Molecular Weight and Polymer Chain Length Determination for Polypropylene Glycol Fatty Ethers

This article is compiled based on the United States Pharmacopeia (USP) – 2025 Edition

Issued and maintained by the United States Pharmacopeial Convention (USP)

DOWNLOAD PDF HERE

Average molecular weight, polydispersity, and average polymer chain length are critical properties in understanding and control of excipient composition and variability during formulation development, manufacturing, and testing of polymeric excipients. The following gel permeation chromatography (GPC)/size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy procedures are used to determine average molecular weight, polydispersity, and average polymer chain length for polypropylene glycol fatty ethers.

These procedures are suitable for the following substances:

• Polypropylene Glycol 11 Stearyl Ether
• Polypropylene Glycol 15 Stearyl Ether

1 METHOD 1: APPARENT WEIGHT-AVERAGE MOLECULAR WEIGHT AND POLYDISPERSITY

Mobile phase: Tetrahydrofuran. [Note—Filter the solvent using a polytetra fluoroethylene (PTFE) filter of 0.45-μm pore size if necessary.]

Standard solutions: Prepare 1.0 mg/mL each of at least 5 polypropylene glycol (PPG) standards with peak molecular weights ranging from approximately 70–4500 Da (g/mol)¹ in Mobile phase separately.

System suitability solution: 1.0 mg/mL of PPG standard with a peak molecular weight of approximately 1000 Da (g/mol)1 in Mobile phase

Sample solution: 1.0 mg/mL of the test substance in Mobile phase

Chromatographic system

(See Chromatography 〈621〉, System Suitability.)

Mode: LC

Detector: Refractive index

Columns: Two 7.5-mm × 30-cm; 3-μm packing L21 columns in series

Temperatures

Detector: 30°

Column: 30°

Flow rate: 1.0 mL/min

Injection volume: 20 μL

Run time: 25 min

System suitability

Samples: Standard solutions and System suitability solution

Calibration curve: Plot the retention times on the x axis against the log M (M : peak molecular weight) on the y axis for the PPG peaks from the Standard solutions to generate a calibration curve using suitable GPC/SEC software. The correlation coefficient for the calibration curve is NLT 0.99.

Determine weight-average molecular weights, M , in g/mol (Da), from the chromatograms of the System suitability solution.

Suitability requirements

Relative standard deviation: NMT 5% for determined weight-average molecular weights of the PPG standard from 6 replicates, System suitability solution

Analysis

Samples: Standard solutions and Sample solution

Compute the data using the same GPC/SEC software from the chromatograms of the Standard solutions and Sample solution and determine the number- and weight-average molecular weights, M and M , in g/mol (Da), respectively, for the tested substance.

Calculate the polydispersity for the tested substance:

Result = M_W/M_n

M_W = weight-average molecular weight from the Sample solution (g/mol)

M_n = number-average molecular weight from the Sample solution (g/mol)

2 METHOD 2: AVERAGE POLYMER LENGTH

Diluent: Deuterated chloroform containing 0.03% (v/v) tetramethylsilane2

Sample solution: Weigh about 100 mg of the tested substance. Dissolve it in 0.6 mL of Diluent.

Instrumental conditions

Mode: An NMR spectrometer that is capable of performing quantitative analysis (see Nuclear Magnetic Resonance Spectroscopy 〈761〉, Qualitative and Quantitative NMR Analysis, Quantitative Applications)

Flip angle: 90°

Analysis: Place the tube in the NMR spectrometer, and record the NMR spectrum. Integrate the peak areas from 0.6–1.8 ppm (A ) and from 2.8–4.2 ppm (A ).

The following are the equations for the total number of protons in the two regions (N and N ):

N₁ = N_N1 + N_N2 × n

N₁ = total number of protons in the region of 0.6–1.8 ppm

N_N1 = total number of protons in the fatty alcohol residual group (stearyl group for the two excipients currently listed in the chapter) not activated by oxygen, 35

N_N2 = number of protons not activated by oxygen in each oxypropylene unit, 3

n = number of oxypropylene units per molecule

N₂ = N_A1 + N_A2 × n

N₂ = total number of protons in the region of 2.8 - 4.2 ppm

N_A1 = total number of oxygen activated protons not included in the oxypropylene unit, 3

N_A2 = number of oxygen activated protons in each oxypropylene unit, 3

n = number of oxypropylene units per molecule

The areas in the two regions (A and A ) are correlated to the total number of protons in the regions (N and N ) as in the following equation:

A₂/A₁ = N₂/N₁

Calculate the number of oxypropylene units per molecule (n):

n = (N_N1 × A ₂− N_A1 × A₁)/(N_A2 × A ₁− N_N2 × A₂)

N_N1 = total number of protons in the fatty alcohol residual group (stearyl group for the two excipients currently listed in the chapter not activated by oxygen, 35

A₂ = area from 2.8–4.2 ppm by integration

N_A1 = total number of oxygen activated protons not included in the oxypropylene unit, 3

A₁ = area from 0.6–1.8 ppm by integration

N_A2 = number of oxygen activated protons in each oxypropylene unit, 3

N_N2= number of protons not activated by oxygen in each oxypropylene unit, 3 (USP 1-Aug-2023)

¹ A PPG standard kit from PSS (Polymer Standards Service; part number PSS-ppgkit) was used. The kit contained a total of 7 PPG standards with the peak molecular weight (M ) range of approximately 70–4500 Da (g/mol). The PPG standard with an M of approximately 1000 Da (g/mol) was used for the System suitability solution preparation. An equivalent standard kit can also be used.

² The reagent from Sigma Aldrich (product #225789) was used. An equivalent reagent can also be used.