Structural Homogeneity of Macromolecular Networks by End-to-End Click Chemistry between Discrete Tetrahedral Star Macromolecules.

Cross-linking via the end-to-end click chemistry of multiarm star polymers creates polymer networks with minimal inhomogeneities. Although it has been suggested that the mechanical and swelling properties of such networks depend on the absence of defects, the structural details of homogeneous networks created by this method have not yet been studied at the molecular level. Here, we report the synthesis of discrete tetrahedral star macromolecules (dTSMs) composed of polylactide (PLA) arms with discrete molecular weight and sequence. Polymer networks prepared by 4 × 4 cross-linking by Cu-free strain-promoted cyclooctyne-azide click chemistry (SPAAC) reaction exhibited a high degree of swelling (>40 fold by weight) in solvents without sacrificing mechanical robustness (elastic modulus >4 kPa). The structural details of the networks were investigated by network disassembly spectrometry (NDS) using MALDI-TOF mass spectrometry. By implementing a cleavable repeating unit in the discrete PLA arms of dTSM in a sequence-specific manner, the networks could be disassembled into fragments having discrete molecular weights precisely representing their connectivity in the network. This NDS analysis confirmed that end-to-end click reactions of dTSM networks resulted in the formation of a homogeneous network above the critical concentration (∼10 w/v%) of building blocks in the solution.

Monodisperse Cyclic Polymer Mechanochemistry: Scission Kinetics and the Dynamic Memory Effect with Ultrasonication and Ball-Mill Grinding.

A series of monodisperse cyclic and linear poly(d,l-lactide)s (c-PLA and l-PLA, respectively) were prepared with various degrees of polymerization (DP) using an iterative convergent synthesis approach. The absence of a molecular weight distribution provided us a chance to study their mechanochemical reactivity without obstructions arising from the size distribution. Additionally, we prepared l- and c-PLAs with identical DPs, which enabled us to attribute differences in scission rates to the cyclic polymer architecture alone. The polymers were subjected to ultrasonication (US) and ball-mill grinding (BMG), and their degradation kinetics were explored. Up to 9.0 times larger scission rates were observed for l-PLA (compared to c-PLA) with US, but the difference was less than 1.9 times with BMG. Fragmentation requires two backbone scission events for c-PLA, and we were able to observe linear intermediates (formed after a single scission) for the first time. We also developed a new method of studying the dynamic memory effect in US by characterizing and comparing the daughter fragment molecular weight distributions of l- and c-PLAs. These results provide new insights into the influence of the cyclic polymer architecture on mechanochemical reactions as well as differences in reactivity observed with US and BMG.

Iterative Convergent Synthesis of Large Cyclic Polymers and Block Copolymers with Discrete Molecular Weights.

We report here the synthesis of cyclic polymers and block copolymers consisting of discrete numbers of repeating units without linear contaminants. The synthesis utilizes the intramolecular cyclization of end-functionalized poly(rac-lactide) (PLA) and its block copolymers with as many as 512 lactic acid units (37 kDa), synthesized by the iterative linear convergence of orthogonally protected building blocks. By exploiting the change in hydrodynamic volume upon cyclization of the linear polymers, macrocyclic polymers were isolated without linear precursors by preparative size-exclusion chromatography as a purification method. Our procedure also allowed the synthesis of a monodisperse cyclic block copolymer in a desired block ratio as a single compound (14 kDa).

High-density information storage in an absolutely defined aperiodic sequence of monodisperse copolyester.

Synthesis of a polymer composed of a large discrete number of chemically distinct monomers in an absolutely defined aperiodic sequence remains a challenge in polymer chemistry. The synthesis has largely been limited to oligomers having a limited number of repeating units due to the difficulties associated with the step-by-step addition of individual monomers to achieve high molecular weights. Here we report the copolymers of α-hydroxy acids, poly(phenyllactic-co-lactic acid) (PcL) built via the cross-convergent method from four dyads of monomers as constituent units. Our proposed method allows scalable synthesis of sequence-defined PcL in a minimal number of coupling steps from reagents in stoichiometric amounts. Digital information can be stored in an aperiodic sequence of PcL, which can be fully retrieved as binary code by mass spectrometry sequencing. The information storage density (bit/Da) of PcL is 50% higher than DNA, and the storage capacity of PcL can also be increased by adjusting the molecular weight (~38 kDa).