Relations on thin layer chromatography of oligomers

Relations of thin layer chromatography of oligomers

1345

6. N. G. MCGRUM, B. E. READ a n d G. WIT.IJT~,MS, Unelastio a n d dielectric effects in Polymeric Solids, London-NewYork-Sydney, 1967 7. I. I. PEREPECHKO, Akusticheskiye metody issledovanlya polimerov (Acoustic Methods of Investigating Polymers), Izd. " K h i m i y a " , 1973 8. A. A. BERLIN, N. G. MATVEYEVA and E. S. PANKOVA, Vysokomol. soyed. A9: 1325, 1967 (Translated in Polymer Sol. U.S.S.R. 9: 6, 1485, 1967) 9. A. A. BERLIN, O. G. SEL'SKAYA, E. S. PANKOVA, E. S. MAMEDOVA and N. G. MATVEYEVA, Vysokomol. soyed. A10: 2642, 1968 (Translated in Polymer Sci. U.S.S.R. 10: 12, 3066, 1968) 10. Yu. S. LIPATOV, Yu. Yu. KERCHA and L. M. SERGEYEVA, Struktura i svoistva po]iuretanov (Structure and Properties of Polyurethanes). l~aukova dumka, 1970

METHODS OF INVESTIGATION RELATIONS OF THIN LAYER CHROMATOGRAPHY OF 0LIGOMERS* E. S.

GANKINA,

i~. D. VAL'CHIKINAand B. G. BELEN'KII

I n s t i t u t e of High Molecular Weight Compounds, U.S.S.R. Academy of Sciences

(Received 17 July 1975) A STUDYwas made of the mechanism of thin layer chromatography of oligomers in relation to the variable absorption activity of end groups and central units characterized by adsorption energy. T h i n layer chromatography (TLC) of oligomers is of considerable interest beth from the point of view of analysing the most important class of polymers and of special features of adsorption chromatography of polyfunctional compounds with different chemical structures of central and end units of the high molecular weight chain. Description are given in the literature of TLC of m a n y classes of oligomers: polyols [1-11], polyesters [12-16], polyolefins [17] and polyamides [18]. I n most studies a classification is made of oligomers of different numbers a n d structures of end groups. I t was shown in m a n y studies [6, 7, 10, 11, 14] t h a t the chromatographic behaviour of cligomers is independent of MW, whilst differences in RI are determined only by the n u m b e r of functional groups present in oligomers. This enables us to organize an extremely important kind of analysis of oligomers -- the analysis of functionality [19], which is responsible for the quality of high polymers--polyurethanes, etc. prepared from oligomers. I t was shown at the same time for substituted polyoxyethylones [9] t h a t these compounds can be classified according to MW, separating individual polymer homologues of up to twelve-fifteen. I t was shown [9] t h a t the efficiency of separating polyols according to molecular weight depends on the type of substituent, which blocks the hydroxyl in the end units. The more hydrophobie the substituent (i.e. the larger the hydrocarbon radical of the substituent), the more efficiently the oligomer is separated into individual polymer homologues. I t is thus interesting to examine the mechanism of TLC of oligomers, in order to determine conditions of classifying oligomers according to MW and functionality a n d examine * Vysokomol. soyed. A18: No. 5, 1170-1172, 1976.

1346

E . S . G~mr_t~rA et al.

these relations for different classes of oligomers. Special features of oligomer adsorption are due to the presence of functional end groups. These end units normally surpass the central oligomer units as regards adsorption a c t i v i t y and the variation of free energy of the oligomer in adsorption is mainly due to a change in the free energy of end groups subject to adsorption. I t is understandable t h a t the greater the differences in a dsorption a c t i v i t y of central and end units, the less the effect of central units on the variation of free energy of adsorption and therefore, the weaker the MW dependence of/~! of the oligomer in adsorption TLC.

