Anal. Chem. 1984, 56,1773-1777 (18)
(17) (18) (19) (20) (21) (22) (23)
Tijssen, R.; Bleumer, J.
P. A,; Smlt, A. L. C.; Van Kreveld, M. E. J . Chromatogr. 1981, 278, 137-185. Takeuchi, T.; Ishll, D. J . Chrometogr. 1982, 240, 51-80. Kucera, P.; Gulochon, G. J . Chromatogr. 1984, 283, 1-20. Snyder, L. R.; Dolan, J. W.; Van Der Wall, S. J . Chromatogr. 1981, 203,3-17. Verzele, M.; Dewaele, C. HRC CC, J . H7gh Resdut. Chromtogr. Chromatogr. Commun. 1982, 5 , 245-249. Bristow, P. A,; Knox, J. H. Chromatographia 1977, IO, 279-289. Cooke, N.; Olsen, K. J . Chromatogr. Sci. 1980, 18, 512-524. Knox, J. H. J . Chromatogr. S d . 1980, 78, 453-481.
1773
(24) Guiochon, G. Anal. Chem. 1981, 53,1318-1325. (25) Gulochon, 0. J . Chromatogr. 1979, 785,3-26. (28) Menet, H.; Gareii, P.; Caude, M.; Rosset, R. Chromatographla 1984, 18, 73-80.
RECEIVED for review February 14,1984. Accepted May 1,1984. The authors acknowledge financial support from Compagnie Franqaise de Raffinage (Paris et Le Havre, France).
Solvent Selectivity in the Liquid Chromatographic Separation of Polystyrene Oligomers on Silica T.H.Mourey* and G. A. Smith Research Laboratories, Eastman Kodak Company, Rochester, New York 14650
L. R. Snyder 2281 William Court, Yorktown Heights, New York 10598
Narrow-molecular-welght polystyrene standards wlth welght-average molecular welghts of 800, 2100, and 4800 were separated on 6 and 50 nm pore diameter slllca wlth n-hexaneltetrahydrofuran, n-hexanehthyl acetate, and nhexane/dlchloromethane gradients. Tetrahydrofuran and ethyl acetate eluents gave separatlons accordlng to the number of ollgomer unlfs, and dlchloromethane eluents further separated the stereoisomers of Individual ollgo-mers. Selectlvlty dlfferences In the oligomer separatlons are dlscussed In terms of solvent localization and preferred solute conlormatlon.
Oligomers are loosely defined by the polymer chemist as compounds consisting of a series of repeat units whose molecular weights total less than -10000. This definition is broad enough to describe the molecular weight region in which small-organic-molecule character disappears and measurable polymer physical properties become evident. Analysis of oligomer molecular weight, constitutional, and configurational distributions has been a long-standing challenge to separation scientists, primarily because of limited molecular weight ranges of traditional separation techniques. Gas chromatography is restricted to volatile oligomers with few repeat units, and the limited peak capacity and resolution in size-exclusion chromatography (SEC) is usually insufficient to adequately separate oligomers on the basis of compositional, constitutional, or configurational differences. Numerous papers on the separation of oligomers have clearly demonstrated the difficulties encountered in this transition region. Low-molecular-weightpolystyrene has been fractionated by recycle SEC, and the isolated oligomers have been characterized by nuclear magnetic resonance (NMR) spectrometry (1-3). This technique is time-consuming and inefficient. Until recently, supercritical fluid chromatography suffered from long separation times as well as complicated, expensive equipment; however, separation of up to 42 styrene oligomers has been reported (4, 5). Several workers have investigated reversed-phase (6-10) and adsorption chromatography (9-12) on microparticulate media as time-saving alternatives. Rapid, efficient separations of several oligomers have been obtained from both modes of high-performance 0003-2700/84/0356-1773$01.50/0
liquid chromatography. Solvent selectivity in reversed-phase separation of oligostyrene stereoisomers has been explained in terms of oligomer solubility in the mobile phase and solvent-induced changes in the conformations of stereoisomers and long-chain hydrocarbon bonded phases (6);however, the subtleties of solvent selectivity in normal phase separations of oligomers cannot be unambiguously elucidated from the reversed-phase case. Our objective is to help identify the sources of solvent selectivity in the adsorption chromatography of oligostyrenes and thus optimize oligomer separations.
