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Relay mechanism of the formation of polypropylene disulphide by anionic polymerization of propylenesulphide and the structure of the polymers formed
202
A . D . ALIY~.v et aL REFERENCES
1. K. A. ANDRIANOV, T. V. VASIL'YEVA, N. M. KATASHUK, T. V. SNIGIREVA and B. I. D'YACHENKO, Vysokomol. soyed. A18: 1270~ 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1976) 2. K. A. ANDRIANOV, T. V. VASIL'YEVA, T. A. PRYAKHINA, N. M. PETROVNINA and B. I. D'YACHENKO, Izv. AN SSSR, ser. khim., 402, 1975 3. Fraktsionirovaniye polimerov (Fractionation of Polymers), edited by Kantov, Mir, 1971
RELAY MECHANISM OF THE FORMATION OF POLYPROPYLENE DISULPHIDE BY ANIONIC POLYMERIZATION OF PROPYLENESULPHIDE AND THE STRUCTURE OF THE POLYMERS FORMED* A. D. ALIY~V, I. P. SOLOMATINA,A. YU. KOSH~VN~,
ZH. ZHUMABAYEVand B. A. KRENTS]~L' A. V. Topchiyev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences
(Received 15 December 1975) Optimum conditions were found and main factors determined for promoting the formation of polypropylene disulphide (PPDS) by anionic polymerization of propylenesulphide (PS) on organo-lithium catalysts. I t was established that PPDS is formed by a relay mechanism by strict alternation of desulphuration of PS by a growing alkylthiolate anion and nucleophilic opening of the episulphide ring of propylene sulphide by the alkyldisulphide anion formed. The method proposed results in the formation of structurally homogeneous and structurally regular PPDS.
TH~ formation of structurally homogeneous and regular disulphide polymers is not only of theoretical but also some practical interest [I]. Oxidation of dithiols, condensation of dihalides with alkali metal disulphides and polymerization of cyclic disulphides [2] are well known methods of obtaining disulphide polymers. However, these methods do not enable structurally regular ("head to tail") and in many cases structurally homogeneous polymers to be obtained because of the varying numbers of sulphur atoms in monomer units [1]. We previously found a new method of obtaining disulphide polymers based on separation of olefm by * Vysokomol. soyed. A19: No. 1, 173-180, 1977.
Formation of PPDS by anionic polymerization of PS
203
anionic, p o l y m e r i z a t i o n of p r o p y l e n e s u l p h i d e (PS) b y t h e s y s t e m [3-5] 2n CH~--CH--CH3./ "~S'
catalyst ~. ,__ [ - - C H ~ - - C H - - S - - S - - ] + n CH~=CtI--CHa k
CH3
I t was a s s u m e d [3-6] t h a t ~he f o r m a t i o n of p o l y p r o p 3 lene disulphide P P D S , a p a r t f r o m d e s u l p h u r a t i o n of PS, also p r o m o t e s t h e d e c o m p o s i t i o n of p o l y m e r s [7] a n d o x i d a t i o n of oligomer t h i o l a t e anions, w h i c h should p r e v e n t t h e f o r m a t i o n of s t r u c t u r a l l y r e g u l a r a n d s t r u c t u r a l l y h o m o g e n e o u s disulphide p o l y m e r s . I t a p p e a r e d [8] t h a t t h e selective f o r m a t i o n of - - S - - S - - b o n d s in p o l y m e r s is b a s i c a l l y due to d e s u l p h u r a t i o n of PS, while t h e effect of o t h e r r e a c t i o n s t h a t m a y t a k e place a f t e r t h e c o m p l e t e c o n v e r s i o n of r e a c t i v e PS, is negligible. I t is t h e r e f o r e possible t h a t s t r u c t u r a l l y h o m o g e n e o u s a n d regular P P D S m a y b e f o r m e d since a 100% c o n t e n t of - - S - - S - - b o n d s is f o u n d in a l m o s t all e x p e r i m e n t s [3-5], while nucleophilic opening of t h e episulphide ring of PS t a k e s place a t t h e C H 2 - - S - - bond. To e x p l a i n t h e s t r u c t u r e o f p o l y m e r s f o r m e d a n d the m e c h a n i s m of t h e selee-. r i v e f o r m a t i o n of P P D S on organic l i t h i u m c a t a l y s t s , we used t h e c a t a l y t i c s y s t e m o f n - C e H 1 3 S L i . x ( - - ) L i O R * ~ ( L i 0 R * - - / - l i t h i u m m e n t h o x i d e ) , since n-CdHsLi• ( - - ) L i O R * a t t h e initiation stage desulphurizes m o r e t h a n t h e e q u i m o l e c u l a r a m o u n t of P S [9], which m a y increase s u l p h u r c o n t e n t in p o l y m e r s a n d complieat~q s t r u c t u r a l identification. R,S-Propylenesulphide (R, S-PS) was obtained from propylene carbonate and potassium rhodanide [10], b.p. 74"4-74.6°; n~° 1.4750, chromatographic purity 99'98~o. PS was dried over CaI-I2 and recondcnsed into reaction ampoules immediately before use. S (--)-Propyleno sulphide with a specific rotation [a]~)°--1.25° (without solvent, optical purity 2.45O/o) was obtained in the form of unreacted monomer with asymmetrical polymerization of R,S-PS with a catalytic system of ZnEt~/L-leucine [11]. Chromatographic purity was 99.9o/0. n-CsH13SLi.xLiOR* was obtained in situ by the interaction of a calculated amount of