymer of MAAm and tert-butyl methacrylate.¹⁰⁵ As mentioned above, the tert-butyl methacrylate units are used as internal foaming agents; they undergo conversion into carboxyl units with the formation of gaseous isobutylene (reaction (8)). Therefore, the authors¹⁰⁵ presented the scheme of thermal imidisation of the said precursor with the intermediate formation of a copolymer containing amide and carboxyl groups. Such copolymers are further imidised by the interaction of the amide and carboxyl units (Figure 7) although their additional imidisation according to Figure 10 is also possible. Thermal imidisation and foaming of the MAAm-tert-butyl methacrylate copolymer are carried out at 190–250°C (0.7–3.2 h). The resulting product contains up to 78% of imide units and, according to the authors, surpasses the standard PMI of the ‘Rohacell’ brand in chemical homogeneity.

In the studies,^106,107 the differences in the IR spectra for copolymers of MAAm-n-butyl methacrylate, AAm-n-butyl methacrylate and products of their heat treatment were analysed. Due to the disappearance of the carbonyl group peak (1740 cm⁻¹) in the spectra of products, the authors conclude that ester and amide groups can directly interact to yield an imide ring (Figure 12).

Figure 12.

Direct interaction of the ester and amide groups to yield the imide fragment.

Thermal imidisation and foaming of the MAAm-n-butyl methacrylate copolymer were carried out with a stepwise increase in temperature: 110°C (2 h), 180°C (3 h) and 160°C (5 h).¹⁰⁶ As a result of heat treatment, high-quality PMI foam was obtained. For thermal imidisation of the AAm-n-butyl methacrylate copolymer, milder conditions were suggested: 90°C (2 h), 180°C (1 h) and 160°C (5 h).¹⁰⁷ Higher availability and the much lower cost of AAm and n-butyl methacrylate than MAN and MAA can open up good prospects for the practical application of the AAm-n-butyl methacrylate copolymer to produce PMI foams. This will be true only if good physical and mechanical properties of the product are achieved; however, such data are not presented by the authors.¹⁰⁷

Amide-containing polymers that already have carboxyl units or these units formed from anhydride fragments can also be used as precursors for thermal imidisation. Examples are P(M)AI foams obtained by heat treatment (170–200°C) of copolymers AAm-MAA, AAm-AA,^66,92 AAm-maleic anhydride, of the terpolymer AAm-AA-maleic anhydride.⁹¹ Dyatlov et al.^39,40 investigated the thermal imidisation of AAm-AA copolymer (mole ratio of 1:2) in the presence and in the absence of foaming agents. Heating the copolymer at different temperatures without a foaming agent made it possible to distinguish between intra- and intermolecular imidisation reactions since the products of these reactions differ in glass transition temperatures. The fraction of neighbouring amide and carboxyl units in the copolymer under study, according to NMR spectroscopy, was 18 mol% of all sequences of the units. These fragments completely undergo intramolecular cyclisation at 120°C. Interchain imidisation begins only at 140°C or higher and manifests itself in an increase in the glass transition temperature of the product. The curing of the polymer is completed at 180°C or above by the intrachain and interchain reactions of the formation of anhydride fragments from residual AA units. Since the polymer precursor contained two times more carboxyl units than the amide ones, a significant part of the carboxyl units turned into anhydride fragments when strongly heated. Therefore, after heating the precursor at 180°C, the content of the anhydride units formed reaches 40 mol%.

Similar temperature patterns are also observed during the thermolysis of the AN-MAA copolymer.^39,97 Consequently, the precursors of the amide-acid and nitrile-acid types obey similar laws under thermolysis. It was repeatedly noted^20,84 that the patterns of thermal imidisation are sufficiently close for different copolymers of the type (meth)acrylonitrile–(meth)acrylic acid. Therefore, it can be assumed that the revealed patterns^39,40,97 can be extended to the most important industrial thermal imidisation of MAN-MAA copolymers.

Thermal imidisation with simultaneous foaming of AAm-AA copolymers (mole ratio of 1:2) was studied.⁴⁰ The release of ammonia as a result of intermolecular imidisation (Figure 11) was not observed, probably, due to the rapid involvement of ammonia in the transformations according to Figure 6. The rate of consumption of amide groups as a result of the intra- and intermolecular imidisations of the copolymer is well described by the general equation

- \frac{d {[CONH]}_{2}}{d t} = k [COOH] [{CONH}_{2}]

where [COOH] and [CONH₂] are the concentrations (mol%) of amide and acid groups in the copolymer and k is the imidisation rate constant ((mol% · min)⁻¹).

