Protecting-Group-Free Iterative Exponential Growth Method for Synthesizing Sequence-Defined Polymers.

As a main synthetic strategy for monodisperse sequence-defined polymers, the known iterative exponential growth (IEG) methods were all developed on protecting-group chemistry, where the additional deprotection reactions increased their synthetic steps and decreased their atom economy. In this study, we developed a protecting-group-free IEG method for the formation of sequence-defined polymers by combining three orthogonal click reactions of copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC), sulfur-fluoride exchange reaction (SuFEx), and Ugi four-component reaction (Ugi-4CR). In this approach, oligomer synthesis began with three parallel CuAAC, SuFEx, and Ugi-4CR couplings among three monomers each with two orthogonal clickable end groups. By iteratively applying parallel CuAAC, SuFEx, and Ugi-4CR to couple three resultant oligomers, each having two orthogonal clickable terminals, this approach could exponentially grow three different sequence-defined polymers simultaneously with high efficiency, requiring no protecting-group chemistry. Additionally, each Ugi-4CR coupling reaction could introduce two external side groups to provide the molecular variation and side-chain functionalization for the resultant sequence-defined polymers.

Step-Growth Polymerization Method for Ultrahigh Molecular Weight Polymers.

The preparation of polymers with an ultrahigh molecular weight (>106 g/mol; UHMW) is always a challenge for homogeneous step-growth polymerization. Herein, a unique homogeneous step-growth polymerization method was developed to prepare various UHMW polymers. In this approach, a double-strain-promoted azide-alkyne click reaction (DSPAAC) with a reactive intermediate was used as the polymerization reaction, and sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) and bis-azide compounds with 2,6-diisopropylphenyl azide terminals were used as the monomer pairs. The DSPAAC reaction, with a reactive intermediate, facilitated this polymerization method to efficiently prepare UHMW polymers under convenient, stoichiometrically imbalanced conditions using a slight excess of DIBOD to bis-azide monomer. In addition, the click nature of the DSPAAC reaction facilitated this polymerization method to synthesize UHMW polymers under ambient conditions, requiring no catalysts. The resultant UHMW polymers presented strong fluorescence peaking at 423 nm, good thermal property with the glass transition temperature up to 270 °C, and good mechanical property with Young's modulus above 1 GPa.