Feb., 19501 HASLAM AND SOPPET : METHACRYLATE POLYMERS 63 The Examination of Methyl Methacrylate Polymers and Co-Polymers BY J. HASLAM AND W. SOPPET SYNoPsIs--hIethods which have proved useful in the examination of methyl methacrylate polymers and co-polymers are described. The method of determination of plasticiser in this type of polymer is described and full details are given of the vacuum depolymerisation of plasticiser-free polymers. Observations are made on the identification of the depolymerisation products with particular reference to polymethyl methacrylate alone, and co-polymers containing polystyrene, polycyclohexyl methacrylate and polyethyl acrylate. The chemical evidence is supported by infra-red data. METHYL methacrylate polymers and co-polymers are of various types.The preparation may contain plasticiser, colouring agent or filler. It may be co-polymerised, i.e., it may consist of co-polymers of methyl methacrylate and styrene, of methyl methacrylate and ethyl acrylate, methyl methacrylate and cyclohexyl methacrylate or of other possible combinations. As the most important question which has to be answered about these preparations is the nature of the polymer or co-polymer, the purpose of this paper is to describe some of the methods which have proved to be most valuable in this connection. When dealing with compositions containing polymethyl methacrylate, the most satis- factory method of identification is to prepare as pure a monomer as possible by depolymerisa- tion of the polymer. The first fact to be realised, however, is that depolymerisation of poly- methyl methacrylate preparations in air, without preliminary treatment, yields a monomer which is distinctly unsatisfactory for the purpose of identification.A monomer prepared in this way from polymethyl methacrylate containing small proportions of plasticiser gave the following figures on examination- Saponification value (mg. of KOH per g.) . . = 527 Refractive index, 20" C. .. .. 1-42 0.11 - I - Aldehydes, as H.CH0, per cent'. .. .. The recovered monomer was shown, by polarographic and dimedone precipitation methods, to contain formaldehyde. This monomer was distilled at 20 mrn. pressure and the fractions were examined separately with the following results. 1st Fraction 2nd Fraction volatile a t 100" C . , (about 30%) (about 60%) 20 mm.pressure) Residue (not Saponification value (mg. of KOH per g. sample) 545 548 390 Refractive index, 20" C . .. . . .. 1-4135 1-4135 1.487 Carbon, per cent. . . .. .. .. - 64.9 - Aldehydes, as H.CH0, per cent. . . .. 0.064 0-016 Hydrogen, per cent. .. . . . . .. - - 7.5 - It should be noted that the accepted values for pure methyl methacrylate are as follows- Carbon, per cent. . , .. . . .. . . 60.0 Hydrogen, per cent. .. . . .. . . 8.0 Refractive index, 20" C. .. . . . . , . 1.4144 Saponification value (mg. of KOH per g.) . . 560 It is therefore desirable to remove the plasticiser as a preliminary to any satisfactory depoly- merisation process and this is best accomplished by solution of the preparation in acetone followed by precipitation of the polymer with light petroleum.This isolation of the plasticiser- free resin forms part of the ordinary method of determination of plasticiser in polymethyl methacrylate compositions and is as follows- One gram of the sample, in the form of finely divided drillings, is weighed into a beaker (250 ml.) provided with a glass stirring-rod. Twenty-five ml. of acetone are added and the liquid is brought just to the boil, with continuous stirring on the water-bath. The solution is then allowed to cool for about 1 hour. By this treatment the polymer in all ordinary64 HASLAM AXD SOPPET: THE EXAMINATIOS OF METHYL [Vol. 7 5 samples will be taken into solution, but with abnormal samples it may be necessary to allow the solution to stand for a longer time. Eighty ml.of light petroleum (b.p. 40" to 60" C.) are now added and the mixture is stirred vigorously and then allowed to settle. After standing for 1 hour the precipitate is filtered on a 1 x G3 sintered-glass Gooch crucible, the filtrate being collected in a flask (250ml.). The resin is washed three times with 5 ml. of light petroleum on each occasion. The washed resin is then dried to constant weight in the oven a t 100" C. In the determination of the plasticiser, the light petroleum solutions are collected and the solvent removed by evaporation on the water-bath, previous to drying at 100" C. Carried out in this way the method gives results for the plasticiser that are slightly on the high side, i.e., 0.0 to 0.3 per cent., owing to the solution of small amounts of low molecular weight polymer in