Introduction

Cationic polymerization is commercially utilized for the production of butyl rubber. Although not as efficient as alternate methods, it can also be used for the polymerization of styrene and its derivtives.

Cationic polymerizations involve the attack of an olefin to a positively charged initiating species. Initiators usually involve the complexation of residual water in the reaction vessle with a Lewis acid. This complex is then attacked by the olefin, and a proton is abstracted. The conjugate Lewis base is then carried along the polymerization as a counter ion. Use of Lewis acids as initiators is important in that a strong acid produces a highly nucleophilic counter ion which simply adds across the olefin instead of facilitating propagation¹.

Cationic polymerizations have also been found to exhibit living character. Although termination and chain transfer mechanisms exist, many of these processes can be frozen out at low temperatures allowing for formation of sequential blocks.

Thin layer chromatography (TLC) can be utilized to determine approximate molecular weights of polymer. Using a solution of standard molecular weight styrene, Rf values coan be found to correlate with molecular weights. Higher molecular weights will not diffuse as far up the TLC plate as low molecular weights, therefore, approximate molecular weights can be determined by comparison to the standards.

Experimental

Cationic polymerization was conducted according to the procedure found in Polymer Synthesis and Characterization: A Laboratory Manual². All quantities were scaled down by a factor of five. Test tubes were not flame dried, however they were purged with nitrogen.

TLC was carried out according to the procedure outlined by Daniel Armstrong in Analytical Chemistry³. Standard molecular weights of 600,000, 400,000, 300,000, and 200,000-90,000 were run in a 50:50 solvent mixture of methanol and methylene chloride. Rf values for the standards are listed in Table 1.

Figure 1

Five milligrams of poly (methyl styrene) was dissolved in one mL of methylene chloride. This was then spotted on the TLC plate and three bands were observed. Table two summarizes the Rf values corresponding to these bands.

Proton NMR showed the presence of residual monomer, but also confirms that poly (methyl styrene) was successfully synthesized via cationic polymerization (See Figure 2).

Figure2

The doublet peak at approximately 1.7 ppm indicates polymer backbone methylene hydrogens. The small peaks at approximately 5.6 ppm indicates residual monomer or oligomer.

TLC indicated a polydisperse sample with strong bands observed at Rf values of 0.74, 0.84, and 0.89. Utilizing the calibration curve, approximate values for the molecular weight of PMS were determined. Molecular weights of 600,000, 388,400, and 288,400 were obtained. The most prominent band corresponded to the 600,000 molecular weight.

Although the flask was not flame dried, relatively high molecular weight was obtained. Higher amounts of water increase the number of initiated chains, thus reducing the molecular weight. Because this effect was not observed, purging the flask with nitrogen was sufficient for water removal. Observation of bands at lower molecular weights could be due to chain transfer reactions, slow initiation, or termination reactions.

Conclusions

Cationic polymerization of PMS proved effective for conversion of monomer to relatively high molecular weight polymer. Analysis of molecular weight by TLC gave a semi-quantitative indication of molecular weight, however for more precise measurements a more rigorous technique such as dilute solution viscometry or SEC would need to be implimented. TLC did indicate a variety of molecular weights although a precise poly dispersity index can not be obtained by this technique. Higher molecular weights could be obtained by utilizing a drier reaction vessel, or reducing the amount of initiator added to the reaction mixture.

References

1 Odian, George. Principles of Polymerization. John Wiley & Sons, Inc., New York: 1991.

2 Sandler, Stanley, et al. Polymer Synthesis and Characterization. Academic Press, San Diego: 1998.

3 Armstrong, Daniel, Bul, K.H. "Nonaqueous Reversed-Phase Liquid Chromatographic Fractionation of Polystyrene," Ananlytical Chemistry. 1982, 54, 706-708.