Oo 0

3 ¸

7

fiUJ ~{)J

:FIG. 1. TLC of polystyrene with f l i r t = 308*, 410", 600, 900 and 2000 in a cyclohexane-benzene system ( 1 4 : 3 ) ( a ) a n d poly-~-methylstyrene fractions (b): 1 - - t e t r a m e r ; 2 - - h e x a m e r ; 3 - - o c t a m e r ; 4 - - d e c a m e r in a carbon tetrachloride-heptane system (2 : 1) on K S K silica gel (Styrerm trimer and t e t r a m e r were obtained in a reeirculatory gel-chromatograph with Woters columns 500, 250 and 100 A from I)S of M n = 6 0 0 ) .

Let us examine in more detail the relations of TLC of oligomers. I f the variation of ¢ordiguration entropy of the oligomer is ignored in adsorption, a change in free energy" A F in oligomer adsorption only involve energy changes during the adsorption of end groups 8e a n d central units 8*.

IzFI =,%1 ÷ (-'¥-'.,)18¢1, where N is the number of monomer units in the oligomer; n - - t h e number of end groups. The. concept of critical adsorption energy e° [20] m a y be extended to oligomers (when entropy losses of the polymer unit Lrt adsorption are equal in accuracy to energy gains). In this case the critical energy 80 should be attributed to central units of the oligomer. Then, according to the ratio of e0 and ec three cases of MW dependences of AF are possible (dependence of A F on the number of oligomer units _N)

1~1>I8°I,

lzFI=~I~oI+(N-~)I~¢I

(1)

i,~o1< i,~ol,

[AFl=nl~el--(N--n)le~l

(2)

18o1=1~°1, Iz~'[=~l~ol

(3)

I n the first case there is a positive MW dependence of A F (and therefore, negative MW *e includes the enthalpy and entropy variation of the polymer unit during adsorption.

Relations of thin layer chromatography of oligomers

1347

dependence of RI of the oligomer), in the second c a s c - - a negative dependence of Ati' and positive dependence of Rf on MW of the oligomer molecule and in the third case- - t h er e is no MW dependence of A F a n d / ? s . TLC can here be used to determine functionality n of the oligomer. I t should he noted t h a t in the absence of functional groups ( n = 0) only relation (1) should hold good since with relation (2) all oligomers will move with the solvent front. Figure 1 shows chromatographic curves of similar non-functional oligomers of polystyrene and poly-e-methylstyrene, where MW relations (1) are clearly observed. I t is quite understandable t h a t for non-functional oligomers MW relations of R I ,sill be most marked. W h e n the oligomer contains functions groups, a sthe differences between ee and ec increase, the MW dependence of /tf will weaken. I t is interesting to show experimentally all the three types of MW dependence of RI examined for oligomers containing functional groups, e. g. polyols.

D

C FIG. 2

]¢'IO. 3

~'m. 2. TLC of polyoxyethylene of Mn = 300,400 and 600 on K S K silica gel in a pyridine--water system (0-1 • 10) (a), on alumina in a chloroform-ethanol system (10 ". 1) (b), on K S K silica gel in a chloroform-pyridh~e system (5 : 7) (c). FIo. 3. TLC of 3,5-dinitrobenzoates of polydimethylsiloxane diols with n = 0 . 5 , 9, 20 or~ K S K silica gel with two fold addition to the layer of 0"075~o fluorescein in a benzene-ethylacetate system (10 • 0.1) (luminescent photography). Figure 2 shows TLC of polyoxyethylene of M n = 300, 400 and 600 in chromatographic systems, where MW dependences of R~ are observed (1)-(3). The agreement of polymer homologues in polyoxyethylenes of different MW is significant. The moon shape of chromatographic spots is due to the specific effect of concentration phenomena on adsorption properties of polyols in TLC. I t is katox~nt t h a t on a convex isothermal curve of adsorption the value of R I increases with an increase in concentration. Consequently, Rf of lateral parts of the chromatographic spot, where the concentration of m a t t e r decreases, will be less t h a n RI of the central part of the polymer zone. As a result, the chromatographic spot acquires a specific shape with a d e a r l y defined "nose", of which the Rf value in relation to _R~ of the sides of the spot m a y be used for quantitative analysis (as shown previously [111

1348

E . S . CxA~rr~A eta/.

