Determination of End Groups

The characterisation of polymer chain ends gives valuable clues to clarify the polymerisation mechanisms, and to design new polymers with improved properties [1-7]. 

Owing to recent developments in highly specific pyrolysis devices, highly efficient separation columns for gas chromatography (GC), and specific identification of the peaks in the pyrograms by GC-mass spectrometry (GC-MS), and pyrolysis-GC (Py-GC), structural characterisation of polymeric materials has become a reality. Particularly, Py-GC is an extremely sensitive, simple and rapid technique. 

The end-groups of a chain are determined by initiation, chain transfer and termination reactions and determination of the end-group structures is 

Quantitatively, the end-group intensity relative to the main-chain intensity gives the number-average degree of polymerisation directly. In practice, because of incomplete resolution of end-group peaks and insufficient sensitivity, this use of NMR is usually restricted to degrees of polymerisation of the order of 100 or less. 

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From Table 6.5 quantitative determination of end groups can be seen to be increasingly uncertain particularly above a value of of 10 000. Nonetheless, end group analysis may be useful in certain circumstances, particularly for lower molar mass polymers and oligomers, where it may be a fairly straightforward approach to obtaining useful data. 

Several techniques for determination of end group structure in polyesters have been described in the literature [25-31]. Many analytical procedures have... 

Absolute methods provide the molecular weight and the degree of polymerization without any calibration. Their calculation from the experimental data requires only universal constants such as the gas constant and Avogadro s number, apart from readily determinable physical properties such as density, refractive index, etc. The most important methods in use today are mass spectrometry, osmometry, light scattering, and - to some extent - sedimentation and diffusion measurements. Also, some chemical and spectroscopic methods (determination of end-groups) are important because of their relative simplicity. 

Exercise 25-17 What problems might be encountered in using the 2,4-dinitrofIuoro-benzene method for determination of end groups on Gly-Lys-Ala Explain. 

However, NMR determination of end-groups is limited to low MM. As the size of the macromolecular chain increases, the signal due to the polymer backbone becomes larger and larger and the intensity of the signal due to end-groups in the NMR spectrum becomes buried in the noise. 

Absolute determination of end-groups in anionically polymerized monodis-perse PS samples with Mw between 1000 and a few million was carried out by Py-GC/MS. Figure 3.12 shows t5q ical p5n ograms of PS samples at 600°C for PS-2 with number average (M ) = 3090 (A), and PS-3 with M - 9000 (B). 

Ohtani, H., Luo, Y. E, Nakashima, Y., Tsukahara, Y, and Tsuge, S., Determination of End Group Functionality in Polyjmethyl methacrylate) Macromonomers by Ppn-olysis Simultaneous Multidetection Gas Chromatography, Anal Chem., 69, 1438,1994. 

Bottrill, A. R., Giannakopulos, A. E., Waterson, C., Haddleton, D. M., Lee, K. S., and Derrick, P. J., Determination of End-Groups of Synthetic Polymers by Matrix-Assisted Laser Desorption/Ionization High-Energy Collision-Induced Dissociation, Anal. Chem., 71, 3637,1999. 

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