menthol and n-CBHlaSLi'xLiOR* with a solution of n-CdHgLi in heptane at --30-0 °, this temperature was then brought to room temperature and beptane and gases removed by evacuation. Cd (S--CH2--CH-----CH~)2 was obtained from cadmimn acetate and allyhnercaptan [12]. Toluene and T H F were purified as described in ar~earlier paper [13] and stored under vacuum over Call 2. Polymerization was carried out in pre-heated and evacuated (10 -4 torr) ampoulcs under conditions which preclude moisture and air. After polymerization the contents of the ampoules were acidified with dilute HC1 to decompose the catalyst, washed with water, evacuated to remove solvent and the residue repeatedly treated with petroleum ether for complete removal of menthol. After decanting the petroleum ether tile residue was dried under vacuum to constant weight. To plot curves of dispersion of optical rotation (DOR}, polymers obtained by asymmetrical polymerization of R, S-PS were reprecipitated twice from benzene solution with methanol for the complete removal of /-meuthol and dried to constant weight in vacuum. The conversion of PS was determined by chromatographic analysis of the reaction mixture using the relative reduction of the cencentration of PS in toluene (LKhM-SM chromatograph, polyethylene glycol adipate).
2{}4
A.D. A•Ivsv et
al.
UV absorption spectra were obtained using a Hitachi EPS-2 spectrophotometer (in dioxane). Curves of DOR were recorded using Jasco ORD-UV-5 and SPU-M speetropolarimeters. PMI~ spectra were obtained in a Varian T-60 spectrometer (solvent CCI,, internal standard--hexamethyldisiloxane). I t was established [3, 14] t h a t lithium alkyl thiolates obtained by desulphuration of PS are the active centres of polymerization of episulphides by the action of lithium alkyls. The RSLi formed, independent of reaction conditions, results in polypropylene sulphide (PPS). However, the active centres formed during bulk polymerization of PS on the n-C4HgLi. LiOR* system and in apolar solvents [3, 5] basically result in PPDS, which underlines the predominant role o f LiOR* in this process. Under similar conditions neither LiOR* nor lithium aleoholates of primary and secondary alcohols cause polymerization of PS or desulphuration due to a marked association [5, 15]: However, lithium tert-alcoholares which cannot exist in the form of stable associates, are relatively active and basically produce PPDS. Active centres of the type of ~ SLi. LiO-tert~Bu take part in all stages of this process of polymerization of PS, since with complete utilization of lithium tert-alcoholate in initiation lithium thiolates were formed which, as is known, only result in PPS. The role of LiOR* is probably firstly to cause steric hindrance to the nucleophilic opening of the episulphide ring of PS in the formation of coordination particles of the type Li+ R--S-
"~,
-OR* °o," l,i +
This is confirmed by the strong induction effect (--) of LiOR* explaining the formation of PPDS with high optical activity ([~]D up tO --63"7 ° [3]). Furthermore, in contrast to RSLi, systems of RSLi.LiOR* and tert-C4HgOLi at temperatures lower than 0 ° have low activity not only during polymerization, but also during desulphuration of PS [3, 5], whicil is due to the formation of stable and inactive associates; secondly, the role of LiOR* is to reduce basicity and nueleopl~ilic properties of growing a]kylthiolate anions, which beconm less active in opening the episulphide ring, but can desulphurize PS by attacking the thiolate anion with a snlphur a t o m of PS of low basicity [6, 9]. The alkyl disulphide anions formed may open the episulphide ring as a result of reduction of steric hindrance and higher thiophilic properties [16] as a consequence of the ~-effect of the heteroa t o m [17]. This process of forming - - S - - S - - bonds by sequentiM reactions of desulphuration and nuc]eophilic opening of PS explains the regular increase previously observed in the amount of propylene separated during polymerization of PS [9]. This suggests t h a t disulphide bonds are formed not only at the early stages of polymerization of PS [15], but also at the stages of polymer chain extension up to complete conversion of PS. For a simple explanation of the mechanism and
Formation of PPDS by anionic polymerization of PS
205
kinetics of formation of PPDS, we examined the relations of polymerization of PS under different conditions (variation of catalyst concentration, type of reaction medium and LiOR* content in the n-CsH13SLi" xLiOR* system). Therefore, to study the structure of polymers formed, which m a y consist of monomer units with mono-, di- and polysulphide bonds, elementary analysis, PMg-spectroscopy and D O g were used, which provide rapid and reliable information about the sulphur con~ents both in polymers and in each monomer unit o f the polymer chain.