EXPERIMENTAL SECTION Narrow-molecular-weightpolystyrene oligomer samples with weight-average molecular weights of 800, 2100, and 4800 were obtained from Pressure Chemicals (Pittsburgh, PA). Samples were dissolved in 3:l n-hexane/B solvent, where solvent B was dichloromethane, tetrahydrofuran (THF), or ethyl acetate. Sample concentrations were 3.3-100 mg/mL. The samples (10 rL) were injected onto a 4.6 mm i.d. X 250 mm column packed with either LiChrosorb Si60 silica (E. Merck, 5-km particle diameter) or Hibar I1 LiChrospher Si500 (10-pm particle diameter). Nominal pore diameters are 6 and 50 nm, respectively, for Si60 and Si500. The LiChrosorb Si60 column was packed by the stirred-slurry method, and the Hibar I1 column was obtained commercially. Both columns were thermostated at 30.00 k 0.05 OC. Polystyrene oligomerswere gradient eluted by use of either two Waters Associates M6000A pumps with a 720 system controller or a Varian 5060 liquid chromatograph. Ultraviolet absorbance of the eluent was monitored at 265 nm with a PerkinElmer LC-55 variable-wavelength detector. Stereoisomerswere semipreparatively separated in two steps. Oligomers 1-6 of polystyrene-800were isolated individually by separations on a 10 mm i d . X 500 mm column of 15-25 pm LiChroprep Si60 (E. Merck). A dichloromethanesolvent gradient beginning at 92/8 (v/v) n-hexane/dichloromethane, programmed at a 0.3%/mL dichloromethane rate of increase, gave base line separation of the first six oligomers. Three injections of 500 mg in 1mL were made, and each oligomer from 1to 6 was collected, and the fractions of each of the three separations were combined and blown to dryness with nitrogen. Stereoisomers in each oligomer fraction were then separated isocratically on a Partisil M-9 (Whatman) 10 mm i.d. X 250 mm column by using n-hexane/ dichloromethane binary eluents at a flow rate of 4.2 mL/min. Oligomer injections of 20 mg/100 pL were repeated until -20 mg of each separable isomer was collected. 0 1984 Arnerlcan Chemical Soclety
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
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Figure 1. PS-2100, 1 mg injected in 10 pL, separated on 4.6 X 250 mm LiChrosorb Si60; linear gradient increasing at 0.2%/mL from 97/3 (v/v) n-hexanelTHF at 1.0 mL/min; detection at 265 nm, 0.05 absorbance units full scale (AUFS).
Figure 3. PS-2100, same conditions as in Figure 1 with linear 0.2% /mL gradient from 97/3 (v/v) n-hexane/dichloromethane; 0.05 AUFS.
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Flgure 4. PS-2100, 1 mg injected in 10 pL, separated on 4.6 X 250 mm LiChrospher Si500; same conditions as Figure 3, n-hexane/ethyi acetate gradient, 0.04 AUFS.
