Polyamides (PA) are a diverse class of highly useful thermoplastic polymers. They are mostly known for their good mechanical and thermal stability, while maintaining good processability and are used as high-quality synthetic fibers, automotive parts and various other fields where plastic materials that can withstand a lot of mechanical and thermal stress are required. A special, high value case among PA are the fully aromatic aramid polymers, which are extremely important to many facets of Industry. They include such well-known high-end materials as Nomex and Kevlar. The partially aromatic PA known as polyphthalamide (PPA) or High-Performance Polyamide class is also of substantial industrial importance. Compared to aliphatic PA, PPA offer increased chemical resistance, better mechanical properties at elevated temperatures, improved fatigue resistance, and lower sensitivity to moisture absorption. Because of those favorable properties as well as their low density and electrical conductivity typical for organic polymers, PAA are used as metal replacement to reduce weight and as housing in high temperature electrical connectors. By experimenting with different partially aromatic dicarboxylic acids and incorporating functional groups in the side chains, while using fully aromatic phenylenediamines as comonomers, TRANSFORMERS-GROUP pushes the diversity of options ever onward.
This document describes a novel industrial process developed by TRANSFORMERS-GROUP. It explains the principles and describes the conditions for polymer synthesis from unconventional starting materials, while explaining the scope of possible conditions in other cases where only experiments can show the best ways to move forward.
Aramids are synthesized from terephthalic acid (Kevlar) and isophthalic acid (Nomex) or their activated derivatives (e.g. acid chlorides), which are reacted with para-phenylenediamine (Kevlar) or meta‑phenylenediamine (Nomex). In PAA, the diamines are aliphatic while >55% of the diacid is, as the name polyphthalamide suggests, terephthalic and/or isophthalic acid. There are, however, currently no commercial PA products derived from fully aromatic diamines like phenylenediamines and (partially) aliphatic diacids or their derivatives.
In our experiments we will use various (di)acids derived from precursors listed in table 1 and shown in scheme 1. The selection of those precursors was based on the presumed properties that the resulting material may have, which include thermal, mechanical, and chemical stability, as well as chemical functionality allowing further tailoring of the resulting properties. Naturally, the availability and price of the precursors also play an integral role in the choice of precursors for future commercial products.
The conversion of the precursors to their corresponding (di)acids will be performed by environmentally benign catalytic oxidation with molecular oxygen. If necessary, the acids will be converted into activated carboxylic acid derivatives such as acid chlorides or esters. Finally, the polymers will be produced by a condensation reaction of these acid derivatives with phenylenediamines (scheme 1) as comonomers, the main focus being on para‑phenylenediamine (PPD) and meta‑phenylenediamine (MPD).
In addition to those AA/BB-type polycondensation reactions, various AB-type monomers will be prepared to be copolymerized with AA/BB-type monomers. The AB-type homopolymers will also be prepared and studied, aiming to better understand the influence of the AB-type comonomers on the resulting copolymers. The use of AB‑type monomers has generally the advantage of allowing to reach higher molecular weights due to the perfect stoichiometry of acid and amine moieties, which follows from Carothers equation.
After careful consideration, diacid precursors with certain combinations of the aromatic substitution position and functional group R were chosen based on suspected solubility and mechanical properties of the resulting comonomers, as well as the reactivity and availability of the precursors.
Of the proposed precursors, XBV01, XBV02, XBV04, YLV01, YLV02, YLV06 and YLV07 are relatively easy to obtain at reasonable prices. The other listed precursors are not currently commercially available and have to be custom synthesized, resulting in long waiting times, high prices and uncertain purity. Therefore, the work on the precursors PX01, PX02, PX03, KGC6, XBAZ01 and XBAZ03 is of lower priority at early stages of the project, even though these precursors remain of considerable interest for our future experiments.
In addition to the discussed acid precursors, another series of precursors shown in scheme 2 and table 2 will be used to obtain AB‑type monomers as well as endcapping agents.
