The online coupling of size-exclusion chromatography and NMR is used to characterize block copolymers consisting of polyisoprene (PI) and poly(methyl methacrylate) (PMMA) regarding their distributions of molar mass (MMD) and chemical composition (CCD). Using the CCD, and are calculated on the basis of PI and PMMA homopolymer calibrations. The microstructure distribution of PMMA and the distribution of isomeric units of PI in dependence of molar mass is also demonstrated. Furthermore, a simulation analysis is presented for a bimodal eluting sample. It allows for full separation, quantifi cation and molar mass determination of the coeluting co-and homopolymer fractions. The quantifi cation of the fractions is verifi ed by liquid chromatography at critical conditions.
A general approach of size exclusion chromatography (SEC)-NMR is introduced for the determination of the classical molar mass parameters M(W), M(N), and M(P). It can be used for the determination of molar mass distributions of homopolymers and copolymers. The main advantage of SEC-NMR of copolymers is the possibility of detecting each monomer unit simultaneously with NMR as a quantitative concentration detector. Therefore, it is possible to provide the chemical compositions of copolymers at any elution volume without calibrations. In this respect, a new method will be presented for getting correct signal quantities of onflow data with sufficient NMR sensitivities. As the consequence, the chemical composition of copolymers can be correctly quantified under typical chromatographic conditions with respect to sample concentration and flow rate. Finally, the molar mass calibrations of the copolymers can be easily adjusted according to their chemical compositions. The methods were applied to polystyrene-b-poly(methyl methacrylate) block copolymers of different molar masses. The results of the molar mass distributions and the chemical composition distributions obtained by SEC-NMR are in very good agreement with the complex SEC multidetector analysis.
EXPERIMENTAL SECTIONSamples. Two series of polyisoprenes of different molar masses were used (Table 1). One series consists of samples containing predominantly 1,4-isoprene units with a small content of 3,4-isoprene units (samples 1-5), and the other series is samples containing predominantly 3,4-isoprene units and additionally 1,2-and 1,4-isoprene units (samples 6-10). The corresponding structures are drawn in Scheme 1.
LCCC.The LCCC experiments were performed with an Agilent 1100 HPLC system (Agilent Technologies GmbH, B€ oblingen, Germany)
For the first time, comprehensive two-dimensional liquid
chromatography (2D-LC) of complex polymers is coupled online to 1H NMR. 2D-LC is used to separate mixtures of poly(ethylene
oxide)s with regard to chemical composition and molar mass. The present
samples contain polymers with different end groups and chain distributions.
In the first LC dimension, liquid chromatography at critical conditions
(LCCC) is used for the selective separation according to the end groups.
Fractions that are then uniform regarding their end groups are automatically
transferred into the second LC dimension which separates the fractions
regarding their chain length distributions using liquid adsorption
chromatography. The eluate from 2D-LC is directly introduced into
the 1H NMR for on-flow analysis. The online coupling of
one- and two-dimensional chromatography with 1H NMR detection
is demonstrated. The NMR is coupled to both individual separations
as well as to the entire two-dimensional separation. As a result of
this multidimensional analysis quantitative information is obtained
on the types and topology of end groups and the chain length distributions
within each functionality fraction.
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