The calculated imidisation rate constants were 0.0004, 0.0147 and 0.0807 (mol% · min)⁻¹ at 140°C, 160°C and 180°C, respectively. The activation energy of imidisation of the AAm-AA copolymer was 208 kJ mol⁻¹.

Other variants of amide-containing precursors are polymers containing simultaneously nitrile, carboxyl and amide units. Thus, the thermal imidisation of AA-AN-AAm copolymers at 170–200°C (20–40 min) to yield PAI foams was suggested in patents^91,108 (the applications were from 1962). In addition to the units of the listed basic monomers, the maleic anhydride, methyl methacrylate and styrene units could also be included in the polymer precursors. The use of such polymer precursors containing only acrylic units at that time did not have practical implementation.

There had been a long pause in the publications on the thermal imidisation of the precursors of the nitrile-acid-amide type after the patents^91,108 were published. But for the last 15 years, interest in precursors which are copolymers of (meth)acrylonitrile, (meth)acrylic acid and AAm has increased dramatically. This can probably be explained by two factors: First, by that time, the good market prospects of P(M)AI (not only the PMI of the ‘Rohacell’ brand) had finally become clear in the case of a decrease in their cost. Secondly, by 2005, many ways of solving various problems in the production of PMI foam of the ‘Rohacell’ brand based on MAN-MAA copolymers had been tested. It turned out that the use of amide-containing polymer precursors in certain circumstances can help solve various problems: to reduce the cost of P(M)AI and to improve the properties of P(M)AI and/or the technology of their production.

The introduction of AAm units into the MAN-MAA precursor can, in fact, be called as a modification of the method for producing PMI foam of the ‘Rohacell’ brand. It was suggested to introduce AAm units (13–15 wt%) as well as maleic anhydride units into MAN-MAA copolymers.⁴⁹ This made it possible to significantly reduce the fraction of defective samples of PMI foam obtained by heating precursors at 160–180°C (heating time up to 21 h). With an increase in the content of AAm units above 15%, the moisture resistance of foam decreased.

Since 2005, studies have been published regularly on the development of PMI foams based on AN-MAA copolymers (see Section 1) and AN-MAA-AAm terpolymers (the fraction of AAm units of 2.0–2.6 wt%)^22,23,109 using the technology similar to that of ‘Rohacell’ PMI. The spectra of AN-MAA-AAm terpolymers after thermal imidisation practically do not differ from those of AN-MAA copolymers (the characteristic bands of the latter are described in Section 1).^22,23 This is not surprising since the content of AAm units in the precursor is low. However, the analysis of the IR spectra of the MAA-AAm copolymer recorded before and after heat treatment allowed authors to confirm that the AAm units are actively involved in the thermal imidisation reaction. This is indicated by the appearance of the C–N–C band typical for the imide or substituted amide groups (1220 cm⁻¹) and by a decrease in the intensity of the band at 930 cm⁻¹ (–OH of the carboxyl group). It should be noted that although the authors^22,23 present four imidisation schemes in their works (Figures 7, 10, 11 and 13), they do not present data on the ratio of the intrachain and interchain imide fragments in PMI foams.

Figure 13.

Direct interaction of the ester and amide groups to yield the imide fragment.

The authors^22,23,109 investigated the physico-mechanical properties of PMI foams. The aim was to carry out additional heat treatment of foam materials to improve their strength characteristics due to additional intermolecular cross links. It was shown that the introduction of AAm units into the polymer precursor ensures the obtaining of more rigid structures and higher heat resistance of foamed products while maintaining high levels of other physico-mechanical properties. Based on this, one can assume a more active participation of AAm units in intermolecular imidisation (Figure 11) than nitrile units (Figure 3). On the basis of a series of tests, the authors^22,23,109 concluded that the products they synthesised are equivalent to ‘Rohacell’ PMI in terms of mechanical properties and heat resistance. The improvement of the properties of PMI foams by introducing a small portion of AAm units into the AN-MAA copolymer was also achieved in the work of Kornienko et al.⁹⁶ All these data indicate good prospects for using AN-MAA-AAm terpolymers as precursors for the production of PMI foams.