The rationale behind using precursors with an ethyl (Et) or propyl (Pr) instead of a methyl side chain, and a methylenedioxy ring (MD) is for the resulting molecules to cause distinct 1H NMR peaks that will not overlap with other peaks assigned to protons in the AA/BB-type comonomers. This will facilitate determining the percentages of AB‑type comonomers in resulting copolymers, or the degree of polymerization if an endcapping agent prepared from precursors Et-MD or Pr is used. The MD function offers additional possibilities because, while being very stable under most conditions, it can be cleaved under certain, carefully chosen conditions. This might be useful for polymer-analogues functionalization reactions at a later stage.
After the oxidation step, the unfunctionalized precursors XDE01 and XMP02 will be converted to the respective carboxylic acids, which can be used to endcap the growing polymer chains at the N-terminus. Similarly, any of the amino-functionalized precursors can be used to endcap the growing polymer chains at the C-terminus before being converted by the oxidation step to AB‑type monomers. The choice of ethylamino over methylamino-groups is also based on the NMR technical considerations mentioned earlier.
The oxidation step to produce the diacid will be performed by a cobalt(II)-catalyzed reaction with molecular oxygen, as shown in Scheme 3.
Depending on the choice of precursor, modification of the reaction conditions or an additional step might be necessary but cannot be accurately predicted without preliminary experiments. The need for further chemical modification prior to the polycondensation reaction also strongly depends on the exact precursor used. Due to the low reactivity of both carboxylic acids and aromatic amines, high temperatures as well as a carefully chosen activating agent and solvent system are crucial to obtain high molecular weight polymers directly from diacid and diamine. Scheme 4 shows the published conditions for a polycondensation from diacid and diamine using copolymerization with m-phenylenediamine as an example.
A more commonly used option is the conversion of the carboxylic acids into activated forms like acid chlorides or esters. Although requiring an additional step, the use of more reactive species reduces the need for high temperatures and otherwise harsh conditions, while making it easier to obtain high molecular weight polymers. The conditions to form such activated diacid derivatives are depicted in scheme 5.
The choice of alcohol for the esterification depends on the temperature that can be tolerated during the polycondensation reaction. Methyl esters are often used because they are cheap and easily obtained when the polymerization can be performed at temperatures sufficiently high for methanol to be distilled off. Whenever milder conditions are required, active ester from alcohols like hydroxybenzotriazole or 4‑nitrophenol are formed instead.
If XBV01 is used as the precursor, the reaction route from diacid to polymers is straightforward. The only matter requiring further investigation will be which of the aforementioned ways to form the polymer will be the most economic and lead to desired molecular weights. In fact, due to the absence of interfering functional groups, a simpler and cheaper esterification procedure using catalytic amounts of sulfuric acid and a Dean–Stark apparatus should be possible. An exemplary route using this method is shown in scheme 6.
If the amine-functionalized precursor YLV01 is used, the synthetic route becomes far less obvious. At the same time the additional amino function offers several ways to influence the structure and properties of the resulting polymers. Additionally, it is possible to obtain an AB-type monomer by modifying the oxidation catalyst to selectively oxidize the aromatic methyl group, as shown in scheme 7. This monomer can either be used to form the homopolymer or copolymerized with other monomers of comparable reactivity. Scheme 7 does not show specific polymerization conditions to demonstrate that any of the methods mentioned earlier as well as others are possible.
If both methyl groups are oxidized to carboxylic acids, a A2B-type monomer is obtained. It is not suitable for homopolymerization by step-growth methods (like typical polycondensations) because it will lead to a low degree of polymerization as a consequence of the Carothers equation. However, if a co-monomer is used to compensate for the inequality in functional group numbers and bring the acid to amine ratio close to 1, it should be possible to synthesize hyperbranched, novel polyamides. The use of secondary amine functions in the comonomer might be advisable to bridge the gap in reactivities between the amine moieties in the two monomers, although this requires experimental verification. Scheme 8 depicts the copolymerization of the A2B-type monomer synthesized from XBV01 with N1,N3-dimethylbenzene-1,3-diamine as an example.
Branching reduces the polymer chains’ ability to stack and form dense and/or crystalline domains, thus lowering the melting point (mp) and glass transition temperature (Tg), while usually increasing the solubility. If carefully controlled, branching can be a useful tool to tune the polymer properties. Another possibility to use the amino-function is protect it with tert-butyloxycarbonyl (Boc) by reacting the amine with di-tert-butyl dicarbonate (Boc2O) under basic conditions prior to the polymerization. The resulting polymer can be deprotected under strongly acidic conditions to form a water-soluble polymer with protonated amine groups in every unit. The resulting polyelectrolyte can be neutralized with an excess of weak base to form a polymer with neutral amine functions in every unit. The whole reaction sequence is shown in scheme 9.
