edlted by
GEORGE L. GILBERT Denisan University Granville, Ohio 43023
A Demonstration of Polymer Crosslinking and Gel Formation Without Heating Submitted by: Joseph H. Ross Indiana University at South Bend South Bend, 46615 Checked By: David Blackman Federal City College Washington, D.C. 20005 The idea of a polymerization process is often hard to grasp, as is the areat of a hizh - difference between the properties . . polymer (or a naturally-occurring macromoleculeJ and those of the simple compounds considered in chemistn courses. The conversion of a soluble linear polymer into crosslinked polymer dramatically illustrates the differences in properties. The experiment described below produces an elastic gel a t room temperature. It is, therefore, also an effective demonstration of gel properties to use in a discussion of lyophilic colloids. The linear polymer used is Gantrez AN, a copolymer of maleic anhydride and methyl vinyl ether, manufactured by GAF Corporation', who kindly furnished samples. In the experiment the polymer chains are crosslinked a t the anhydride groups by addition of the hydroxyl groups of triethanolamine (2,2',2"-nitrilotriethanol),which prohably also acts as a basic catalyst.
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that although the linear polymer starting material was prepared by an addition polymerization, the crosslinking process is less easily classified; polyesters are normally regarded as condensation polymers, but the crosslinking reaction is not strictly a condensation, since no small molecule is eliminated. A further advantage is that the two solutions can be prepared ahead of time and simply mixed in class. Common methods of forming crosslinked polymers (for example, w l canized rubber, phenolformaldehyde and alkyd types) require heating or irradiation, and they may release irritating fumes. It is probable that some other maleic anhydride copolymers, such as thme listed by Aldrich Chemical Co. and Polysciences, can be used. A few experiments witb styrene-maleic anhydride copolymers (SMA lOOOA and SMA 3000A) kindly supplied by the ARC0 Chemical Division of Atlantic Richfield Company did not give smooth gels under the conditions described above. The demonstration has been performed a number of times with Gantrez AN-169 at one-tenth the scale described above; Gantrez AN-119 and AN-139, polymers of lower molecular weight, were also tried with just as good results. Although it is not necessary to use specially dried tetrahydrofuran, the Gantrez and thk solutions should hekept stoppered to minimize hydrolysis of the anhydride groups. ~ c e & n ecan be used as the solvent, but the gelation maybe somewhat slower. If other solvents are employed, for example in trying other copolymers, it should be noted tbat the solvent must dissolve the very polar triethanolamine as well as the copolymer, but should not have reactive -OH or -N-H groups. In disposing of the gel i t must he rememhered that covalentlv cross-linked nolvmers are insoluble hv nature. Neither hbt water nor organic solvents will dissolve them. The product of this demonetratiun isa hizhlv .. . swnllen eel u,hirh can he 1)roki.n up hy vlpnrnus shaking. After the curds have heen dumord nut. the container can t ~ easil!. e cleaned h\. Iwushine immkdiatel; with soap and water. ~ e c a u s ethe polymer swollen with a flammable solvent, it is recommended that the lumps be let dry under a hood before they are disposed of as solid waste.