Conclusion

The development of research on the synthesis of P(M)AIs by intramolecular thermal cyclisation of (meth)acrylic polymer precursors in the last 30 years has followed two main avenues:

– searching for ways to reduce the cost of obtaining PMI foams with excellent physico-mechanical characteristics for the use in mission-critical applications (primarily, in products for aviation and astronautics);

– searching for ways of obtaining relatively cheap P(M)AI with good physico-mechanical characteristics for wider practical application.

To date, great progress has been made in the first avenue in terms of improving the properties of PMI foams of the ‘Rohacell’ brand obtained by the thermal imidisation of MAN-MAA copolymers; the number of defective products in the production of this material has decreased. In addition, the scientific and technical bases have been created for the production of new PMI (similar in properties to ‘Rohacell’ PMI) using cheaper AN-MAA copolymers and AN-MAA-AAm terpolymers as precursors. Numerous studies have been published on the optimisation of the physico-mechanical properties of PMI foams containing simultaneously acrylic and methacrylic units.^{22,23,98,99,109,110} Based on such PMIs, materials with reduced combustibility,^111

-114 promising composite materials such as silicate-containing composites,^115,116 composites with carbon nanotubes¹¹⁷ have been developed. The development of sandwich materials and various products based on PMI of this type has been started (including those for use in aviation and astronautics).^{59,61,118,119}

In the second avenue, the aim was to obtain PAI foams with good heat resistance and strength characteristics on the basis of AN-AA polymer precursors. Such materials are inferior in properties to ‘Rohacell’ PMI but are cheaper even in comparison with PMI obtained from AN-MAA and AN-MAA-AAm copolymer precursors. This is due to the lower cost of AA than MAA. In addition, the copolymerisation of two acrylic monomers (AN and AA) makes it easier to obtain copolymers with a higher degree of unit alternation and compositional homogeneity as compared to copolymers of AN and MAA. This is due to the closer reactivities of the comonomers. Therefore, PAI can be used in many less mission-critical applications compared to aviation and astronautics; however, this requires the solving of the problems of synthesis of AN-AA copolymers. Bulk copolymerisation of AN and AA cannot be used because of strong heat release, uncontrolled intermolecular cross links and strong adhesion of copolymers to silicate forms. This may be resolved by abandoning bulk polymerisation in favour of other more technologically advanced and high-performance methods for obtaining acrylic polymer precursor powders or granules.³⁹

The use of polymer powders or granules as precursors instead of monolithic polymer blocks can also be applied to acrylic–methacrylic polymer precursors. Changing the form of polymer precursors will require a change in the technology for obtaining P(M)AI foam. The results of studies of temperature patterns of imidisation and other transformations of nitrogen-containing polymer precursors have shown the principal possibility of development of new technologies for obtaining P(M)AI foams in the future. In such technologies, the stage of moulding polymeric half-finished materials from powders or granules (with additives of foaming and other agents), the stage of intramolecular imidisation, the stage of intermolecular imidisation (strong cross linking of macromolecules) and foaming can be separated.⁴⁰ Technologies of this type can expand the area of application of P(M)AI and, most importantly, will dramatically reduce their cost by increasing the productivity of the stage of obtaining polymer precursors.

The main general conclusion from the last 15 years’ work on the synthesis of P(M)AIs is the experimental confirmation that the formation of imide fragments during the thermal transformations of nitrogen-containing (meth)acrylic polymers depends little on the type of units in precursors: methacrylic or acrylic. Imidisation is influenced more strongly by the microstructure of polymer precursors (the degree of alternation of carboxylic and nitrile or amide units). Other important factors for the competition of different directions of imidisation and side transformations are temperature and the ratio between nitrogen-containing (nitrile, amide) and carboxylic (or carboxyl-forming) units. This means that P(M)AI foams can be efficiently obtained using both more expensive methacrylic monomers (MAN, MAA, MAAm) and cheaper acrylic monomers (AN, AA, AAm) as raw materials. Thus, the implementation of various options for obtaining P(M)AI with the required properties will be largely determined by the technological capabilities of the synthesis of precursors with a given structure and molecular weight characteristics.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Russian Foundation for Basic Research and the Ministry of Industry, Trade and Entrepreneurship of the Nizhny Novgorod region [Research Project No. 18-43-520041 r_a].

ORCID iD

Alexey P Sivokhin

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