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a b

F i gu re 1 . The appa ra t us : ( a ) r e f l ux as s em b l y ; ( b ) d i s t i l l a t i on

assembly.

An Undergradua te Ex perim ent in Poly ester (PET) Sy nthesis

Andrew N. Camm idge*

School o f Chemi ca l Sc iences, Un ivers i ty o f East A ng l ia , N orw ich N R4 7 TJ, UK

At UE A, first-year und ergraduate chem istry student s areencouraged to take an extra laboratory course if their time-table allows. The current course entitled Macromoleculesand Molecular Assemblies is also compulsory for studentsenrolled on our degree course Chemistry with AdvancedMaterials. All experiments are chosen to reinforce somefundamental principles of chemistry under this commontheme. Experiments in this unit include liquid crystal syn-thesis, inorganic and organic polymers and colloids. Eachexperiment is introdu ced by a on e-hour lecture, in whichth e science and in du strial relevance of th e experimen t is putinto context. The laboratory experiments themselves aimto introduce some more advanced practical techniques andencourage independent investigation.

An important part of the course is the introduction of

polymer science through the synthesis of both organic andinorganic polymers. T he polymers chosen are intend ed to beof relevance to industry (silicones, nylons, etc.). For thisreason a laboratory experiment introducing polyesters (1 ) wasrequired. To our surp rise, no suitable und ergradu ate experi-ment could be found. This paper details the experiment wedeveloped and incorporated successfully into the first-yearlaboratory program.

The Experiment

Description

The most important polyester manufactured industrially

is PET (polyethyleneterephthalate). T he ind ustrial preparationis a two-step procedure from terephthalic acid and ethanediol (ED). This procedure is not suitable for direct modifica-tion to a laboratory experiment (high temperature and pressure),but can be mimicked using a transesterification reactionstarting from dimeth ylterephth alate (D M T P) (Scheme I).

OO

OCH3H3CO

HO OHOO

OO

OHHO

HO OHOO

OOOHHO

OO

OOO

2

2 CH3OH

nn n

H

heat

Scheme I

The intermediate formed in the first step (bis-(2-hydroxy-ethyl)terephthalate (2 , 3 ) is identical to that formed in theindustrial synthesis. In the first step the students assemblereflux apparatus inserting a Y adapter (with t herm om eter)between the roun d-bottom flask and condenser. T he roun d-bottom flask is charged with DMTP and excess ED. A smallpiece of sodium metal is added and the heating mantle isswitched on (Fig. 1).

At this stage the students are assembled around thechalkboard to d iscuss the procedure. Th ey are asked to p redictthe tem perature reading on the th ermometer at reflux. Theanswer is usually ~190 C (bp of ED ). By working throughthe equilibria setup in the reaction, a consensus is reachedthat the expected temperature is ~65 C (bp of MeOH).T he students return to th eir apparatus to find th at the refluxtemperature is indeed ~65 C .

After fluxing for 30 min the apparatus is cooled andconverted to distillation (the Y adapter is kept in place as acrude fractionation column). Heat is reapplied and am easur ing cylinder is used to collect meth anol. T he stud ent sare asked to predict the volume of methanol expected(reinforcing basic calculations). When the head temperaturereaches 180 C the heat is turned off and the mixture is

allowed to cool slightly. The hot solution is decanted fromthe an tibum ping granules and left to crystallize. (Th e ex-perimen t can be left at this point.) C rystallization can b einduced by scratching the flask or adding water, if necessary.The crystals are filtered off under vacuum for analysis by1H N M R spectroscopy.

(N O T E: another base such as sodium methoxide couldbe employed, but use of sodium metal makes the procedureconceptually simp le and does not in terfere with the subsequent

calculation of th e expected volum e of meth anol distillate).

Polymerization

Small samples of the intermediate are dried betweenfilter pap ers and p laced in crucibles or on microscope slides.The samples are heated carefully on a hot-plate, ED isevaporated, and the physical properties of the sample aremo nitored as the sample is re-cooled. To som e preparations,a small crystal ofp-toluene sulfonic acid is added during theheating process. This polymerization requires some care tobe successful. Low-molecular-weight polymer (oligomer)cools to form waxy, semicrystalline material. Samples leftlonger (with acid catalyst) form sticky polymeric masses,which cool further to give clear glassy solids. These properties

* Em a i l : a .cammidge@uea .ac .uk .

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are used to qualitatively monitor polymerization. (Samplesleft too long simply char.) Students wash the bulk samplewith water and dry it thoroughly. A sample is submitted forN M R, which the student s discuss in their report.

Procedure

Place dimethyl terephthalate (5 g), ethane diol (80 mL)and some antibumping granules in a 250-mL round-bottom

flask. Add a small (~0.1-g) piece of sodium metal (CAUTION!).Fit a Y adapter, thermometer, and reflux condenser and heatthe mixture to reflux for 45 minutes using a heating mantle.Cool the mixture somewhat and modify the apparatus fordistillation, leaving the Y adapter in place for fractionaldistillation. D istill off the m ethan ol, stopping t he collectionwhen the head temperature reaches 180 C. Decant thesolution from antibum ping granules and allow it to cool.Collect the crystals that form by suction filtration. Dry asample thoroughly between filter papers and place it in acrucible or on a microscope slide. Heat this strongly on ahot-plate, carefully removing samples from time to tim e to checktheir physical properties as they cool. Repeat with anotherdry sample, this time adding a small crystal ofp -toluenesulfonic acid to t he m olten m aterial. Wash the bu lk samp lewith water and dry thoroughly before submitting a samplefor 1H NMR analysis.

Conclusion

T his experiment is designed to reinforce impo rtan t con-cepts in organic chemistry and introd uce polymer synthesis.Stud ents are able to o bserve, in real time, equ ilibration of areaction mixture and see how product composition can bedetermined by removal of one component (methanol) or useof excess reagent (E D ). T hey perform fractional distillationsand apply knowledge of the reaction to calculate predictedvolume of distillate. (This was the first time these studentshad performed a distillation.) The N M R of the int ermediateallows the concepts of coupling, chemical shift, and integrationto be demonstrated. More advanced experiments could in-volve D 2O exchange.

Acknowledgments

I would like to thank the students of CHE-1H04 forproviding feedback on this experiment, and Amy King forrunning NMR spectra.

Literature Cited

1. Billmeyer, F. W. Textb ook of Polymer Science, 3rd ed.; W iley: New

York, 19 84.2. Zahn, H .; Krzikalla, R. M ak rom ol. C hem . 1957 , 23 , 3153.3. Allan, R. J. P.; Iengar, H . V. R.; Ritchie, P. D .J. C hem . Soc. 1957 ,

21072113.