Intelligent monitoring of solid state polymerization via molecular rotors: The case of poly(butylene succinate)

•Solid state polymerization (SSP) was performed on poly(butylene succinate) (PBS).•SSP is monitored for the first time by a fluorescent probe (CCVJ).•Labelled PBS shows a decrease in the CCVJ fluorescence intensity and a blue shift during SSP process.•CCVJ is able to act as a molecular rotor as well as a solvatochromic probe.•Probe fluorescence lifetimes would show that 70% of CCVJ were localized in the amorphous regions of the PBS.