the light petroleum extract.The behaviour of a methacrylate polymer in the plasticiser determination can on occasion yield evidence of the presence of abnormal additives. In the determination of the plasticiser in a recent competitive product, the light petroleum extract of the plasticiser had a strong odour of "oil of wintergreen," and the presence of salicylate was confirmed on hydrolysis of the resulting plasticiser. Before depolymerisation experiments are carried out, it is often desirable to make exact carbon and hydrogen determinations on the deplasticised resin ; in this connection the theoretical figures for the carbon and hydrogen contents of polymethyl methacrylate, poly- styrene, polycyclohexyl methacrylate and polyethyl acrylate are of value.They are as follows- Carbon Hydrogen Y O % Polymethyl methacrylate . . . . .. 60.0 8.0 Polyst -ene . . . . . . .. .. 92.26 7-74 Polyc- lohexyl methacrylate . . .. 71.4 9.5 Polye yl acrylate . . . . .. . . 60.0 8.0 It will be no d from the above figures that co-polymerisation of methyl methacrylate with 10 per cent. ( polystyrene leads to production of a polymer with the following elementary andysis- Per cent. Carbon . . .. . . . . . . . . 63.2 Hydrogen . . .. .. . . .. . . 7.95 Co-polymerisation with 50 per cent. of cyclohexyl methacrylate produces a polymer with the following elementary analysis- Per cent. Carbon . . . . . . .. * . . . 66.7 Hydrogen . . . . .. .. .. . , 8.75 It is important, however, to realise that co-polymerisation with ethyl acrylate produces a co-polymer with carbon and hydrogen figures exactly similar to those of polymethyl met hacrylate. Depolymerisation in air, i.e., by heating the plasticiser-free resin prepared in the above way in a small flask and subsequent distillation of the monomer, does not yield a very satis- factory monomer for identification purposes, as is shown by the following figures which were obtained by heating plasticiser-free polymethyl methacrylate in a side-arm distillation flask at 360" C., using a Wood's metal bath, for an hour.The distillate had the following characteristics- Saponification value (mg. of KOH per g.) . . 540 Refractive index, 20" C. . . . . .. . . 1.416 Recovery . . . . . . . . . . . . about 60% Appearance * . . .. . . . . . .Yellow Our experience with methacrylate polymers goes to show that the only satisfactory method of depolymerisation involves heating in vacuo under conditions which ensure the immediate distillation of the monomer as soon as it is formed. We are indebted to Dr. J. W. C. Crawford for details of an apparatus for this purpose, and the method we have found most satisfactory is given below in full. In this method, as applied to the depolymerisation of polymethyl methacrylate, 1 g. of the plasticiser-free sample is weighed into the tube A of the Pyrex glass vacuum depoly- mensation apparatus shown in Fig. 1.Feb., 19501 METHACRYLATE POLYMERS AXD CO-POLYMERS 65 The upper portion of tube A is then sealed off at C in the blowpipe flame, leaving a capillary tip. Tube B is now connected at E to a high vacuum pump and when the pressure on the manometer connected to the pump has been reduced to 1 mm.of mercury and with the pump still connected, the apparatus is sealed off in the blowpipe flame at D. I 'i Fig. 1. Vacuum depolymerisation apparatus of pyrex glass. Dimensions are in centimeters The apparatus is now assembled with tube A immersed in a bath of Wood's metal at 100" to 120" C . This Wood's metal is contained in a small metal bath, l-inch internal diameter and 34 inches long. *Tube B is immersed in a solid carbon dioxide - methanol freezing mixture at -70" to -80" C. contained in a Dewar flask (capacity 1 litre) and the levels of A and B are so arranged that the point G is at a higher level than point F. The temperature of the Wood's metal bath is now raised to 340" to 350" C.over a period of 15minutes and is maintained at this temperature for 1 hour. The apparatus is then withdrawn from the hot Wood's metal bath and cold solid carbon dioxide - methanol mixture, the capillary tip at C broken and the tube B cut off near the top, i.e., at a point G. The analytical examination of the monomer in tube B is carried out without delay in order to avoid polymerisation. The following figures are typical of those obtained on depolymerisation of polymethyl methacrylate by the above procedure. The figures obtained on examination of pure methyl methacrylate are also given for purposes of comparison. Monomer recovered from polymethyl methacrylate by Pure methyl depolymerisation methacrylate in vacuo monomer ... . 90-97 - .. .. . . .. . . Water white - Recovery, per cent. .. .. .. Saponification value (mg. of KOH per g.) . . .. 