!
J
I
I
I
z/
3
2
I
!
I
# ~, p.p.m.
PMR spectra (in CC14) of PPS (1), PPDS (2) and 90.5% PPS+9"5~o PPDS (3); gctt,= 1-37 (1, 3) and 1.43 (2); I--HMDS. Thus, %he P M g spectroscopic method which is used successfully for the analysis of low molecular weight mono-, di- and polysulphide compound mixtures [16, 18] enabled a clear distinction to be made between P P S and P P D S samples a n d a qualitative interpretation of the information was obtained (Figure). A noticeable displacement is observed in PMI~ spectra of P P D S (in CCld) in %he chemical shift of the CH 3 proton doublet in the region of weak field $~= 1-43 p.p.m, in relation to CHa protons of P P S g~= 1.37 p.p.m. (A5=0.06 p.p.m.). An
206
A.D. ~IYEV et al.
empirical correlation between the ratio of 5~/~1 and that of mono- and disulphide compounds [16, 18] points to the accuracy of attributing ~ to disulphide polymers. CC14 is the most suitable solvent and not benzene or CHCI 8 (AS=0.02 and 0.03 p.p.m., respectively). Even more considerable shifts in the direction of weak field are observed for CH~ and CH protons, however, t h e y provide little information in view of the multiplet nature of singals and can only be used to detect small numbers of diand polysulphide bonds. To evaluate the sensitivity of this method when determining the proportion of mono- and disulphide units, PMR spectra were obtained of mechanical mixtures of mono- and disulphide polymers. I t appeared that chemical shifts of CH3 protons only become noticeable with PPS or PPDS contents of over 10-15% in the mixture. Using the method of DOR is based on the difference of rotation and form of DOR curves of PPS and PPDS, which conform to the monomial Drude equation [3, 5], the dispersion constant ).c of which characterizes the lange of arrangement of optically active absorption bands - - S - - of chromophores in PPS and - - S - - S chromophores in PPDS [5, 15]. For PPS and PPDS values of tc are within the range of 203-209 and 245-258 rim, respectively, which is in satisfactory agreement with UV spectroscopic data: for PPDS a clear band of the disulphide bond is observed at 255 n m with ~=-400-450, which is absent in PPS samples [5, 15]. Dispersion coefficients [~]480/[a]D which are within the range of 2.13-2.17 for PPS and 2.36-2.40 for PPDS also differ noticeably. With a combination of these structures in the polymer, values of Ic and [a]430/[a]D have intermediate values and with polysulphide bonds, higher values, since po]ysulphides show an even greater bathochrome shift of UV bands and therefore have higher rotation values [19]. The Table indicates t h a t in the initial stage of the process (in 5 hr) an increase in x (experiments I-IV) noticeably increases the conversion of PS and the intrinsic viscosity of the polymers obtained. This is not due to the pal% of LiOR* as active centres of polymerization of PS since PMR spectra of polymers (experiments I - I V ) in the strong field only contain proton signals of n-C6H13S groups and not R 0 groups of the catalytic system. The addition of LiOR* to the catalytic system apparently increases the probability of decomposition of less stable associates of (C6H13SLi)n and thus the proportion of active centres of the t y p e of RSLi.LiOR* in view of the strong tendency of LiOR* to associate. Although the accurate stoichiometry, the degree of association and the equilibrium of RSLi.xLiOR* complexes are unknown it m a y be assumed t h a t t h e y consist of associates of mixed structure (RSLi)n; (RSLi)n" (LiOR*)m and (LiOR*)m in dynamic equilibrium, the activity of which decreases during polymerization of PS considering the increased ability of lithium alcoholates to associate and the inertia of LiOR* itself in desulphuration and polymerization o f PS in bulk and in apolar solvents [3, 5, 15]. Nevertheless, assuming t h a t the mix-
Formation of PPDS by anionic polymerization of PS
207
