Unexpected fluorescence from maleimide-containing polyhedral oligomeric silsesquioxanes: nanoparticle and sequence distribution analyses of polystyrene-based alternating copolymers

In this study, we synthesized unusual fluorescent polyhedral oligomeric silsesquioxane (POSS)-containing polymers lacking any common fluorescent units (e.g., phenyl or heterocyclic rings): a poly(maleimide isobutyl POSS) [poly(MIPOSS)] homopolymer and poly(styrene-alt-maleimide isobutyl POSS) [poly(S-alt-MIPOSS] and poly(4-acetoxystyrene-alt-maleimide isobutyl POSS) [poly(AS-alt-MIPOSS)] alternating copolymers, through free radical polymerization, and a poly(4-hydroxystyrene-alt-maleimide isobutyl POSS) [poly(HS-alt-MIPOSS)] alternating copolymer, through acetoxy hydrazinolysis of poly(AS-alt-MIPOSS). We used 1H, 13C, and 29Si nuclear magnetic resonance spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and MALDI-TOF mass spectrometry to examine the chemical structures and sequence distributions of these POSS-containing polymers. The FTIR spectra revealed the existence of specific intermolecular interactions, namely dipole–dipole interactions between the CO groups in poly(MIPOSS) and poly(AS-alt-MIPOSS) and intermolecular hydrogen bonding between the CO groups of the MIPOSS units and the OH groups of the HS units in poly(HS-alt-MIPOSS). Differential scanning calorimetry and thermogravimetric analyses revealed that the incorporation of MIPOSS units could enhance the thermal stability, but decrease the glass transition temperatures, of these alternating copolymers. The photoluminescence emission of poly(MIPOSS) was greater than those of the POSS-containing alternating copolymers, presumably because of the former's crystallinity and clustering of locked CO groups of POSS units.