556-560 560 Colour . . Refractive index, 20" C. .. .. .. . . 1.4142-1*4144 1.4144 Acid value . . .. . . .. . . . . nil nil On occasion, valuable evidence about other additives in the polymer may be obtained from the odour of the recovered monomer. A recent competitive sample had a strong odour of mercaptan, presumably added in the polymerisation process. Although the amount present was extremely small it was quite sufficient to mask the very characteristic odour of methyl met hacrylate. BEHAVIOUR OF METHYL METHACRYLATE - POLYSTYRENE INTERPOLYMERS ON DEPOLYMERISATION in vacuo In our experience polymethyl methacrylate - polystyrene interpolymers may be depoly- merised by vacuum depolymerisation exactly as indicated above for polymethyl methacrylate alone.Although it might be expected that because methyl methacrylate boils at 100" C. and styrene a t 143" C., preferential depolymerisation of the polymethyl methacrylate would take The yield of monomer is rather low.66 HASLAM AND SOPPET: THE EXAMINATION OF METHYL [Vol. 75 place, our analytical evidence hardly supports such a view and the recovered monomer mixture always contains styrene. The following results were obtained on depolymerisation of a polymethyl methacrylate - polystyrene interpolymer containing 10 per cent. of polystyrene. Recovery, per cent. .. . . .. .. .. 70-76 Colour .. . . .. . . . . . . Water white Refractive index, 20" C. ... . . . Saponification value (mg. of KOH per g.) . . . . . . 1.4242 510 High carbon figures on the original deplasticised resin, together with figures similar to those given above for the recovered mixed monomer, point to the probable presence of styrene. I t is our practice to confirm the presence of styrene in such samples by heating the monomer recovered from 2 to 2.5 g. of polymer with 5 ml. of 50 per cent. w/v KOH solution for 1 hour under reflux. The mixture is cooled and diluted to 25ml. with water and the solution then extracted with 5ml. of ether. The ether layer is washed with 5ml. of water and then filtered through a dry filter paper into a test tube (5 x + inch). The ether is removed by evaporation on the water-bath, the last traces of the solvent being removed by a stream of compressed air.Eight to ten drops of the residue are measured into a clean test tube (5 x 4 inch) and liquid bromine is added dropwise until a definite excess is indicated by the colour of the mixture. The excess of bromine is removed by heating on the water-bath and the mixture cooled. The solid residue is broken up with a glass rod and dissolved by heating with 5 ml. of 80 per cent. v/v ethyl alcohol. The filtered solution is cooled and the crystalline deposit filtered off and recrystallised from 3 ml. of 80 per cent. v/v ethyl alcohol. The crystallised product is dried in air and its melting-point determined. Styrene dibromide melts at 73" C . (uncorrected). BEHAVIOUR OF POLYMETHYL METHACRYLATE - POLYETHYL XCRYLATE INTERPOLYMERS ON DEPOLYMERISATION in vacuo Polymethyl methacrylate - polyethyl acrylate interpolymers may be depolymerised by vacuum depolymerisation as indicated above for polymethyl methacrylate alone.The recovered monomer usually possesses an odour resembling to a certain extent that of ethyl acrylate and is quite different from that of methyl methacrylate alone, but the yield of recovered monomer is low, and the saponification value and refractive index do not differ appreciably from the corresponding figures for pure methyl methacrylate, as the following results indicate- Monomer recovered Monomer recovered from polymethyl from polymethyl methacrylate with methacrylate with 10 per cent. polyethyl 5 per cent. polyethyl acrylate acrylate .. .. about 60 about 60 ..Recovery, per cent. . . .. 1.4135 .. 1.4141 Refractive index, 20" C. .. .. Saponification value (mg. of KOH per g.) . . 648 557 Colour . . .. .. a . .. . . Water white Water white In material such as this it is desirable to supplement the information by carrying out a test for the detection of the ethyl ester in the recovered monomer as follows- One ml. of the monomer is heated under reffux for 1 hour with 2.5 ml. of 50 per cent. w/v KOH solution. After cooling, 2ml. of water are added and the solution is distilled until 2 ml. of distillate have been collected. To this distillate are added 5 ml. dichromate mixture (100 g. of potassium dichromate dissolved in a mixture of 280 ml. of concentrated sulphuric acid and 750ml. of water) and the solution redistilled into a 15-1111.centrifuge tube until 0.5 ml. of distillate has been collected. One pellet of solid potassium hydroxide is added and