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Figure 5. PS-2100, 1 mg injected in 10 pL, separated on 4.6 X 250 RESULTS AND DISCUSSION mm LiChrospher Si500; same conditions as Figure 4, n-hexaneldiThe anionically polymerized polystyrene samples examined chloromethane gradient, 0.08 AUFS. have the general structure CH3(CH2)3(CH2CHPh),CH2CH2Ph. The n-butyl end group is a result of initiation with n-butylprofiles obtained from eluents containing THF and ethyl lithium. The polymerization is terminated by abstraction of acetate. There is an apparent loss in resolution of individual oligomers, and an additional splitting of oligomer peaks gives a proton upon addition of methanol, which results in a satuthe appearance of fine structure or oligomer packets, particrated terminal styrene unit (13). The above structure was ularly in oligomers 1-10. confirmed by electron-impact mass spectrometry. Oligomers longer than 8-10 units were insoluble in the Separations of the 2100 weight average molecular weight initial gradient compositions used to obtain Figures 1-3. The oligomer sample on 6 nm pore diameter silica are shown for insoluble fraction of the samples precipitated onto the column solvent gradients of n-hexane and THF (Figure l), n-hexane upon injection and produced a noticeable increase in column and ethyl acetate (Figure 2), and n-hexane and dichloroback pressure, which slowly dissipated during the gradient. methane (Figure 3). Each gradient increased at 0.2%/mL Precipitation of a fraction of the sample onto the column head in B solvent, beginning at 97% n-hexane for THF and diis unavoidable when eluents that contain THF and ethyl chloromethane gradients and at 99% n-hexane with the acetate are used. The volume fraction of these solvents restrongest polar eluent, ethyl acetate. Oligomer retention times quired for solubilization of the longest oligomers results in with the ethyl acetate gradient were shorter than those obsolvent strengths that are too strong for the separation of the tained with THF, but the distribution profiles obtained from low-molecular-weight portion of the sample. Separation solely both solvents are similar, and in both examples individual by selective resolubilization of progressively longer oligomers oligomer peaks are sharp and symmetrical. We have resolved would depend on oligomer solvation at a given solvent comas many as 65 oligomers in a 4800 weight average molecular position along the gradient and should be largely independent weight polystyrene using the T H F gradient, which is comof adsorbent type or surface area. Oligomer retention times parable to the resolution of 48 oligomers reported by Curtis with ethyl acetate (Figure 4) and dichloromethane (Figure 5 ) et al. for similar separations on 10-pm silica (10) and to the gradients on 50 nm pore diameter silica were less than those resolution obtained in supercritical-fluid methods ( 4 , 5 ) . The obtained from equivalent gradients on 6 nm pore diameter distributional profile obtained from the n-hexaneldichlorosilica (Figures 2 and 3). The reductions in retention times methane gradient (Figure 3) is noticeably different from
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
are proportional to the differences between the phase ratios of these adsorbents. A similar proportionality between retention and packing surface area was noted in reversed-phase separations of polystyrenes (7). Sensitivity of oligomer retention to a change in the adsorbent surface area is inconsistent with the sequential resolubilization model, and it may be assumed that resolubilization precedes elution and that retention is determined by the solvent composition required to desorb the solubilized oligomer from the adsorbent surface. One annoying consequence of sample precipitation onto the column head is the elution of as little as 90% of the sample with binary mobile phases of hexane with T H F or ethyl acetate. Complete sample recovery was obtained with any dichloromethane gradient, as well as with T H F and ethyl acetate gradients in which only oligomer-solubilizingconditions were chosen. Resolution of the first 20 oligomers must be sacrificed to obtain these solubilizing conditions. Reducing sample sizes and using shallow solvent gradients increased the percentage of recovered sample when ethyl acetate and T H F initial solvent conditions that precipitate oligomers were used. The rate of resolubilization on the adsorbent surface is predicted to be dependent upon sample size and accessibility of the mobile phase to oligomers that have precipitated in a layer of variable thickness on the column packing. Our findings imply that the kinetics rather than the selectivity of resolubilization may be important in the operational aspect of these separations. A longer time passes between the minimum solvent compositon for dissolution and the composition needed for oligomer displacement from the adsorbent when shallow gradients are used. Similarly, the amount of dichloromethane required for oligomer dissolution is substantially less than that needed for elution, which apparently provides adequate time for the dissolution before elution. Dissolution and elution occur at similar compositions when eluents