Such polymers with protonated or basic amine functions in each repeating unit have multiple application due to their ability to bind small molecules, either by forming a complex in the case of free amine groups, or by electrostatic interaction when the amine is charged. Such applications include among other ion exchange, drug delivery and slow release, as membrane materials in microfiltration, as superabsorbers and functional coatings. Combined with the known favorable mechanical and thermal properties of polyamides, such functional polymers could open a whole new scope of possible uses. It is also possible to use a defined mixture of protected and unprotected monomer for copolymerization and calculate the needed stoichiometric amounts of comonomer accordingly. While following the general path of scheme 9, the additional free amine groups of the unprotected monomer would lead to branching as was shown in scheme 8 and discussed above. However, this would allow to choose the degree of branching by adjusting the proportion of unprotected monomer instead of getting the hyperbranched polymer that has been discussed before. This will allow further tuning of the final polymer properties.
The dimethylamino-functionalized precursor YLV02 offers fewer possibilities than YLV02 because the tertiary group cannot participate in the polycondensation reaction but is also less prone to side reactions. The reaction sequence from precursor to polymer is shown in scheme 10.
The resulting basic polymer can bind acids and metal ions by forming dimethylamino-complexes, while retaining the favorable properties of PA discussed above. This makes it a good candidate for applications as a protective material or filter material. The dimethylamino-groups can also be quaternized with long alkyl chains to create antibacterial activity. This is especially promising as a surface modification method because this way antibacterial surfaces can be created without modifying the bulk of the material and thus influencing its mechanical or thermic properties.
The precursors PX, KGC6, KGC5 and XBAZ are not currently available and will therefore not be discussed in detail. In general, the same procedures discussed before can be applied to PX, KGC6 and KGC5 du to the lack of additional reactive functional groups. The reactions should therefore be analogous to XBV01, which was shown in scheme 6. The azide-functionalized precursor XBAZ, on the other hand, might require milder conditions, especially during the polymerization step. This would necessitate to form a reactive acid derivative (probably acyl chloride) to prevent the azide group from degradation. The azide group in each unit can be used for a wide array of further functionalization by copper(I)-catalyzed azide-alkyne cycloaddition (click chemistry). The functionalization can also be done on the monomer or precursor instead of the polymer, should the stability of the azide function prove to be insufficient at some point.
From initial experiments it became apparent that creating and using AB-type monomers as was shown in scheme 7 is among the most promising ways to create useful novel materials. The advantages are the relative ease of controlling the reactions involved, as well as the versatility of such monomers in copolymerization reactions. In contrast to AA-type monomers which lead to alternating copolymers, AB‑type monomers can be incorporated statistically by adding them to a copolymerization reaction mixture. Since the percentage of AB-comonomer used in such a manner is proportional to its share in the final product, a high degree of control over the composition of the target polymer is easily possible. This led us to examen additional precursors with slightly different structures than those mentioned in tables and schemes 1 and 2, such as CLV03 (scheme 11).
The preparation of the AB-type monomer from CLV03 and its homopolymerizaion are shown in scheme 12, While there are no particular expectations for the homopolymer as such, its structure is depicted to illustrate the resulting polymer unit, which will be the same in copolymers.
Furthermore, the chlorine functionalization of the aromatic ring can be used to further functionalize the monomer and thus the resulting polymer by means of Suzuki coupling. Although such reactions can sometimes be performed on finished polymers, such polymer analogous reactions require the polymer to be sufficiently soluble in the reaction solvent and are often non-quantitative, leaving a part of the functional groups unreacted or degraded due to side reactions. Therefore, the chemical modification of the monomer or its precursor is usually preferable. The reaction conditions and a few potentially interesting precursors obtainable from CLV03 are shown in scheme 13.
Since Suzuki coupling has a large scope of possible substrates, it gives access to a wide variety of precursors for AB-monomers, underscoring the promise of CLV03 and similar compounds for further study.