ture of these complexes has an equilibrium composition and considering tha~ P P D S is basically formed with an equimolecular composition of C6H13SLi and LiOR* (x----l), it m a y be assumed that R S L i . L i O R * particles have a higher activity during the formation of P P D S than associates of RSLi resulting in PPS. This is confirmed b y tabulated results which show that formation kinetics of - - S - - S - - bonds exist in polymers both with the conversion of PS and with increase in x. Thus, a slight increase in sulphur content with the conversion of PS, observed when x z 0 (experiment 1 ) does not involve the formation of -- S-- S - bonds in polymers (propylene is not separated) b u t is caused b y an increase in molecular weight at which the proportion of terminal n-C6H13S groups decreases, which is in agreement with results of viscosity and P M R spectroscopy. However, when x > 0 propylene is separated and P P D S formed in every case (experiments l I - I V ) . The marked effect of monomer concentration on the formation of P P D S is here clearly apparent. Thus, with a molar ratio of PS/RSLi ----50 (experiments II and ]II) P P S is mainly formed at the beginning up to about 20% conversion of PS and only on reaching a given ratio of PS/RSLi (_~ 40) does selective formation of P P D S become possible. With a ratio of PS/RSLi-~35 (experiment VII), P P D S is formed with different degrees of conversion of PS. With an increase of this ratio up to 100 P P S is mainly formed and only with advanced conversion of PS is some separation of propylene observed, which is sho~vn b y a slight change in the type of PMI~ spectra of sample 2, experiment V ( - - S - - S - bonds are present in a proportion of minimum 5(~b). This behaviour may be explained by the basicity of PS which is relatively high and comparable with the basicity of some ethers with a fairly high solvaring power: a shift in the band of the OD-deuteromethano] in the Ii~ spectrum (in relation to the band of OD in CCI4) z~YODis 90, 99, 112 and 117 cm -1 for PS, propylene oxide, l-menthyl ethyl ester and T H F , respectively. Therefore, with an increase of the molar ratio of PS/RSLi the contact of the ionic pair decreases and the L i - - S - - bond is solvated, which is covalent in apolar solvents and determines the association of lithium thiolates [20]. Solvation of the L i - - S - - bond does not assist the preliminary coordination of R- or S-antipode of PS either at the stage of polymerization or desu]phuration of PS and thus explains the negligible optical activity of polymers obtained with low degrees of conversion of PS (experiment II, samples 1 and 2, experiments III, IV, sample 1). During polymerization of PS in T H F all organo-lithium compolmds, including lithium alcoholates only produce P P S (experiment VI) with low optical activity. LiOR* has an equally strong effect on the formation of PPDS. Thus, on increasing x to 3 (experiment IV) the rate of polymerization increases noticeably, conversion of PS increases and so does the viscosity of polymers, which contain - - S - - S in the main bond, in spite of the high ratio of P S / R S L i = 5 0 . These results suggest that with a~ increase of x not only the proportion of active eentres increases, producing PPDS, b u t the structure of active centres
~08
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Formation of PPDS by anionic polymerization of PS
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A.D. ALIY~.v et al.
changes since a more highly associated complex of the type Li..O--R* : : --CH--S... Li I
:
:
CHa Li..0--R* m a y form. Steric hindrance increases and the polarizability of the growing thiolate anion decreases (three lithium atoms which do not favour nucleophilie opening of the PS ring in view of the possible reduction of the polarizability of Li--S bond, but can promote desulphuration of PS). After desulphuration an alkyldisulphide anion is formed which is more active in polymerization of PS in view of the a~ffect of the hetero-atom [17] and a reduction of steric hindrance for the opening of PS at the CH~--S-- bond. The sequence of these reactions may account for the selective formation of PPDS. Thus, the formation of disulphide bonds in polymers is considerably influenced by solvation and lithium alcoholates, which promote association and thus clearly change the activity of growing thiolate anions in desulphuration of PS and opening of the episulphide ring. I$ follows from results that PPDS is formed by a "relay mechanism" as a consequence of a strict alternation of desulphuration of PS and nucleophilic opening of the episulphide ring of PS by the alkyldisulphide anion formed CHa
CH3
I
PS d e s u l p h u r a t i o n
,~CH--S...Li "
"
propylen¢
I
, ~ CH--S--S...Li *
:
Li.. ()--R*
PS
opening
~-.>
:
I~i..O--R*
CH3 CH, I I --, ~ C H S S C H ~ C t t S . . . • Li.