containing T H F and ethyl acetate are used. The examples presented for identical adsorbents and volumetrically equivalent rates of increase in B solvent demonstrate fundamental differences among ethyl acetate, THF, and dichloromethane in terms of the adsorption behavior of polystyrene repeat units on silica. When 10% B solvent is leaving the gradient device (35 min after injection), oligomers of 4, 16, and 35 repeat units are being eluted by dichloromethane, THF, and ethyl acetate, respectively. This implies that these B solvents increase in strength from dichloromethane (weakest) to ethyl acetate (strongest). This order of solvent strengths differs from that predicted from literature values of dichloromethane (eo = 0.30), THF (to = 0.53), and ethyl acetate (eo = 0.48). The reversal of T H F and ethyl acetate strengths may be unique to nonpolar solutes such as polystyrenes, in view of the fact that the literature solvent strengths were calculated from the retention of polar compounds (14-16). The gradients used to obtain chromatograms 1-4 did not increase equally in solvent strength; they were volumetrically equivalent gradients containing B solvents differing in strengths. Solvent effects on selectivity and resolution are better compared by adjusting B solvent rates of increase to give equivalent solvent-strength gradients. The resolution of individual oligomers decreased as expected when the dichloromethane gradient was increased from 0.2 % /mL to 0.5%/mL (Figure 6 ) (18)and produced retention times of oligomers 14-20 similar to those obtained with the 0.2% /mL THF gradient (Figure 1). The splitting of peaks in the slower dichloromethane gradient, which is not apparent in the THF separation (Figure l),is still evident with the steeper dichloromethane gradient. This peak splitting arises from the separation of stereochemical oligomer isomers. It is not a result of end-group differences, since all oligomer chains have equivalent n-butyl and proton terminal groups. We were
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Figure 6. PS-2100 low-molecular-welght detail; 1 mg injected in 10 pL; linear gradient increasing at 0.5%/mL from 89/11 (v/v) n-hexane/dichloromethane;detection at 265 nm, 0.05 AUFS.
unable to separate stereoisomers in oligomers larger than hexamer with any combination of THF or ethyl acetate gradient rates, but we routinely saw these isomer separations in oligomers containing as many as 20 repeat units with various dichloromethane gradients. We interpret this behavior as a difference between the selectivity of dichloromethane and THF or ethyl acetate in the separation of stereochemical isomers. An analogous example of solvent selectivity in the adsorption chromatography of isomers has been presented for polycyclic aromatic hydrocarbons and halogenated aromatic isomers on alumina (19). The source of isomer selectivity arises from, among other things, weak localization of solute aromatic rings. Typically, a flexible molecule such as biphenyl provides greater localization of each aromatic ring on a surface adsorption site than a less flexible fused-ring system such as naphthalene. Biphenyl is adsorbed more strongly because of this weak localization effect. Optimum selectivity for the separation of isomers on alumina is greatest when weak ring localization is favored, typically with weak solvents, and can be perturbed or completely lost by adsorbent deactivation or with eluents containing small amounts of strong solvents (19, 20). A similar solvent effect on separation selectivity, via changes in the ability of polystyrene stereoisomers to weakly localize on silica, is feasible, considering two pieces of evidence on this adsorbent. Examples of weak aromatic-ring localization on silica have been reported (21), and large differences in the separation of polar isomers on silica have been quantitated in terms of the degree of solvent localization (22). The preceding observations (see further discussion of ref 23 on solvent-solute localization effects) then lead to the following picture of solvent-selectivitydifferences as observed in Figures 1-5. A given pair of stereoisomers will tend to localize weakly onto silica. Because conformational differences exist among the isomers, their degree of weak localization will also differ. The use of binary solvents that are nonlocalizing (e.g., hexane/dichloromethane) will not interfere with weak localization of these stereoisomers, because there is no competition between the solvent and sample molecules for adsorption onto strong sites where localization is favored (there is a general competition for adsorption onto the surface, but this will not affect differences in isomer retention as a result of localization). When localizing solvents such as THF or ethyl acetate are used in place of dichloromethane, however, these
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984 i
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F@re 8. Isocratic, semipreparative separations of (a) trimer with 93/7 (v/v) n-hexane/dichloromethane eluent, (b) tetramer, 92/8 (v/v) nhexane/dichloromethane, (c) pentamer, 9 119 (v/v) n -hexane/dichloromethane. Stereolsomers: s, syndiotactic; i, isotactic; h, heterotactic.
