Determination of Acrylate and Maleate Esters in Polymers by Combined Zeisel and Gas Chromatographic Analysis D. L MILLER,
E.
P. SAMSEl, and 1. G. COBLER
Special Services Laboratory, The Dow Chemical Co., Midland, Mi&.
b Methods have been developed for the determination of olkoxyl groups (methyl through butyl) in polymers and copolymers containing alkyl acrylates and maleates. First the total alkoxyl is determined using a modified Zeisel hydriodic acid hydrolysis method. Secondly, the various alkoxyl groups, after being converted to the corresponding alkyl iodides, are collected in a cold trap and separated by gas chromatography. These methods have also been used for the determination of ester plasticizers in various plastic formulations. Ester plasticizers such as ethyl phthalyl ethyl glycolate, and dibutyl sebacate have been determined in the presence of each other.
- 32 m d 0 . D .
T
HE DETERMINATION of poly(ethy1 esters) in methyl methacrylate polymers is a difficult analytical problem. A similar problem exists when propyl and butyl esters are present. In the past, the poly(acry1ic acid esters) were hydrolyzed with potassium hydroxide to form alcohols which could be isolated and characterized by standard methods; however, in the case of poly(methyl methacrylate) this hydrolysis is extremely difficult. W e l l and Strutz (5) and Strasaburger, Brauer, and Forzisti (6),have pyrolyzed acrylate and methacrylate polymer mixtures and analyzed thepyrolysis products by gas chromatography. Haslam, Hamilton, and Jeffs (8) published a method for determining blends of poly(alky1 acrylates). The alkoxy1 groups are converted to their corresponding iodides, which are absorbed in a cold trap containing nheptane. An aliquot is injectcd into a gaa chromatographic apparatus and the alkyl acrylates are determined by an internal standard method. We applied the above method to the analysis of acrylate polymers but had difficulty in obtaining satisfactory recoveries from known mixtures. The methods described in this paper show how gas chromatography in conjunction with hydriodic acid hydrolysis can be used to charackrize and determine polymers and copolymers
F Bmrn.
Figure 1.
Apparatus for olkoxyl determination
containing alkyl acrylates, methacrylatee, and maleates. The method of Haslam and coworkers waa modified so that propyl and butyl groups could be determined w well as methyl and ethyl groups. The method is based upon the quantitative determination of the total ester groups present in the polymer, followed by an effective separation of the individual iodides formed. The total alkoxyl is determined using a modification of the Zeisel method described by Samsel and McHard (4) for cellulose ethers. In the second determination, the various alkoxyl groups after b i g converted to alkyl iodides are collected
in a cold trap contttining n-heptane instead of the bromine solution which is used in the modified &isel method, The alkyl iodides are then eeparated by gaa chromatography. APPARATUS
Distillation apparatus for the alkoxyl determination (Figure 1). Distillation apparatus for the alkyl iodide absorption (Figure 2). The chromatograph consists of a detector unit described by Gohlke (I), coupled with a standard recorder. The column is a 9-foot length of inch stainless steel tubing packed with di-kthylhexyl sebacate on firebrick. I t is operated at a temperature of 70° C. VOL 33,
NO. 6, MAY 1961
677
p;
boiling mixture. Add 6 ml. of constant boiling hydriodic acid to the flask and attach at once to the apparatus aa shown. Connect the side arm to a source of nitrogen and pass a current of the as into the apparatus a t a rate of 2 bubhea r second. Immerse the flask in an ogebath, maintained a t 150' C., and heat for 3 hours. Wash the contents of the receiver into a flask containing 15 mi. of sodium acetate solution. Reduce the exceas bromine with formic acid. Add 3 r a m of potassium iodide and 16 ml. of L u t e sulfuric acid. Titrate the liberated iodine with 0.1N sodium thiosulfate. Make a blank determination on the Separation of A&?f%%es. The hydriodic acid hydrolysis of the polymeric sample is carried out in exactly the same manner as before exce t that a cold trap is substituted for t i e bromine receiver. The details of the cold trap are shown in Fi ure 2. Since moisture interferes witeh gsa chromatography systems, place a small amount of drying agent (magnesium erchlorate into the inlet section o r the abeor r trap. With a hypodermic syringe, add 0.25 mi. of n-heptane to the outlet section and assemble the apparatus aa shown. Surround the trap with a dry ice bath at - 8 O O C. and allow the reaction to proceed at 1 M ) O C. for 90 minutes. At completion of the hydrolysis, dieconnect the trap and allow it to warm ta mom temperature. By meam of a microsyringe, inject a small amomt of the heptane solution into the gas chromatograph system. The quantities of the alkyl iodides are determined b measuring the area of their corresponi ing peaks on the resulting chromato-
b
Figure 2.
Apparatus for alkyl iodide absorption
and a pregsure of 8 ps.i. (flow rate, 62 ml. per minute).
Dilute Sulfuric Acid. Carefully mix 1 volume of reagent grade sulfuric acid with 9 volumes of water.