et¢
Li.. b-n* is due to steric factors and a Varying activity of sec. alkylthiolate- and sec. alkyldisulphide-anions. It should be noted that sulphur content found in every case in the polymers does not reach 60.3%--the calculated value for PPDS. This may be due to the effect of SH and C~H13S end groups, considering the low molecular weight of PPDS. This fact and the absence of polysulphide bonds from PPDS, according to elementary analysis, DOR, UV and PMR spectroscopy, may point to the formation of structurally homogeneous polymers. The relay mechanism of polymerization of PS does not exclude the possible formation of structurally regular PPDS. This is underlined by a slight change in the structure and chiroptical properties of PPDS (experiment VII), obtained with different conversions of S (--) PS on the achiral system of n-C~HlaSLi, xLiOR (LiOR was obtained from d and/-menthol). However, the possibility of two similar reactions should be considered which may cause structural irregularity in which
Formation of PPDS by anionic polymerization of PS
21I
P P D S : a) e x c h a n g e reactions o f t h e t y p e of t h i o l - d i s u l p h i d e a n d d i s u l p h i d e - d i sulphide [21], t h e effect o f w h i c h is insignificant in t h e presence of reactive P S in t h e system; b) nucleophilic s u b s t i t u t i o n [22] CH--SS-Li+ + Li+-S--CH -- --~ ~ CH--S--S--CH ~ -~ Li2S
I
CH3
i
CH3
I
CH3
I
CH~
H o w e v e r , this reaction which c o m p e t e s w i t h t h e r e a c t i o n of extension, is n o t v e r y likely since a significant r e d u c t i o n in t h e r a t e of p o l y m e r i z a t i o n a n d p o l y m e r molecular w e i g h t is f o u n d as a result o f t h e r u p t u r e of t w o g r o w i n g chains to f o r m Li2S inactive in p o l y m e r i z a t i o n of P S [8]. Translated by E. SEMERE
REFERENCES
1. R. M. FITCH and D. C. HELGESON, J. Polymer Sci. C22: 1101, 1968 2. E. M. FETTES, Organic Sulphur Compounds, Ed. N. Kharaseh, Oxford, 1961 3. A. D. ALIEV, B. A. KRENTSEL', G. M. MAMEDIAROV, I. P. SOLOMATINA and Ye. P. TIURINA, Kinetics and Mechanism of Polyreactions, Budapest, 1969 4. I. P. SOLOMATINA, Ye. P. TYURINA, A. D. AL1YEV and B. A. KRENTSEL', Sb. l~iziologicheski i opticheski aktivnye polimernye veshchestva (Physiologically and Optically Active Polymeric Substances). Riga, 1971 5. I. P. SOLOMATINA, Dissertation, 1971 6. A. D. ALIEV, I. P. SOLOMATINA and B. A. KRENTSEL, International Conference on Chemical Transformation of Polymers, Bratislava, 1971 7. I. P. SOLOMATINA, A. D. ALIYEV and B. A. KRENTSEL', Vysokomol. soyed. B13: 252, 1971 (Not traalslated in Polymer Sci. U.S.S.R.) 8. I. P. SOLOMATINA, Zh. ZHUMABIYEV, A. D. ALIYEV and B. A. KRENTSEL', Doklady I I I respublikanskoi nauchno-tekhnicheskoi konferentsii po neftekhimii, Guryev, 1974 9. A. D. ALIEV, I. P. SOLOMATINA and B. A. KRENTSEL, Macromolecules 6: 738, ]973 10. S. SEARLE, H. R. HAYS and E. F. LUTZ, J. Organ. Chem. 27: 2832, 1962 