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Flgure 7. PS-800, 50 pg injected in 10 pL, separated on two 4.6 X 250 mm LiChrosorb Si60 columns in series: ilnear gradient increasing at 0.1 %/mL from 89/11 (v/v) n-hexane/dichioromethane at 1.0 mL/mln; detection at 265 nm, 0.05 AUFS.
localizing solvents compete directly with sample molecules for attachment to strong adsorption sites (silanol groups in the case of silica). This competition shifts the adsorption of sample molecules to other parts of the surface (no attachment to silanol groups with localization), with loss in the stereospecific retention that arises from weak localization of sample molecules. A similar effect has been noted in the adsorption separation of threo/erythro diastereomers (22). The effect of localizing solvents as noted above is also dependent on the concentration of the solvent and the resulting relative coverage of the adsorbent surface by the localizing solvent. The displacement of localized sample molecules by localizing solvent molecules is most pronounced when the localized solvent molecules have covered about 75% of the adsorbent surface. Likewise the effect of localizing solvents on separation (solvent selectivity) will change markedly with the concentration of the localizing solvent in the mobile phase, corresponding to solvent concentrations on the surface that are less than or greater than 75%. This appears to occur in the separations of Figures 1and 2, where isomers smaller than the hexamer are partially resolved into individual diastereomers, and larger oligomers are unseparated. This transition from separated to unseparated isomers occurs at about 4 vol % T H F and 2 vol % ethyl acetate, which is in rough agreement with 75% surface coverage by these solvents for 3 vol % solvent in the mobile phase (23, 24). This implies that selectivity for the separation of polystyrene isomers is greatest with solvents that permit weak localization of aromatic rings. We have found dichloromethane,along with other delocalizing B solvents such as chloroform and carbon tetrachloride, to be very suitable for these isomer separations, whereas localizing 2-butanone resembles THF and ethyl acetate and does not provide isomer information above 75% localization on the adsorbent surface, corresponding to 3% by volume in a binary hexane eluent. The fine structure of low-molecular-weightoligomers can be examined in detail by changing dichloromethane gradient
conditions. An example is shown in Figure 7 for the lowmolecular-weight fraction of PS-800 separated on two LiChrosorb Si60 columns coupled in series. The number of peaks per oligomer increases with increasing number of repeat units. Significant amounts of constitutional isomers other than head-to-tail repeat-unit arrangement and end-group differences are not predicted from a knowledge of the polymerization procedure and have not been observed in NMR characterization. Separation of the anionically polymerized trimer into meso and racemic isomers has been reported in both the recycle SEC method (1-3) and adsorption chromatography (9) and is the major source of fine structure. The total number of repeat-unit arrangements with two possible stereochemical configurations (meso and racemic) can be calculated from a binomial expansion under the constraint that the terminal repeat unit is achiral. In this case, the number of repeat units that can be arranged into meso or racemic configurations is reduced by one to n = N - 1,where N is the number of repeat units in the oligomer. The predicted number of observable stereochemical isomers becomes the binomial expansion coefficient n!
(n)
=j!
(n- j ) !
