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Polymers: Defining Molecular Weight01:01

Polymers: Defining Molecular Weight

3.0K
Unlike small molecules with definite molecular weights, polymers are a mixture of individual polymer chains of varying lengths, each with a unique molecular weight.  So, the molecular weight of a polymer is expressed as an average value based on the average size of the polymer chains. The two most common forms of averages used for polymers are the number average molecular weight and weight average molecular weight.
The number average molecular weight (Mn) is the summation of the number...
3.0K
Polymers: Molecular Weight Distribution01:10

Polymers: Molecular Weight Distribution

3.5K
For any given polymer, the weight average molecular weight (Mw) is higher than, if not equal to, the number average molecular weight (Mn). The only situation in which the weight average molecular weight and the number average molecular weight are equal is when a polymer consists only of chains with equal molecular weight. However, this never happens in a synthetic polymer, since it is difficult to control the polymerization process up to a molecular level with accuracy to a hundred percent.
3.5K
Molecular Weight of Step-Growth Polymers01:08

Molecular Weight of Step-Growth Polymers

2.3K
Step growth polymerization involves bi or multifunctional monomers. Bifunctional monomers react to form linear step growth polymers, whereas multifunctional monomers react to form non-linear or branched polymers.
As the step-growth polymerization involves step-wise condensation of monomers, the molecular weight also builds up eventually. Consequently, high molecular weight polymers are obtained at the late stages of the polymerization, where 99% of monomers have been consumed.
The extent of the...
2.3K
High-Resolution Mass Spectrometry (HRMS)01:15

High-Resolution Mass Spectrometry (HRMS)

1.5K
The resolution of a mass spectrometer depends on the efficiency of separating ions with different ion masses. The mass of an atom is approximated to the sum of the masses of protons and neutrons inside, considering the masses of protons and neutrons as equal. However, the masses of the proton (1.6726 × 10−24 g) and neutron (1.6749 × 10−24 g) are not truly equal. There is a minor error in the expression of atomic masses relative to the simplest atom of hydrogen. For...
1.5K
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.1K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.1K
NMR Spectroscopy of Aromatic Compounds01:14

NMR Spectroscopy of Aromatic Compounds

4.8K
Aromatic compounds can be identified or analyzed using proton NMR and carbon‐13 NMR. Typically, aromatic hydrogens or hydrogens directly bonded to the aromatic rings are strongly deshielded by the aromatic ring current. Therefore, they absorb in the range of 6.5–8.0 ppm in proton NMR spectra. For instance, aromatic hydrogens directly bonded to the benzene ring absorb at 7.3 ppm. However, aromatic hydrogens of larger rings absorb farther upfield or downfield than the ideal range.
4.8K

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Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight MALDI-TOF Mass Spectrometry
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Polymer Molecular Weights via DOSY NMR.

Eric Ruzicka1, Perry Pellechia1, Brian C Benicewicz1

  • 1Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St, Columbia, South Carolina 29203, United States.

Analytical Chemistry
|May 5, 2023
PubMed
Summary

Diffusion-ordered spectroscopy (DOSY) 1H nuclear magnetic resonance (1H NMR) offers a fast, solvent-efficient method for polymer molecular weight determination. This technique provides an alternative to size exclusion chromatography (SEC), requiring no sample purification.

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Area of Science:

  • Polymer Chemistry
  • Analytical Chemistry
  • Spectroscopy

Background:

  • Diffusion-ordered spectroscopy (DOSY) 1H nuclear magnetic resonance (1H NMR) is a valuable technique for polymer characterization.
  • Traditional methods like size exclusion chromatography (SEC) have limitations in speed, solvent usage, and sample purity requirements.

Purpose of the Study:

  • To establish and validate DOSY 1H NMR as a method for determining polymer molecular weights.
  • To investigate the critical parameters for generating accurate DOSY calibration curves.
  • To explore the development of a universal calibration curve for polymer molecular weight determination.

Main Methods:

  • Utilized DOSY 1H NMR to measure diffusion coefficients (D) of polymers including poly(methyl methacrylate) (PMMA), polystyrene (PS), and polybutadiene (PB).
  • Established linear correlations between log(D) and log(Molecular Weight) using SEC data for calibration.
  • Investigated the impact of polymer dispersity on calibration curve accuracy and explored solvent viscosity effects for a universal curve.

Main Results:

  • Demonstrated a linear relationship between diffusion coefficients and molecular weights for PMMA, PS, and PB.
  • Highlighted the importance of proper pulse sequence selection, parameter optimization, and sample preparation for reliable calibration curves.
  • Showcased the potential for a universal calibration curve for PMMA by incorporating viscosity corrections.

Conclusions:

  • DOSY 1H NMR is a rapid, low-solvent, and versatile alternative to SEC for polymer molecular weight analysis.
  • Careful method development, including calibration curve generation, is crucial for accurate DOSY-based molecular weight determination.
  • DOSY NMR is becoming an increasingly important tool in polymer chemistry research.