11. J. FURUKAWA, N. KAWABATA and A. KATO, J. Polymer Sei. B5: 1073, 1967 12. D. R. MORGAN and R. T. WRAGG, Makromolek. Chem. 125: 220, 1969 13. A. WEISBERGER, E. PROSCAUER, G. RIDDEY and E. TOOPS, Organicheskiye rastvoriteli (Organic Solvents). Izd. inostr, lit., 1958 14. R. CAMMERECK, C. J. FETTES and M. MORTON, Polymer Preprints 11: 72, 197{) 15. A. D. ALIEV, B. A. KRENTSEL, G. M. MAMEDIAROV, I. P. SOLOMATINA and E. P. TIURINA, Europ. Polymer J. 7: 1721, 1971 16. B. D. VINEYARD, J. Organ. Chem. 32: 3833, 1967 17. J. O. EDWARDS and R. G. PEARSON, J. Amer. Chem. Soc. 84: 16, 1962 18. D. GRANT and J. R. VAN WASER, J. Amer. Chem. Soc. 86: 3012, 1964 19. K. BALEHOVIC and B. GASPERT, Chem. Ind., 624, 1960 20. E. CAMPOS-LOPER, A. LEON-GROSS and M. A. PONCE-VELEZ, J. Polymer Sol., Polymer Chem. Ed. 11: 3021, 1973 21. Khimicheskiye reaktsii polimerov (Chemical Reactions of Polymers). Edited by E_ Fettes, Mir, 1967 22. W. A. PRYOR, Mechanisms of Sulphur Reactions, New York, 1962
A . D . ALIY~.v et aL REFERENCES
1. K. A. ANDRIANOV, T. V. VASIL'YEVA, N. M. KATASHUK, T. V. SNIGIREVA and B. I. D'YACHENKO, Vysokomol. soyed. A18: 1270~ 1976 (Translated in Polymer Sci. U.S.S.R. 18: 6, 1976) 2. K. A. ANDRIANOV, T. V. VASIL'YEVA, T. A. PRYAKHINA, N. M. PETROVNINA and B. I. D'YACHENKO, Izv. AN SSSR, ser. khim., 402, 1975 3. Fraktsionirovaniye polimerov (Fractionation of Polymers), edited by Kantov, Mir, 1971
RELAY MECHANISM OF THE FORMATION OF POLYPROPYLENE DISULPHIDE BY ANIONIC POLYMERIZATION OF PROPYLENESULPHIDE AND THE STRUCTURE OF THE POLYMERS FORMED* A. D. ALIY~V, I. P. SOLOMATINA,A. YU. KOSH~VN~,
ZH. ZHUMABAYEVand B. A. KRENTS]~L' A. V. Topchiyev Institute of Petrochemical Synthesis, U.S.S.R. Academy of Sciences
(Received 15 December 1975) Optimum conditions were found and main factors determined for promoting the formation of polypropylene disulphide (PPDS) by anionic polymerization of propylenesulphide (PS) on organo-lithium catalysts. I t was established that PPDS is formed by a relay mechanism by strict alternation of desulphuration of PS by a growing alkylthiolate anion and nucleophilic opening of the episulphide ring of propylene sulphide by the alkyldisulphide anion formed. The method proposed results in the formation of structurally homogeneous and structurally regular PPDS.
TH~ formation of structurally homogeneous and regular disulphide polymers is not only of theoretical but also some practical interest [I]. Oxidation of dithiols, condensation of dihalides with alkali metal disulphides and polymerization of cyclic disulphides [2] are well known methods of obtaining disulphide polymers. However, these methods do not enable structurally regular ("head to tail") and in many cases structurally homogeneous polymers to be obtained because of the varying numbers of sulphur atoms in monomer units [1]. We previously found a new method of obtaining disulphide polymers based on separation of olefm by * Vysokomol. soyed. A19: No. 1, 173-180, 1977.