Setting the factor j = n/2 to either nearest whole integer prevents redundant counting of stereoisomers that become equivalent upon rotation about the chain axis. Values of (?) can be conveniently read from a Laplace triangle and are included in Figure 7 . The predicted number of dimer through pentamer isomers is resolved, but only 9 of 10 possible hexamer isomers and only 11of 20 possible heptamer isomers are clearly separated. It is unlikely that the correlation between calculated and separable isomers in oligomers longer than pentamer and certainly in oligomers longer than hexamer could be improved by further optimizing this separation. The differences in weak localization and thus retention between the large number of similar stereoisomers in long oligomers are small and produce broadening of the oligomer peak into a packet. It is this packet broadening that contributes to the appearance of poorer resolution of long oligomers compared to the high resolution of oligomers according to number of repeat units obtained with localizing, stereoisomer-insensitive solvents such as THF and ethyl acetate. Conditions used to obtain isocratic semipreparative separations of stereoisomers (Figure 8) are not optimal but do provide sufficient resolution and amounts of each isomer for identification by l3C and 'H NMR spectrometry. Assignments based on previous NMR studies of oligomers ( 8 , 2 4 )helped in the identification of each stereoisomer and established an elution order for this separation to be, by polymer convention, syndiotactic (s), isotactic (i), followed by heterotactic (h) isomers of equivalent repeat unit number. The amounts of
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Anal. Chem. 1984, 56, 1777-1781
syndiotactic and isotactic isomers (first two peaks in each oligomer packet of Figure 7) decrease steadily with increasing oligomer length, until only heterotactic isomers are observed in oligomers longer than decamer.
CONCLUSIONS The marked differences between localizing and nonlocalking B solvents can be applied to specific polymer problems. Molecular weight distributions can be obtained by using THF (as demonstrated in ref 10)or ethyl acetate gradients without complications arising from tadicity or end-group differences. Quantitation of molecular weight distributions must be approached cautiously, however, because large amounts of sample and steep solvent gradients may lead to incomplete sample elution. It is presumed that in both localizing-solvent examples, polystyrene oligomers do not have a preferred adsorption configuration because of the inability of aromatic rings to weakly localize. Preferred adsorption configuration with nonlocalizing solvents such as dichloromethane produces isomeric fine structure and effectively separates oligomers according to molecular weight, stereochemistry, and chemical compositional differences that may typically arise from endgroup differences or comonomers. This adsorption mode is best applied to oligomer problems that require more information than a molecular weight distribution. ACKNOWLEDGMENT We thank J. Uebel for his assistance in the characterization of stereoisomers by NMR and E. Otocka and D. Wonnacott for their helpful comments in the preparation of this manuscript. Registry No. Polystyrene (homopolymer),9003-53-6; n-hexane, 110-54-3; THF, 109-99-9; ethyl acetate, 141-78-6; dichloromethane, 75-09-2.