a n d there is a linear relation between ARs and the a m o u n t of material in the oligomer spot). We note that b y substituting hydroxyls of end groups of polyoxyethylene b y groups of lower adsorption activity, it is easier to obtain a positive MW dependence of AF of adsorption (and therefore, negative MW dependence of Rs). I f we take oligomers with central units weak in adsorption activity, e. g. polydimethylsiloxane diols, without blocking end groups these oligomers cannot be effectively identified according to MW (Fig. 3). I t is quite understandable that by blocking end hydroxyls (e. g. b y the residue of dinitrobenzoic acid), it is easy to establish enly a MW dependence (2), when according to increasing the n u m b e r of weakly adsorbed siloxane units in the oligomer, the value of R! increases. Results indicate that special features of TLC of oligomers are due to the relation between adsorption activity of end and central units. Changing these ratios b y appropriate selection of separating systems types of TLC m a y be brought into practice in the absence of a MW dependence of RI with separation of oligomers according to functionality and with positive a n d negative MW dependence of R I. The latter two versions of TLC m a y be used to determine MWD of oligomers. The authors are grateful to L. I. Makarov a n d A. F. Podol'skii for providing polydimethylsiloxano diol a n d poly-a-methylstyreno samples a n d P. P. Nefedov for preparing styrene trimers and tetramers. Translated by E. S ~ E ~ REFERENCES 1. K. BURGER, Z. Analyt. Chem. 196: 259, 1963 2. I. A. VAKHTINA, P. A. OKUNEV and O. G. TARAKANOV, Zh. anallY, khimii 21: 630, 1966 3. K. BURGER, Z. Ansly~. Chem. 224: 421, 1967 4. L. FAVRETTO, G. PERTOLDI MARIETTA and L. FAVRETTO GABRIELLI, J. Chromatogr. 46: 255, 1970 5. T. SALVAGE, Analyst 95: 363, 1970 b. I. A. VAKHTINA, R. I. KHRENOVA and O. G. TARAKANOV, Zh. analit, khimii 28: 1625, 1973 7. I. A. VAKHTINA, O. G. TARAKANOV and R. I. KHRENOVA, Vysokomol. soyed. A16 2598, 1974 (Translated in Polymer Sci. U.S.S.R. 16: 11, 3022, 1974) 8. M. S. J. DALLAS and M. F. STEWART, Analyst 92: 634, 1967 9. L. FAVRETTO, L. FAVRETTO GABRIELLI and G. PERTOLDI MARIETTA, J. Chromatogr. 66: 167, 1972 10. W. A. MANENSKH, J. Appl. Polymer Sel. 14: 1189, 1970 11. B. G. BELEN'KII, I. A. VAKHTINA and O. G. TARAKANOV, Vysokomol. soyed. 17: 1116, 1975 (Not translated in Polymer Sci. U.S.S.R.) 12. K. KONINSHI and S. YAMAGUCHI, Analyt. Chem. 38: 1755, 1966 13. S. HAYANO, N. NIHONGI and T. ASAHARA, Tenside 5: 80, 1968 14. K. KONDO, M. MIYAZAKI, M. HORI and M. HATTORI, J a p a n Analyst. 16: 419, 1967 15. Th. PYE and U. WUNTUE, Plaste u n d Kautsehuk 15: 274, 1961 16. V. A. DORMAN-SMITH, J. Chromatography 29: 265, 1967 17. M. T. BRYK, A. S. SHEVLYAKOV and O. B. KREZUB, Vysokomol. soyed. B1O: 893, 1968 (Not translated in Polymer Sci. U.S.S.R.) 18. Y. KOBAYASHI, J. Chromatogr. 24: 447, 1966 19. S. G. ENTELIS, V. V. YEVREINOV, A. P. KUZAYEV. Book: Uspekhi khimli i fiziki polimerov (Progress in the Chemistry and Physics of Polymers). Izd. " K h i m i y a " , 1973 20. E. A. DIMARZIO and F. L. MCCRAKIN, J. Chem. Phys. 43: 539, 1965