Sequence analysis of synthetic polymers

Monitoring the relative abundance of comonomer sequences is a common technique and is used, for example, to observe the progress of transesterification reactions between polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) in their blends.

During such a transesterification reaction, three resonances representing four diads can be distinguished via ¹H NMR spectroscopy by different chemical shifts of the oxyethylene units: The diads -terephthalate-oxyethylene-terephthalate- (TET) and -naphthalate-oxyethylene-naphthalate- (NEN), which are also present in the homopolymers polyethylene naphthalate und polyethylene terephthalate, as well as the (indistinguishable) diads -terephthalate-oxyethylene-naphthalate- (TEN) and -naphthalate-oxyethylene-terephthalate- (NET), which are exclusively present in the copolymer. In the spectrum of a 1:1 physical PET/PEN mixture, only the resonances corresponding to the diads TET and NEN are present at 4.90 and 5.00 ppm, respectively. Once a transesterification reaction occurs, a new resonance at 4.95 ppm emerges that increases in intensity with the reaction time, corresponding to the TEN / NET sequences.[2]

The example of polyethylene naphthalate and polyethylene terephthalate is relatively simple, as only the aromatic part of the polymers differ (naphthalate vs. terephthalate). In a blend of polyethylene naphthalate and polytrimethylene terephthalate, already six resonances can be distinguished, since both, oxyethylene and oxypropylene, form three resonances.[3] The sequence patterns can become even more complex, when triads can be distinguished spectroscopically.[2]⁠ The extractable information is limited by the difference in chemical shift and the resonance width. In addition to ¹H NMR spectroscopy, also ¹³C NMR spectroscopy is a common method for the sequencing shown above, which is characterized in particular by a very narrow resonance width.

Deconvolution and assignment of these triad-based resonances allows a quantitative determination of the degree of randomness and the average block length via integration of the distinguishable resonances. In a 1:1 mixture of two linear two-component 1:1 polycondensates (A₁B₁)_n and (A₂B₂)_n (with molecular weight high enough to neglected chain-ends), the following two equations are valid:

[ A_i] = [B_i], wherein (i = 1,2) (1)

[ A₁B₂ ] = [ A₂B₁] (2)

Equation 1 states that the molar ratio of all four repeat units is identical and equation 2 states that both types of copolymer are of identical concentration. In this case, the degree of randomness χ is calculated as given by equation 3:

${\displaystyle \chi =[{\frac {A_{i}B_{j}}{A_{1}A_{2}}}]}$, wherein (i, j = 1, 2) (3)

In the beginning of a transreaction process (e. g. transesterification or transamidation), the degree of randomness χ ≈ 0 as the system comprises a physical mixture of homopolymers or block copolymers. During the transreaction process χ increases up to χ = 1 for a fully random copolymer. If χ > 1 it indicates a tendency of the monomers to form alternating structure, up to χ = 2 for a completely alternating copolymer.[4]⁠ The degree of randomness χ gives thereby statistical information about the polymer sequence. The calculation can be modified for three-component[5]⁠ and four-component[6]⁠ polycondensates.

NMR spectroscopy is used in industrially relevant systems to study the sequence distribution of copolymers or the occurrence of transesterification in polyester blends. A change in sequence distribution can effect the crystallinity, and transesterification can affect the compatibility of two otherwise incompatible polyesters. Depending on their degree of randomness, copolyesters can show different thermal transitions and behaviours.[7]