End-group characterization of poloxamers

Tetronic® 701 is a tetrafunctional poloxamer predominantly functionalized with primary hydroxyl groups. However, in order to establish the exact proportion of isocyanates, the basic structure of this poloxamer was more deeply analyzed. The propylene:ethylene oxide ratio of Tetronic was determined to be 4:1 (1H-NMR), corresponding to a PEO content of roughly 15%. The primary/secondary nature of the hydroxyl end-groups was determined through analysis of the displacement produced by trichloroacetyl isocyanate (TAIC) on the NMR peaks of terminal groups.

Terminal group / integration / multiplicity / δ (ppm)
no TAIC / TAIC
PEO / R1 = R2 = H / 2H / t / 3.5-3.6 / 4.4-4.5
PPO / R1= H / 1H / m / 3.3-3.4 / 4.4-4.5
R2= CH3 / 3H / dd / 1.1 / 1.3

Table 1 : 1H-RMN of terminal groups of poloxamers before and after coupling with TAIC displacement agent

Before reaction with the displacement reagent TAIC, the terminal groups of the poloxamer are not clearly visible since they arise approximately in the same area as the signals from the non-terminal PEO/PPO groups of the poloxamer.

However, reaction of TAIC with the terminal hydroxyl groups of Tetronic (either from PEO or PPO) causes the displacement of the adjacent methylene groups. As a result, clearly discernible new signals arise in the 1H-NMR spectra at 4.4-4.5ppm. In addition, for terminal secondary hydroxil groups (terminal PPO units) another signal appears at 1.3ppm corresponding to the methyl group of PPO units.

Since each PPO unit produces a signal at 4.4-4.5 accounting a third of the signal at 1.3ppm, the proportion of this signal derived from PEO units can be determined. By comparing the relative integration of the duplet appearing at 1.3 with the multiplet at 4.4-4.5ppm the proportion of terminal groups with primary:secondary alcohol units (PEO:PPO) can be determined.

Figure above presents a typical 1H-RMN of a Tetronic before and after adding the displacement reagent (TAIC). Even though the theoretical structure of Tetronic® 701 is reported to have PEO end-groups, the presence of new peaks arising at 1,3 ppm after adding TAIC, evidences the presence of PPO terminal groups. In fact, analysis of the relative integration of the signal at 4,3-4,4 suggests that the proportion of primary:secondary hydroxyl groups was around 60:40.