REAGENTS
Potassium Acetate Solution. Diesolve 100 g r a m of potassium acetate in 1 liter of a solution containing 900 ml. of glacial acetic acid and 100 ml. of acetic anhydride. Bromine Solution. Diseolve 6 ml. of bromine in 146 ml. of the potesSium acetate solution. Pre are fresh daily. Sodium Acetate Sorution. .Diseolve 220 am of eodium acetate m water and %ute to 1 liter.
Table 1 Calculations
PROCEDURE
Determination of Total Alkoxyl. Add 3 ml. of water to the scrubbing trap and 10 mi. of bromine solution to receiver, A. Attach the receiver to the distillation apparatus, B. Weigh a Wmg. a m le into the a i d s ml. of melted arm flak, C, add
! phenol, and warm on a steam bath to
dissolve. The beat raulta are obtained when the olymer Sam le is dissolved com letery in phenol {efore the addition orhydriodic acid. Occasionally it is newssag to add a small amount of propionic anhydride to the phenol mixture and heat again to com lete the dieeolution of the aam le. some t r ofr!wm,. the a dition of hydri c aci wdl precipitate the sample. If thie occura, gently shake the reaction h k from time to time so that the precipitate flab free in the
B
wig
g-.
Calculations of the various c o m p nenta of a methyl methacrylateethyl acrylate-butyl acrylate copolymer are deacribed in Table I. After the individual peak areas have been meaeured on the chromatogram, the total area ie computed. Then the ratio of the area of an individual peak to the total area of the peaks is multiplied by Vt, the volume of standard t h i d a t e required for the total alkoxy1 titration. This volume ie the portion of the total titration due to the individual alkyl iodide.
Table IL Structural Formulas Methyl MethaorylateButyl Maleate Polymer Ethyl Aorylate Copolymer where
V; Arne Aet Abu Vme
Vet Mu
678
N&& required for total alko 1 = area under methyl iodxe = -under ethyl iodide = area under butyl i d d e peak = volume of 0.lN N&D required to tltrate CHaI = volume of 0.1N Na&O, re quired to titrate = volume of 0.W NsBOc required to t~trabGHJ = volume of 0.1N
peaeea]r
w
A W m a CHEMISTRY
\
MMA
When multiplied by the appropriate factor, the amount of the bound monomer can be calculated. The other alkyl groups are calculated in the same manner. RESULTS AND DISCUSSION
The structural formules for B methyl methacrylate-ethyl acrylate copolymer and a butyl maleate polymer are shown in Table 11. The alkyl eaters are attached to alternate carbons of the copolymer and to adjacent carbons of the maleate polymer. It is these alkyl eaters which react with hydriodic acid to form the corresponding iodides. A typical chromatogram is shown in Figure 3. The methyl iodide peak is recorded after 5 minutea, the ethyl peak a few minutea later, and the n-butyl iodide peak after 45 minutes. The propyl iodides and the other butyl iodides emerge individually between the n-heptane and the n-butyl iodide. Thus this becomes a rapid method for characterizing the structure of the propyl and butyl groups. For instance, one can readily determine if the polymer contains a secondary, iso, or tertiary butyl group. Isopropyl iodide emerges just beyond n-heptane and although there is sufficient peak to indicate its presence, a quantitative measurement cannot be made. If this situation occurs, n-octane is substituted for n-heptane aa the absorbant and the run is repeated. Caution must be used to keep the cold bath above -56' C. aa n-octane freezes a t that temperature. If it becomes necessary to determine very small amounts of the alkyl esters, the sensitivity of the system can be increrwed by using less absorbant in the micro trap and recording the chromatogram at 0- to 1-mv. sensitivity. Most of the chromatograms have been recorded at 0 to 10 mv. Known mixtures of methyl methacrylate and ethyl acrylate were analyzed by the above method. The results are shown in Table 111. In each case, the amount of methyl methacrylate found was slightly less than the amount added, while on the other hand, the amount of ethyl acrylate found was greater than the amount added. Them differences were corrected aa follows: Ratios of methyl methacrylate to the s u m of methyl methacrylate plus ethyl acrylate were calculated for both polymer added and polymer found. A graph was prepared by plotting the ratio of methyl methacrylate added against the ratio of methyl methacrylate found. This is shown in Figure 4. The amount of methyl methacrylate calculated is measured on the abscissa of the graph and the corrected amount taken from the ordinate. Results of the analyses of various polymers are given in Table IV. The
8
H -1 I c w
45
I5
5
10
0
RETENTION TINE IN MINUTES
Figure 3.
Gas chromatogram of alkyl iodides
Column, 10 foot di-2-othyhoxyl mbacator hoHum preuwo, 1 1 p.J. (Row rate 79 cc/min)r tomparature, 70" CJcurrent, 200 ma.) &art rpood, 24 in./how
Table 111.
