End group
One example of an end group is in the polymer poly(ethylene glycol) diacrylate where the end-groups are circled.
The end groups that are found on polymers formed through free radical polymerization are a result from the initiators and termination method used.
[4] Furthermore, end groups play an important role in the process of initiation, propagation and termination of graft polymers.
There are various methods to synthesize graft polymers some of the more common include redox reaction to produce free radicals, by free radical polymerization techniques avoiding chain termination (ATRP, RAFT, nitroxide mediated, for example) and step-growth polymerization.
These examples show us the potential of fine tuning end groups of polymer chains to target certain copolymer structures.
The three main methods for analyzing the identity of the end group are by NMR, mass spectrometry (MS) or vibrational spectroscopy (IR or Raman).
Although chemists utilize many kinds of MS, the two that are used most typically are matrix-assisted laser desorption ionization/time of flight (MALDI-TOF) and electrospray ionization-mass spectroscopy (ESI-MS).
The disadvantage of using ESI is that the data obtained can be very complex due to the mechanism of the ionization and thus can be difficult to interpret.
These methods are useful in fact that the polymers do not need to be soluble in a solvent and spectra can be obtained simply from solid material.
The end groups can subsequently be modified by the reaction of the thiocarbonylthio compounds with nucleophiles and ionic reducing agents.
[11] The method for removal of thiocarbonyl containing end groups includes reacting the polymers containing the end-groups with en excess of radicals which add to the reactive C=S bond of the end group forming an intermediate radical (shown below).
One method of monitoring thermolysis of RAFT polymers is by thermogravimetric analysis resulting in a weight-loss of the end group.
These dithioesters can be reduced to the thiol which can be immobilized on a metal surface; this is important for applications in electronics, sensing and catalysis.
The schematic below demonstrates the immobilization of copolymers onto a gold surface as reported for poly(sodium 4-styrenesulfonate) by the McCormick group at the University of Southern Mississippi.
