of Polystyrene Danlel W. Armstrong,' John N. Marx, Don Kyle, and Ala Alak Texas Tech University, Lubbock, TX 79409 Because of the tremendous importance of polymers and plastics in today's society there has been an impetus for including polymerization experiments in modern chemistry teachine laboratories. Indeed. manv laboratorv courses and textbooks now contain one or more polymerization experiments (1). While all of these experiments can teach the student how to make a polymer, very often little or no information is obtained on the molecular weight or molecular weight distribution of the polymer. In essence, the student has little idea as to what he has made and conseauentlv of the factors that control polymer formation. The reason for this state of affairs is amarent when one considers traditional methods ofohtainin;pulymer molecular weights and (listrihutions (i.e., lirhr scatterina, osmometry, srd~mentation,small angle X-ray scattering, vi&osity, and-gel permeation chromatography). It is apparent that the required equipment, time, and expertise to obtain this information is considerable. I t is, in fact, beyond the scope of most laboratory courses. Recently a simple, efficient, high-resolution thin-layer chromatographic (TLC) method has been developed that allows one to evaluate inexpensively polymer molecular weights and distributions. This technique has been used with great t make ~ success in coniunction with an e x ~ e r i m e nto . o l.v s-t v rene. Not onlyian the student follow the cowseof the reaction with time but he can also evaluate the effects of different parameters (such as concentration, temperature, amount of initiator, type of initiator, catalysts, etc.) on the polymerization reaction and the polymer formed. The TLC technique that makes this possible utilizes a reversed phase plate and a binary solvent mobile phase consisting of methylene chloride and methanol (see experimental section). The mechanism of separation is akin ro a selective precipitation process, which is \,cry different frvm traditional chrwuittryraphic mechanisms 12.3). . , , What a(:tunIIv hnuneni is that durine develoument a mobile phase gradient spontaneously forms on the TLC nlate. The less nolar methvlene chloride comoonent of the developing solveht is select&ely absorbed by tke hydrophobic stationary phase thereby enriching the remaining mobile phase in methanol. Compared to methylene chloride, methanol is a thermodvnamicallv Doorer solvent for ~ o l v styrene and therefore tends to pre&kate it. Higher molicuiar weieht ~olvstvrenes are more sensitive to methanol than those .. . . . of lower molecular weights, and thertfore tend to be rerno\.ed from thr mol)ik phase lirst. More detailed discussions of this process as it pert& to chromatography have been published (2-7). The use of this simple TLC fractionation in conjuuction with the synthesis of polystyrene has resulted in a very successful and popular experiment. This experiment has been used and evaluated in our sophmore level organic chemistry lahoratory (vide infra).
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Discussion Natural and synthetic macromolecules are formed by joining together (by polymerization or polycondensation) many small relatively simple molecules (monomers). The molecular weight of a synthetic polymer is dependent on both Author to whom correspondence should be addressed.
the reaction mechanism and the experimental conditions through which it was formed. Most synthetic polymers are not monodisperse (i.e., all of the same molecular weight) but rather consist of a distributon of molecular weights. Consequently the "molecular weight" of most polymers is really an average of the molecular weights of many homologous molecules. For linear synthetic polymers, the average molecular weight and the distribution of molecular weight can be correlated to several mechanical and physical properties of the polymer such as: strength, toughness, solubility, viscosity, elongation, abrasion resistance, etc. (8).For example, the fiber strength of polyester (a common polymer used in clothing) increases with increasine averaee molecular weieht (9,101 and the strength and tougkkess oipolyethylene increases as its molecular weight distribution becomes more narrow (I1). Experimental In this experiment styrene will he polymerized
where the value of n can vary considerably with reaction conditions. (The molecular weight of polystyrene would by 104 X n.) Polystyrenes have been produced with molecular weights that range from a few hundred to 100million. You will follow the molecular weight and molecular weight distrihution of polystyrene with time. Keep in mind that the average molecular weight and distrihution determines the properties of the polymer and thus its commercial applications. Materials Toluene, styrene, benzoyl peroxide, methanol, and methylene chloride can he obtained from Aldrieh, Baker andlor Fisher Chemical Companies. Five polystyrene molecular weight standards ranging from 600to 100,000g M-I can be obtained from Palysciences Polymer Laboratories andlor Waters Ine. The narrow disperse polystyrene standards used in this study were: MW's = 600,2000,9050,35,000, and 100,000. Equipment One 100-ml3-neckedflask, one reflux condenser, one Pasteur pipet with squeeze bulb, one steam bath or heating mantle, one 10 X 10-em reversed-phase TLC plate impregnated with fluorescent indicator (from Whatman, Inc.; 5 X 10-cmplates can also he used), three small test tubes with corks, a test tube holder or clamps,and eight capillary spotters are needed. One developing chamber (e.g., a suitably sized beaker or jar with cover is needed for every two to four students. In addition,one ultraviolet lamp (254 nm) will be needed per lahoratory section. Method Lightly label a 10 X 10-cmreverse-phase TLC plate with a soft lead pencil as indicated in the figure. Prepare capillary tubes for spotting by heating open tuhes in the center in a burner flame, drawing them out and breaking off the thinnest part. You will need 8 such capillary spotters. Make sure all Volume 62
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time interval, determine the Rr value of the center of each streak. From your calibration curve, determine the average molecular weight. From the H r vnlurq 01 the brginniny and end oleach etreak, uetermine thr distribution of AfIl"s at rarh time interval during the reartiun After the reaction mixture is cwl, isolate the polysty&ne by pouring the solution into 75 mlaf methanol. Collect the~recinitatebvfiltration with a nurhner funnel and a preweighed piece
