Configurationally Chiral SuFEx-Based Polymers

Abstract

Novel methods to make synthetic chiral polymers are highly desirable given their potential in a rapidly increasing number of bio-inspired applications. The enantiospecific sulfur-fluorine exchange (SuFEx) reaction of chiral di-sulfonimidoyl fluorides (di-SFs) with diphenols, was used to produce high-molecular-weight chiral polymers with configurational backbone chirality. The resulting new class of polymers, polysulfonimidates, can be efficiently produced via this step-growth mechanism for a wide range of di-SFs and diphenols, yielding M_n^PS up to 283 kDa with a typical dispersity Đ around 1.6. The optical activity of the resulting chiral polymers is largely due to the intrinsic asymmetry of the S atoms (configurational chirality). Finally, the enantiospecificity (ee>98 %) of the polymerization reaction was demonstrated by the degradation of a disulfide-containing polysulfonimidate. This novel route towards configurational main-chain chirality opens up new approaches towards tailor-made chiral polymers with precisely defined properties.

Keywords: Chiral Polymer; Click Chemistry; Enantiospecific; Polysulfonimidates; SuFEx Reaction.

Figure 1

SuFEx‐based polymerizations: Previous work focused on accomplishing SuFEx polymerizations under mild reaction conditions, and on introducing SuFEx‐based post‐polymerization modifications. The current work introduces chirality in SuFEx‐based polymerizations.

Figure 2

Synthesis of chiral disulfonimidoyl fluorides 1. a) Both enantiomers of 5 could be obtained by reacting arylsulfinates 4 with (R)‐(+)‐N‐benzyl‐1‐phenylethylamine, to yield diastereomeric toluenesulfinamides that could be separated, and subsequently transformed into methyl arylsulfinates, and finally into enantiopure (R)‐ or(S)‐arylsulfinamides 5. b) dr=ratio of diastereomers; ee=enantiomeric excess. Both dr and ee were determined by chiral HPLC with a UV detector, since the diastereomers could not be separated by flash column chromatography or distinguished by ¹H NMR.

Figure 3

GPC elution profiles: From left to right: (R,R)‐3  b; (R,R)‐3  g, (R,R)‐3  h, and (R,R)‐3  m. Note: GPC results (UV absorption at 270 nm) display molecular weights calibrated relative to linear polystyrene standards in the elution solvent (DMF+LiBr (0.1 % w/w)).

Figure 4

UV/Vis absorption and CD spectra of chiral polymer and monomers: a) CD spectra for chiral monomers (R,R)‐1  a and (S,S)‐1  a, measured (for structure see Figure 2). b) CD spectra for chiral polymers (R,R)‐3  a (formed from (R,R)‐1  a) and (S,S)‐3  a (formed from (S,S)‐1  a). c) UV/Vis absorption spectra of the polymer (R,R)‐3  p. d) CD spectra for (R,R)‐3  p; the insert shows linear correlation between the CD signal versus concentration (R ²=0.998 at 234 nm, and R ²=0.996 at 278 nm). e) Zoomed‐in section of Figure 4d between 250–300 nm versus the concentration of (R,R)‐3  p. f) CD and UV absorption asymmetry factor (g=0.1×CD [mdeg]/(3298.2×UV [abs])) of the obtained for chiral polymer (R,R)‐3  p at 234 nm and 278 nm for different concentrations.

Figure 5

Disulfide reduction of polymer (R,R)‐3  u to demonstrate the enantiospecificity of the SuFEx‐based polymerization reaction.