59th Annual Report on Research 2014

Figure 1. Synthesis of the monomers I and II from sorbic acid and leaf aldehyde, respectively.

 The powder of monomer I and II polymerized slowly at room temperature under UV irradiation using a high-pressure Hg lamp. The reactions can be accelerated. For example, the polymerization of I was finished in 99% yield within 16 h by UV irradiation at 120 ^oC, which was well below the melting point of monomer I (178–180 ^oC). Raising the temperature increased thermal vibration of the monomers in the solid state; therefore, heating sped up the topochemical polymerization. FT-IR spectra of monomer I compared with its polymers showed a shift of C=O stretching vibration from 1717 to 1740 cm^-1. The anti-symmetric stretching of C–C at 1008 cm^-1 shifted to a higher wavenumber 1014 cm^-1, and the relative intensity of the peak for the isolated trans C=C at 970 cm^-1 increased after polymerization. These changes showed that the polymerization broke the conjugation between the carbonyl and diene in monomer I.      .

Figure 2. Crystal structures of the monomers I and II showing their 1D packing.

To further investigate the solid state polymerization, high quality crystals of monomers I and II were obtained, respectively. Their structures were determined by single crystal X-ray diffraction. The monomers self-assembled into columnar complexes based on p-p stacking as shown in Figure 2. Within the assemblies, the closest reactive atoms are the 1^st diene carbon and 4^th diene carbon in the nearest neighboring monomer as marked in Figure 1. These two adjacent reactive sp² hybridized carbons are only separated from each other by about 3.6 Å and their p orbitals are well overlapping each other. The distance and orientation are ideal to form a new carbon-carbon bond in the solid state upon UV irradiation or heating. UV irradiation and/or heating of the self-assemblies furnished polymeric products. As outlined in Figure 3, the 1,4-photopolymerization is proposed.  Essentially, this process occurs via an intra-assembly reaction. Meanwhile, it was also noticed that the reactive alkene sp² carbons between self-assemblies are too distant for polymerization and their p orbitals are not overlapping. The formation of the polymeric ladder was further confirmed by solid state ¹³C NMR, which showed the disappearance of two sp² hybridized carbons of the diene concurrent with the appearance of two sp³ hybridized carbons around 43.6 and 56.4 ppm. The powder XRD pattern of the synthesized monomer I without further processing was nearly identical to that of the calculated one from the single crystals making this crystal structure suitable to analyze and interpret the solid state polymerization for both crystals and powder.  

Figure 3. Stereoregular polymeric ladders constructed by solid-state polymerization.

Solid-state reactions offer a unique opportunity to synthesize molecules and macromolecules with regio- and stereo-specificity. Because the solid-state polymerization proceeds with minimum movement of atoms, stereoregular polymeric ladders are proposed as products (Figure 3). The four carbon-carbon bonds that each monomer forms with its two adjacent neighbors all involve chiral centers that are generated stereo-specifically. Besides their stereoregularity, the two polymeric ladders have shown excellent chemical stability, which is also important for future applications.  They are insoluble in all the common organic solvents. The ladders can tolerate strong acids such as concentrated HCl and TFA, and organic bases such as Et₃N (even when refluxed overnight). However, these ladder polymers are degradable in aqueous potassium hydroxide and can be oxidized by concentrated sulfuric acid.    Our first polymeric ladder paper entitled “Synthesis of Polymeric Ladders by Topochemical Polymerization” has been published on Chemical Communication in 2014.  We are currently preparing a follow-up full paper - “Gemini Monomers for High-performance Polymeric Materials”. Another recent exciting development in my research is that we were able to synthesize a 2D polymer using sunlight and cinnamic acid, which can be derived from the byproduct of growing biofuel manufactory on large scale. The novel polymer contains 81% by mass sustainable raw materials. The solvent-free polymerization was able to be carried out by using sunlight or UV lamp. The critical assemblies with multiple pre-organized reactive centers were characterized by powder and single crystal X-ray diffraction.  The product structure was further confirmed by hydrolysis of the 2D polyester: the crystal structure of the isolated hydrolysis product α-truxillic acid directly revealed the newly formed C-C bonds in the 2D polymer.  Extremely thin sheets of the 2D polymer were observed under SEM and TEM after simple exfoliation.  The 2D polymer shows high chemical stability; moreover, a deepened exploration of this novel polymer may expose innovative properties and subsequent possibilities.  The manuscript “Synthesis of a Stereoregular Two-dimensional Polymer Using Biomass-derived Starting Material and Sunlight” is currently under review.  In summary, this ASC PRF supported Doctoral New Investigator (DNI) project has been focusing on the synthesis of novel organic polymers, such as ladder and two-dimensional (2D) polymers. These polymeric structures are of great interest for their basic scientific richness and potential applications in sustainable chemistry. My research team has published three papers during the past academic year, and two more papers are coming out soon.