Determination of polymer molecular weight and composition using picoSpin NMR spectroscopy

Significance of the Topic

The accurate determination of polymer molecular weight and copolymer composition is critical for designing materials with precise physical properties. Nuclear magnetic resonance (NMR) end-group analysis provides a direct and robust approach for quantifying number average molecular weight (Mn) and monomer ratios without relying on matrix-based separations. Low-field, compact NMR spectrometers—such as the picoSpin 80—combine cost efficiency with ease of operation, making advanced polymer characterization more accessible for research, quality control, and industrial laboratories.

Study Objectives and Overview

This study demonstrates how a Thermo Scientific™ picoSpin™ 80 NMR spectrometer (82 MHz) can be employed for:

• Determining the Mn of poly(ethylene glycol) acetyl triarm via 1H end-group integration
• Performing compositional analysis of a PEG-PPG-PEG block copolymer (Pluronic® L-35)
• Comparing results from a low-field (82 MHz) instrument with those from a high-field (300 MHz) spectrometer

Methodology and Instrumentation

Samples were analyzed neat or diluted in CDCl₃/CHCl₃ and introduced into a 40 µL capillary cartridge. Key acquisition parameters included a 90° pulse, 750 ms acquisition time, 5 s recycle delay, and 16–64 scans. Spectral processing used zero filling and phase correction without apodization. End-group signals (acetyl or methyl) were identified and integrated alongside broad repeating-unit resonances to calculate Mn and monomer ratios.

Instrumentation Used

• Thermo Scientific picoSpin 80 NMR spectrometer (82 MHz, 2 T permanent magnet, 40 µL capillary sample)
• High-field 300 MHz 1H NMR spectrometer (for comparison)

Main Results and Discussion

Mn determination of PEG acetyl triarm yielded 1 142 g/mol on the 82 MHz instrument, matching expectations. Copolymer analysis of Pluronic L-35 produced a PEG/PPG weight ratio of 46.8:53.2% at 82 MHz and 47.4:52.6% at 300 MHz, in excellent agreement with manufacturer specifications. Slight chemical-shift shifts between spectra were attributed to concentration differences; overall, low-field and high-field spectra provided consistent integration of broad polymer signals.

Contributions and Practical Applications

This work highlights the picoSpin 80 as a cost-effective alternative to high-field NMR for routine polymer characterization. Laboratories can deploy compact NMR systems for:

• Fast molecular-weight measurements during synthesis optimization
• Quality control of commercial polymers and copolymers
• Educational demonstrations of end-group analysis

Future Trends and Opportunities

Emerging trends include the integration of low-field NMR with automated sample handling for high-throughput polymer screening and the development of advanced pulse sequences to improve resolution of closely overlapping signals. Combining compact NMR with chemometric modeling may further extend capabilities in real-time process monitoring and in situ polymerization studies.

Conclusion

NMR end-group analysis on the picoSpin 80 provides reliable Mn determination and compositional analysis comparable to conventional high-field instruments. Its ease of use, low operating cost, and small footprint make it an attractive tool for diverse polymer research and quality-control applications.

References

1. Higginbotham, C. L.; Izunobi, J. U. J. Chem. Ed. 2011, 88, 1098–1104.
2. Skoog, D. A.; Holler, J. F.; Crouch, S. R. Principles of Instrumental Analysis, 6th ed.; Brooks Cole: Pacific Grove, 2006.
3. Bovey, F. A.; Mirau, P. A. NMR of Polymers, 1st ed.; Academic Press: San Diego, 1996.
4. Sigma-Aldrich. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic L-35; CAS 9003-11-6).