Living polyaramids with a twist
A new polymerization method was developed that gives access to living polyaramids for example poly(phenylene terephthalamide) or helical poly(aramid) foldamers. A highly reactive activating agent and slow amino acid addition are key to polymerize monomers and even oligomers in a living fashion.
Polymer syntheses are typically divided into two types of mechanisms, chain growth and step growth. Examples for chain growth polymers are polyethylene, polypropylene, polystyrene and all polyacrylates. Many advances have been made in the field of chain growth polymerization over the last few decades. The ability to control the molecular weight and make block copolymers while keeping a low dispersity was once the exclusive domain of carbanionic polymerization. Today, radical, cationic, coordination polymerizations and others can all be carried out with similar degrees of control.
Far fewer advances have been made in the area of step growth polymers. Typical step growth monomers are those which contain two (sometimes more) functional groups that can react with each other such as a carboxylic acid and an alcohol in the case of an AB-type monomer for polyester synthesis. Polycondensation reactions (for example polyester or polyamide formation) and also polyaddition reactions (for example urethane or triazol formation) form the polymer chain in a step growth fashion if all reactive groups in the system are allowed to react with each other irrespective of whether they are attached to a monomer, oligomer or polymer.
This total equality of reactive groups comes at a high price as Wallace Carothers, after whom the famous equation is named, showed early on. Any block-copolymer formation is impossible to achieve in principle and low dispersities and high degrees of polymerization are mutually exclusive.
In a non-egalitarian world of functional group reactivity, more control can be exerted over the polymer formation. If, for example, the reactive group at the polymer chain end was always more reactive than any other functional group in the system, even condensation or addition monomers could react according to a chain growth mechanism. This was demonstrated by Yokozawa in 2000 for aromatic polyamides. In his case, aromatic rings were substituted with strongly electron donating aminyl anions rendering the esters on the same aromatic ring unreactive. This prevented all reactions of monomers with each other and favored the reaction of monomers with a growing polymer chain end thereby achieving a chain growth mechanism.
In our current manuscript, we take inspiration from Yokozawa's work but reverse the polarity of the monomers. We describe two new phosphonium reagents that allow the formation of aromatic acid chlorides in the presence of aromatic primary or secondary amines. Due to the strong electron withdrawing effect of the acid chlorides, such amino carbonyl chlorides can also be self-deactivated, thereby avoiding all reactions among the monomers and favoring the reaction with the end of a growing polymer chain.
However, the activation of the aromatic amino acids with the phosphonium reagent is very fast and forms a highly reactive acid chloride. If only very small quantities of monomer are activated at a time, their high dilution will prevent bimolecular self-condensation (the beginning of the step-growth mechanism) and favor the reaction with an initiator or growing polymer chain end present at a higher concentration.
This way of polymerizing condensation monomers in a chain growth fashion is different from the previously reported mechanisms as it no longer relies on any electronic self-deactivation. We demonstrate this by polymerizing dimers of aromatic amino acids in which the two functional groups are spatially separated from each other so that electronic deactivation is inconceivable. In this way, we prepared a narrow dispersity poly(phenylene terephthalamide), the polymer from which Kevlar is made. Furthermore, we were able to polymerize a 5-amino-2,4-bis(alkoxy)-benzoic acid which folds into tubular helical structures driven by intramolecular three-center hydrogen bonding.
The general principle of drastically increasing the monomer reactivity while lowering its concentration to favor a chain growth mechanism could theoretically be extended to other condensation/addition monomers if suitable activation reagents can be found.
This type of mechanism allows the living chain growth polymerization of sequence-controlled oligomers which might be important for the synthesis of supramolecular polymers and foldamers.