Analysis of Polymer Standards
Pesk
Peakof Area 1Iodide
Ar+
%t'O
Ratio
Alkoxy1 Titer, Polymer, Mg. M1. Added Found
MMA
EA Polymer Added Found Methyl methacrylate. 9 . 6 86/14 28.3 41.3 40.3 88/12 88/14 Ethyl acrylate' 1.6 5.8 6.8 Meth 1 methacrylate" 11.4 68/42 24.7 24.6 23.8 59/41 68/42 Ethyfacrylatd 8.3 16.7 17.4 Methyl methacrylate' 7 . 8 44/66 26.3 19.6 18.5 46/54 44/66 Ethyl acrylatd 10.0 22.7 23.7 Meth 1 methaorylate' 3.6 18/82 22.8 8.2 7.0 21/79 18/82 Ethy [acrylate' 16.2 29.9 31.0 a Plexiglas 6209XP obtained from R o b & Haan, 99.8% methyl methacrylate by alkoxy1 determination. * Pure ethyl acrylate monomer waa eolution pol erized 24 houre at 90' C. with 2% benzo 1 peroxide catelyet. The olymer waa dri& constant wei t. The concentration or ethyl acrylate, aa obtaineaby the alkoxy1 procedure was 98.1%. MMA EA
Table IV.
% Methyl
Polymer A
B
C
D E F Q
H
Methacrylate
44.4 61.1 74.3 16.5 88.0 99.8 26.0
Analysis of Polymers
% Ethyl Acrylate
MonoMethyl Maleate
% I-
Butyl Maleate
%
Styrene 65.2 37.3 26.0
84.6 12.0 0.2 max. 74.0
7.6
33.8
VOL 33, NO. 6, MAY I961
63.0
679
TIME MINUTES
Figure 5.
PER CENT FOUND
Figure 4. Ratio of methyl methacrylate-ethyl acrylate added to methyl methacrylate-ethyl acrylate found
Plastic formulations
the n-butyl iodide peak shown in the lower lefbhand comer of the chromatogram. The small methyl peak ie UDaccounted for. WERANRE n
first three Ramples are copolymers of methyl methacrylate and styrene in which the styrene was determined by an ultraviolet method. Polymer D was polymerized to contain 15% methyl methacrylate and 85% ethyl acrylate. Polymer H is a commercial product believed to be a copolymer of monomethyl maleate, isobutyl maleate, and styrene. This method is also useful for the determination of alkyl ester plasticizers in polymers. To be successful, it is necessary that the alkyl group in the
ester react with hydriodic acid to form a volatile iodide. For example, Santiciaer B 1 6 (butyl phthalyl butyl glycolate) and combinations of ethyl phthalyl ethyl glycolate and dibutyl sebacate can be determined in various plastic formulations. Figure 5 illustrates how two different plasticizers are determined in the presence of each other. The ethyl groups in the glycolate react with hydriodic acid to give the ethyl iodide peak shown at the lower right. Likewise, hydriodic acid reacta with butyl groups in the sebacate to produce
D
(1) Gohlke, B S., ANAL.CHQY.29, 1723 (1967). (2) Haelam, J., Hamilton, J. B., Jeffs, A R A d @ 83,6&71(1958).
(3) d e l l , E. A., Struts, H. C., ANAL
CHleM. 31,1890 (1969). (4) Samsel, E. P., McHard, J. A., IND. ENO. CHEW, ANAL. ED. 14, 760-4 (1942).
( 6 ) Stramburger, John, Brauer, G. M., Forziati, A. F., ANAL. C ~ M32, . 454 (1960).
RE~CEIVED for review September 30, 1960. Accepted March 2,1961. Regional Meeting, ACS, Detroit, Mich., February %, 1960.
Investigation of the Linearity of a Stream Splitter for Capillary Gas Chromatography L. S. ETTRE a
d WARREN AVERILL
The Perkin-Elmer Corp., Nomalk, Conn.
b When using capillary columns in gas chromatographs, an indirect sample procedure is used by splitting the injected sample volume into two parts, and introducing the smaller part into the column. It is absolutely necessary that this splitting device should not be discriminatory for any sample component-i.e., it should be linear. The criteria of the linearity of the splitting device are discussed. A system is described, and its linearity is demonstrated over a wide range.
680
0
ANALYTICAL CHEMISTRY
S
INCE the
introduction of the capillary column by Golay (4, 6) the problem of the sampling technique hss been discussed many times. It is well known that the sample capacity of the Golay columns is far below that of packed cDlumns, which makes the usual injection technique difficult. A few months ago, a special microsyringe with 1-pl, capacity was introduced by the Hamilton Co., Inc., Whittier, Calif.; however, the possible miume of the sample to be injected is, even in this
case, larger than that permissible with Golay columns. To overcome this difficulty, an indirect sampling procedure was reported by several authors (1, 9, 7, 8). A relatively large sample is injected into the carrier gss flow and the flow is then split into two parts, one of which is much smaller (1% or leas) than the other. In this way, the sample will also be split in the ratio given by the two flow rates and the sample volume entering the Golay column will be only a