the tube placed in a beaker of boiling water. If an ethyl ester was present in the original recovered monomer, the caustic pellet will turn yellow and the solution will boil, turn yellow and emit the obnoxious odour charac- teristic of aldehyde resins. BEHAVIOUR OF POLYCYCLOHEXYL METHACRYLATE - POLYMETHYL METHACRYLATE INTERPOLYMERS ON VACUUM DEPOLYMERISATION The behaviour of polycyclohexyl methacrylate - polymethyl methacrylate interpolymers on depolymerisation in vacuo is entirely different from that of polymethyl methacrylate itself.Feb., 19501 METHACRYLATE POLYMERS AX D C 0-POLYME RS 67 The procedure is that of the general method except that where this kind of interpolymer is expected the depolymerisation is carried out for a period of 2 hours.With this type of interpolymer the depolymerisation product consists of two layers, the lower one of which is chiefly water. The figures for saponification and refractive index of the product are, therefore, of little diagnostic value. The following figures, which were obtained on the depolymerisation product of an inter- polymer containing 46 per cent. of polycyclohexyl methacrylate and 54 per cent. of polymethyl methacrylate, are typical of the results that may be obtained. Saponification value (mg. of KOH per g.) . . . . 120-200 Refractive index, 20" C . . . . . . . . . 1'42-1.43 Yield (2 hours), per cent. . . . . . . . . 40-50 ,4cid value (mg.of KOH per g.) . . . . .. 3 Water (Fischer), per cent. . . . . . . . . 7-10 Behaviour similafto that recorded above usually indicates the presence of a co-polymer of cyclohexyl and methyl methacrylates, but it is always necessary to proceed further in order to obtain evidence of this. Tests which are of great value in this connection involve the production of cyclohexene from the depolymerisation product and its subsequent identification. In this identification the cyclohexene is oxidised to adipic acid, the melting-point of which is determined. \ .WIRE SPACER F5rnm. DlAM c Fig. 3. Continuous ether extractor Fig. 2. Apparatus for continuous steam distillation The methacrylic acid in the depolymerisation product is isolated and may be identified as its fi-phenyl phenacyl bromide derivative; the presence of a methyl ester is confirmed by isolation of the corresponding alcohol, oxidation of this to formaldehyde, and the identifica- tion of the latter by the chromotropic acid reaction.Full details of the method are given below. One ml. of the depolymerisation product is heated under reflux for an hour with 5 ml. of 50 per cent. w/v potash solution. The flask containing the solution is then connected to the continuous steam distillation apparatus The mixture is cooled and 5 ml. of water added.€!ASLAM AND SOPPET: THE EXAMINATION OF METHYL [Vol. 75 Polr methyl fltthorrylQt P Fig. 4. Infra-red spectra of methyl I Pal 7 s t y re neFeb., 19501 METHACRYLATE POLYMERS ASD CO-POLYMERS 69 methacrylate polymers and co-polymers70 HASLAM AND SOPPET: THE EXAMINATIOS OF METHYL [Vol. 75 illustrated in Fig.2 and the steam distillation carried out for half an hour, by which time an oilyjayer will have collected in the detachable tube A. The aqueous layer in the detachable tube is reserved for the detection of methyl alcohol. The oily layer is transferred to a small separating funnel, and after being washed with 5 ml. of water, transferred to a 1-mi. centrifuge tube containing a small quantity of anhydrous potassium carbonate. After the solution has stood overnight, the boiling-point, refractive index and specific gravity of the clear upper layer of cyclohexene are determined. A portion of this cyclohexene is then converted to adipic acid by taking 0.2 ml. of the suspected hydrocarbon and heating it gently under reflux for 10 to 15 minutes with 5 ml. of dichromate - sulphuric acid mixture (100 g.of potassium dichromate dissolved in a mixture of 250 ml. of concentrated sulphuric acid and 750 ml. of water), The product is diluted with 5ml. of water and then extracted with ether for 15 minutes in a small continuous ether extractor (Fig. 3). The ether is evaporated to low bulk and the solution transferred t o a 1-ml. centrifuge tube previous to final evaporation to dryness. The resulting adipic acid is recrystallised from 0.5 ml. of water and the product dried at 100" C. previous to the determination of its melting-point. For the detection of methyl alcohol, the water in the detachable tube, referred to above and reserved for this test, is transferred back to the original hydrolysis flask containing potash, etc.To this distillate are added 5ml. of dichromate oxidation mixture and the solution distilled until 0.5 ml. of distillate