LITERATURE CITED (1) Sato, H.; Saito, K.; Miyashtta, K.; Tanaka, Y . Makromol. Chem. 1981, 782, 2259. (2) Fujishige, S.;Ando, I. Makromo/. Chem. 1978, 777, 2195. (3) Fujishige, S.; Ohguri, N. Makromol. Chem. 1975, 178, 233. (4) Kiesper, E.; Hartmann, W. J . Polym. Sci., Po/ym. Len. Ed. 1977, 75, 707. (5) Klesper, E.; Hartmann, W. J . Polym. Sci., Polym. Left. Ed. 1977, 75, 9. (6)Lewis, J. J.; Rogers, L. B.; Pauls. R. E. J. Chromatcgr. 1983, 284, 339. (7) Larmann, J. P.; DeStefano, J. J.; Goidberg, A. P.; Stout, R. W.; Snyder, L. R.; Stadalius, M. A. J. Chromatogr. 1983, 255, 183. (8) Lattimer, R. P.; Harmon, D. J.; Welch, K. R. Anal. Chem. 1979, 57, 1293. (9) Eigert, K. F.; Henschei, R.; Schorn, H.; Kosfeld, R. Polym. Bull. 1881, 4, 105. (10) Curtis, M. A.; Webb, J. W.; Warren, D. C.; Brandt, V. 0.; Gerberich, F. G.; Raut, K. B.; Rogers, L. 8. Sep. Sci. 1980, 75, 1413. (11) Sackett. P. H.; Hannah, R. W.; Slavin, W. Chromatographie 1978, 7 1 , 834. (12) Tennikov, M. 6.; Nefyedov, P. P. Polym. Sci. USSR (Engl. Trans/.) 1978, 22, 513. (13) Rosen, L., Pressure Chemicals, private communication, March 1982. (14) Snyder, L. R.; Giajch, J. L.; Kirkland, J. J. J. Chromatcgr. 1981, 278,
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9QQ
(15) Hara, S.;Fujii, M.; Hirasawa, M.; Miyamoto, S. J. Chromatogr. 1979, 749, 143. (16) Hara, S.; Ohsawa, A.; Dobashi, A. J . Liq. Chromatogr. 1981, 4 . 409. (17) Snyder, L. R.; Glajch, J. L. J . Chromatogr. 1982, 248, 165. (18) Snyder, L. R.; Kirkland, J. J. "Introduction to Modern Liquid Chromatography"; Wiley: New York, 1979; Chapter 16. (19) Snyder, L. R. J . Chromatogr. 1985, 20, 463. (20) Snyder, L. R. "Principles of Adsorption Chromatography"; Marcel Dekker: New York, 1968. (21) Snyder, L. R. J. Chromatogr. 1983, 7 7 , 195. (22) Snyder, L. R. J . Chromatogr. 1982, 245, 165. (23) Snyder, L. R. High-Perform. Llq. Chrom8togr. 1983, 3 , 157. (24) Snyder, L. R.; Giajch, J. L.; Kirkland, J. J. J . Chromatogr. 1981, 278, 299. (25) Jasse, B.; Laupretre, F.; Monnerie, L. Makromol. Chem. 1977, 178, 1987.
RECEIVED for review April 21,1983. Resubmitted February 6,1984. Accepted April 19,1984.
Liquid Chromatographic Retention of Polystyrene Oligomers Thomas H. Mourey
Research Laboratories, Eastman Kodak Company, Rochester, New York 14650
The adsorption chromatography of anlonlcally and catlonlcally prepared polystyrene ollgomers was lnvestlgated on 8-nm porediameter, 5-pm slllca wlth n-hexane/dlchloromethane eluents. Endgroup dlfferences between the two samples produced slgntflcant differences In the retentlon of oligomers that are equlvalent In length. Measured adsorption energles and occupatlonal areas of ollgomers 2-14 lndlcate tilted repeat-unlt/adsorbent contact, whlch Is a result of chaln stlffness. Methylene backborn, carbon atoms In ollgomers longer than pentamer make only mlnor contrlbutlons to the adsorp tlon process, as demonstrated by greater adsorptlon energy per unlt of occupled adsorbent surface area for these ollgomers than Is calculated for planar solute Conformation on the slllca surface.
The solvent-displacement model, originally developed for the adsorption chromatography of small organic molecules (1-3), adequately explains the effects of solvents on the se0003-2700/84/0356-1777$01.50/0
lectivity of oligostyrene stereoisomer separations (4) without considering in detail the effects of oligomer length and structural complexity on the adsorption. This paper defines the fundamentals of a potentially practical approach to the separation and analysis of homologues such as polystyrene oligomers and testa the adsorption theory for small-molecule separations when applied to large molecules. It focuses, in particular, on the adsorbed solute conformation and its influence on chromatographic retention. A general expression for the chromatographic distribution coefficient K of a polystyrene oligomer can be written as log K = r
log
v, + 4CQ"r +
r &"e,
+ Q",, - d C a r + a,, + a,JI
(1) where Q".,Q",,, and Q",,are the standard adsorption potentials for adsorbed repeat units r and end groups e, and e2, respectively, 'ab is the mobile-phase solvent strength, and a,, a,,, and aszare the molecular areas of adsorption for adsorbed repeat units rand the oligomer end groups el and e2. Standard 0 1984 American Chemical Society