Formation of PPDS by anionic polymerization of PS
203
anionic, p o l y m e r i z a t i o n of p r o p y l e n e s u l p h i d e (PS) b y t h e s y s t e m [3-5] 2n CH~--CH--CH3./ "~S'
catalyst ~. ,__ [ - - C H ~ - - C H - - S - - S - - ] + n CH~=CtI--CHa k
CH3
I t was a s s u m e d [3-6] t h a t ~he f o r m a t i o n of p o l y p r o p 3 lene disulphide P P D S , a p a r t f r o m d e s u l p h u r a t i o n of PS, also p r o m o t e s t h e d e c o m p o s i t i o n of p o l y m e r s [7] a n d o x i d a t i o n of oligomer t h i o l a t e anions, w h i c h should p r e v e n t t h e f o r m a t i o n of s t r u c t u r a l l y r e g u l a r a n d s t r u c t u r a l l y h o m o g e n e o u s disulphide p o l y m e r s . I t a p p e a r e d [8] t h a t t h e selective f o r m a t i o n of - - S - - S - - b o n d s in p o l y m e r s is b a s i c a l l y due to d e s u l p h u r a t i o n of PS, while t h e effect of o t h e r r e a c t i o n s t h a t m a y t a k e place a f t e r t h e c o m p l e t e c o n v e r s i o n of r e a c t i v e PS, is negligible. I t is t h e r e f o r e possible t h a t s t r u c t u r a l l y h o m o g e n e o u s a n d regular P P D S m a y b e f o r m e d since a 100% c o n t e n t of - - S - - S - - b o n d s is f o u n d in a l m o s t all e x p e r i m e n t s [3-5], while nucleophilic opening of t h e episulphide ring of PS t a k e s place a t t h e C H 2 - - S - - bond. To e x p l a i n t h e s t r u c t u r e o f p o l y m e r s f o r m e d a n d the m e c h a n i s m of t h e selee-. r i v e f o r m a t i o n of P P D S on organic l i t h i u m c a t a l y s t s , we used t h e c a t a l y t i c s y s t e m o f n - C e H 1 3 S L i . x ( - - ) L i O R * ~ ( L i 0 R * - - / - l i t h i u m m e n t h o x i d e ) , since n-CdHsLi• ( - - ) L i O R * a t t h e initiation stage desulphurizes m o r e t h a n t h e e q u i m o l e c u l a r a m o u n t of P S [9], which m a y increase s u l p h u r c o n t e n t in p o l y m e r s a n d complieat~q s t r u c t u r a l identification. R,S-Propylenesulphide (R, S-PS) was obtained from propylene carbonate and potassium rhodanide [10], b.p. 74"4-74.6°; n~° 1.4750, chromatographic purity 99'98~o. PS was dried over CaI-I2 and recondcnsed into reaction ampoules immediately before use. S (--)-Propyleno sulphide with a specific rotation [a]~)°--1.25° (without solvent, optical purity 2.45O/o) was obtained in the form of unreacted monomer with asymmetrical polymerization of R,S-PS with a catalytic system of ZnEt~/L-leucine [11]. Chromatographic purity was 99.9o/0. n-CsH13SLi.xLiOR* was obtained in situ by the interaction of a calculated amount of menthol and n-CBHlaSLi'xLiOR* with a solution of n-CdHgLi in heptane at --30-0 °, this temperature was then brought to room temperature and beptane and gases removed by evacuation. Cd (S--CH2--CH-----CH~)2 was obtained from cadmimn acetate and allyhnercaptan [12]. Toluene and T H F were purified as described in ar~earlier paper [13] and stored under vacuum over Call 2. Polymerization was carried out in pre-heated and evacuated (10 -4 torr) ampoulcs under conditions which preclude moisture and air. After polymerization the contents of the ampoules were acidified with dilute HC1 to decompose the catalyst, washed with water, evacuated to remove solvent and the residue repeatedly treated with petroleum ether for complete removal of menthol. After decanting the petroleum ether tile residue was dried under vacuum to constant weight. To plot curves of dispersion of optical rotation (DOR}, polymers obtained by asymmetrical polymerization of R, S-PS were reprecipitated twice from benzene solution with methanol for the complete removal of /-meuthol and dried to constant weight in vacuum. The conversion of PS was determined by chromatographic analysis of the reaction mixture using the relative reduction of the cencentration of PS in toluene (LKhM-SM chromatograph, polyethylene glycol adipate).
2{}4
A.D. A•Ivsv et
al.
