Synthesis and characterization of telechelic hydroxyl functional poly (N-vinylpyrrolidone)
Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2008.
Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization has emerged as a versatile method for preparing polymers with control over molecular weight and polydispersity. Inherent in its mechanism is the retention of the chain transfer agent, the RAFT agent, at the polymer chain ends. Typically RAFT agents are made up of two parts, the so called R (leaving) and Z (thiocarbonyl thio, stabilizing) groups. These are retained as the a-and the w-end groups of the final polymer, respectively. RAFT polymerization offers a ready method for preparing polymers with well defined end functionalities. The a-end functionality can easily be built into the R group. The Z group, however, is thermally unstable and can impart color and smell to the polymer. Hence, two new methods for Z end group removal were introduced. Both methods take advantage of the facile reaction between thiocarbonyl thio compounds and radicals. By matching the functionalities of the R group (a-end group) with that of the end modified w-chain end, both methods offer an easy route to accessing telechelic functional polymers. End functional polymers have many important uses in industry and in the biomedical field. An alcohol functional xanthate RAFT agent was synthesized and successfully used to conduct the RAFT-mediated polymerization of N-vinylpyrrolidone (NVP). Characterization by NMR and MALDI ToF MS confirmed that a-hydroxyl-w-xanthate-functional PVP was easily produced. In the first end group modification method radicals were generated as in atom transfer radical addition (ATRA). A hydroxyl functional a-haloester was used as the ATRA initiator with a Cu catalyst system. The alkyl radical produced by this ATRA initiator then replaced the Z group giving a telechelic hydroxyl functional polymer. NMR analysis showed that the thiocarbonyl thio end group was completely removed. The hydroxyl functionality was quantified by derivatizing with trichloro acetyl isocyanate and subsequent analysis by NMR. MALDI ToF MS analysis, however, was inconclusive. In the second method the thiocarbonyl thio end group was removed by simply heating the polymer with hydrogen peroxide, thereby replacing the Z group with a hydroxyl end group at the w-chain end, giving a telechelic functional polymer. The telechelic hydroxyl functional polymer was subsequently crosslinked with a trifunctional isocyanate to make a PVP hydrogel. This confirmed that the end-modified polymer was indeed telechelic. The swelling kinetics of this hydrogel were determined in water at 37 oC.