has been collected. Three to four drops of this distillate, contained in a 6 x 8 inch test tube, are mixed with 2 ml. of diluted sulphuric acid solution (150 ml. of concentrated sulphuric acid + 100 ml. of water) and a few crystals of the sodium salt of chromotropic acid. The tube is then heated for 10 minutes by immersion in a water-bath at 60" to 70" C. The production of a violet colour indicates the presence of a polymethyl ester in the original polymer. The presence of rnethacrylic acid in the depolymerisation products is confirmed as follows- The alkaline solution obtained in the small flask, after the removal of any methyl alcohol referred to above, is acidified with concentrated hydrochloric acid and the solution saturated with sodium chloride.This saturated solution is then extracted twice with 5-ml. portions of ether. The ether extracts are combined, washed with 2 ml. of saturated sodium chloride solution and transferred to a small flask. One ml. of 10 per cent. w/v sodium hydroxide solution is now added, the ether is evaporated off on the water-bath and the residual solution neutralised with N hydrochloric acid solution. P-Phenyl phenacyl bromide (0.1 g.) is added and the mixture heated under reflux for 30 minutes. The hot solution is filtered and the filtrate allowed to crystallise. The product is recrystallised from 2 ml.of ethyl alcohol and dried in air previous to determination of its melting-point. Some idea of the kind of figures that are obtained in this kind of test may be seen from the results of the examination of two polymers, one of which was polycyclohexyl methacrylate itself, whilst the other consisted of an interpolymer containing 46 per cent. polycyclohexyl methacrylate and 54 per cent. of polymethyl methacrylate. The cyclohexene obtained from the two polymers had the following properties- Interpolymer containing 46 per cent. polycyclohexyl methacrylate and Polycyclohexyl 54 per cent. polymethyl The mixture is now distilled until 2 ml. of distillate have been collected. methacrylate methacrylate Refractive index, 20" C. .. .. .. 1.449 1.451 Boiling-point . . .. .. .. .. 84" C . 84" C. Specific gravity, 25/4' C. . . . . .. 0.838 0.846 In both, the adipic acid prepared by oxidation of the cyclohexene melted at 148" C. The melting-points of the 9-phenyl phenacyl bromide derivatives of the methacrylic acid obtained from the two depolymerisation products were respectively 121" C. for the inter- polymer and 118" C. for the cyclohexyl methacrylate. It is always desirable to supplement chemical examinations similar to those outlined above with infra-red evidence obtained on known polymers and co-polymers, e.g., a substance submitted for analysis was shown to contain Per cent. Carbon . . .. . . .. . . . . 66.3 Hydrogen . . .. . . . . . . * . 8.9 Oxygen .. 0 . , I 3 , . . . . 25.8Feb., 19501 METHACRYLATE POLYMERS AND CO-POLYMERS 71 On solution in acetone and precipitation with light petroleum followed by evaporation of the light petroleum solution, the substance yielded a residue amounting to 2.2 per cent. This residue was a hard, clear solid which possessed the smell of a monomer. It gave no evidence of the presence of a phthalate and was almost certainly low molecular weight polymer.Subsequent examination of this polymer by the chemical methods outlined above indicated that it was a co-polymer of polycyclohexyl methacrylate and polymethyl meth- acrylate and the carbon, hydrogen and oxygen figures indicated that the approximate proportions were 46 per cent. polycyclohexyl methacrylate and 54 per cent. polymethyl met h acrylat e. Support for this view was obtained when an interpolymer was prepared containing 50 per cent. of polymethyl methacrylate and 50 per cent. of polycyclohexyl methacrylate. The infra-red spectra of these two specimens, the sample and the comparison mixture were very similar indeed, as is shown in Fig; 4. On pages 68 and 69 are given the infra-red spectra of the above two specimens, together with those of- (a) Polymethyl methacrylate. (b) Polystyrene. (c) Polycyclohexyl methacrylate. (a) Interpolymer of polymethyl methacrylate containing 10 per cent. of polystyrene. (e) Interpolymer of polymethyl methacrylate containing 7 per cent. of polyethyl acrylate. (f) Interpolymer of polycyclohexyl methacrylate containing 20 per cent. of poly- methacrylic acid. We should like to take this opportunity of expressing our indebtedness to Mr. Willis of our Research Department for the preparation of the infra-red spectra and for his general interest in this investigation. IMPERIAL CHEMICAL INDUSTRIES LIMITED WELWYN GARDEN CITY PLASTICS DIVISION HERTS. June, 1949