Synthesis, characterization, and potential application of linear bis-MPA
Description
There are three major architectural categories of polymeric materials: linear, branched, and dendritic. Each of these architectures have specific advantages that make them suitable for different applications. However, some of these architectures are inaccessible depending on the monomer system employed. In fact, monomers that can be synthesized into all three of these architectures are extremely rare. This is due to several factors including the lack of a monomer branch point or the poor chemoselectivity of the monomer in use. As a result, there are few monomers, such as 3,5-dihydroxybenzoic acid, that can be synthesized into homopolymers of all three categories. Demonstrations of this level of monomer adaptability are few, but with some recent advancements, another monomer, known as 2,2’-bis(hydroxymethyl)propionic acid (bis-MPA), can be added to this short list of versatile monomers. Bis-MPA is a biocompatible monomer that has been used to synthesize linear, branched, and dendritic materials over the last few decades. Its popularity has grown in academic and industrial settings which has led to numerous publications that show the range of applications it can be used for. However, most of these publications use branched and dendritic bis-MPA architectures. These architectures of bis-MPA have existed for some time and have been well studied. Unlike dendritic and branched bis-MPA, linear bis-MPA has only recently been discovered and detailed (last ~20 years) which has led to the publication of several new applications of bis-MPA materials. While linear bis-MPA has been used to synthesize all three architectures, it is important to note that these linear bis-MPA materials all contain a polycarbonate backbone. This is in stark contrast to traditional bis-MPA materials which contain polyester backbones. Though bis-MPA has been used to synthesize polymeric materials of all three architectures, its important to note that the linear bis-MPA materials published thus far are dissimilar to traditional branched and dendritic bis-MPA. To firmly cement bis-MPA as a versatile monomer, much like 3,5-dihydroxybenzoic acid, a linear polyester bis-MPA material would need to be synthesized. Therefore, the goal of this work is to not only develop a synthetic method for producing linear, polyester bis-MPA but use it to better understand the behavior of dendritic bis-MPA materials and explore its potential application. Herein, linear, polyester bis-MPA was synthesized using a brominated bis-MPA monomer derivative followed by a chain-addition polymerization in mildly basic conditions. This polymerization can take place in a range of solvents and unexpectedly occurs through a chain growth mechanism, which will be discussed. Linear, polyester bis-MPA also proved to be a great analog for traditional bis-MPA dendrimers due to their similar atom economy and repeat unit molecular weight. As such, linear and dendritic bis-MPA analogs were compared in a solution size variation investigation along with poly(caprolactone). These analogs were analyzed using size-measuring techniques including gel permeation chromatography (GPC) and diffusion ordered spectroscopy-nuclear magnetic resonance (DOSY-1H NMR). Finally in this work, a potential application for linear, polyester bis-MPA was explored through the use of triazole chemistry via copper catalyzed azide alkyne cycloaddition (CuAAC). The incorporation of triazole moieties into the linear bis-MPA backbone could be of interest due to the antimicrobial properties of the triazole functional groups. Through the discovery of linear, polyester bis-MPA, bis-MPA can not only be cemented as one of a few highly versatile monomers, but this work opens the door for the exploration of a new class of bis-MPA material.