UV absorption spectra were obtained using a Hitachi EPS-2 spectrophotometer (in dioxane). Curves of DOR were recorded using Jasco ORD-UV-5 and SPU-M speetropolarimeters. PMI~ spectra were obtained in a Varian T-60 spectrometer (solvent CCI,, internal standard--hexamethyldisiloxane). I t was established [3, 14] t h a t lithium alkyl thiolates obtained by desulphuration of PS are the active centres of polymerization of episulphides by the action of lithium alkyls. The RSLi formed, independent of reaction conditions, results in polypropylene sulphide (PPS). However, the active centres formed during bulk polymerization of PS on the n-C4HgLi. LiOR* system and in apolar solvents [3, 5] basically result in PPDS, which underlines the predominant role o f LiOR* in this process. Under similar conditions neither LiOR* nor lithium aleoholates of primary and secondary alcohols cause polymerization of PS or desulphuration due to a marked association [5, 15]: However, lithium tert-alcoholares which cannot exist in the form of stable associates, are relatively active and basically produce PPDS. Active centres of the type of ~ SLi. LiO-tert~Bu take part in all stages of this process of polymerization of PS, since with complete utilization of lithium tert-alcoholate in initiation lithium thiolates were formed which, as is known, only result in PPS. The role of LiOR* is probably firstly to cause steric hindrance to the nucleophilic opening of the episulphide ring of PS in the formation of coordination particles of the type Li+ R--S-
"~,
-OR* °o," l,i +
This is confirmed by the strong induction effect (--) of LiOR* explaining the formation of PPDS with high optical activity ([~]D up tO --63"7 ° [3]). Furthermore, in contrast to RSLi, systems of RSLi.LiOR* and tert-C4HgOLi at temperatures lower than 0 ° have low activity not only during polymerization, but also during desulphuration of PS [3, 5], whicil is due to the formation of stable and inactive associates; secondly, the role of LiOR* is to reduce basicity and nueleopl~ilic properties of growing a]kylthiolate anions, which beconm less active in opening the episulphide ring, but can desulphurize PS by attacking the thiolate anion with a snlphur a t o m of PS of low basicity [6, 9]. The alkyl disulphide anions formed may open the episulphide ring as a result of reduction of steric hindrance and higher thiophilic properties [16] as a consequence of the ~-effect of the heteroa t o m [17]. This process of forming - - S - - S - - bonds by sequentiM reactions of desulphuration and nuc]eophilic opening of PS explains the regular increase previously observed in the amount of propylene separated during polymerization of PS [9]. This suggests t h a t disulphide bonds are formed not only at the early stages of polymerization of PS [15], but also at the stages of polymer chain extension up to complete conversion of PS. For a simple explanation of the mechanism and
Formation of PPDS by anionic polymerization of PS
205
kinetics of formation of PPDS, we examined the relations of polymerization of PS under different conditions (variation of catalyst concentration, type of reaction medium and LiOR* content in the n-CsH13SLi" xLiOR* system). Therefore, to study the structure of polymers formed, which m a y consist of monomer units with mono-, di- and polysulphide bonds, elementary analysis, PMg-spectroscopy and D O g were used, which provide rapid and reliable information about the sulphur con~ents both in polymers and in each monomer unit o f the polymer chain.
!
J
I
I
I
z/
3
2
I
!
I
# ~, p.p.m.
PMR spectra (in CC14) of PPS (1), PPDS (2) and 90.5% PPS+9"5~o PPDS (3); gctt,= 1-37 (1, 3) and 1.43 (2); I--HMDS. Thus, %he P M g spectroscopic method which is used successfully for the analysis of low molecular weight mono-, di- and polysulphide compound mixtures [16, 18] enabled a clear distinction to be made between P P S and P P D S samples a n d a qualitative interpretation of the information was obtained (Figure). A noticeable displacement is observed in PMI~ spectra of P P D S (in CCld) in %he chemical shift of the CH 3 proton doublet in the region of weak field $~= 1-43 p.p.m, in relation to CHa protons of P P S g~= 1.37 p.p.m. (A5=0.06 p.p.m.). An
206
A.D. ~IYEV et al.
empirical correlation between the ratio of 5~/~1 and that of mono- and disulphide compounds [16, 18] points to the accuracy of attributing ~ to disulphide polymers. CC14 is the most suitable solvent and not benzene or CHCI 8 (AS=0.02 and 0.03 p.p.m., respectively). Even more considerable shifts in the direction of weak field are observed for CH~ and CH protons, however, t h e y provide little information in view of the multiplet nature of singals and can only be used to detect small numbers of diand polysulphide bonds. To evaluate the sensitivity of this method when determining the proportion of mono- and disulphide units, PMR spectra were obtained of mechanical mixtures of mono- and disulphide polymers. I t appeared that chemical shifts of CH3 protons only become noticeable with PPS or PPDS contents of over 10-15% in the mixture. Using the method of DOR is based on the difference of rotation and form of DOR curves of PPS and PPDS, which conform to the monomial Drude equation [3, 5], the dispersion constant ).c of which characterizes the lange of arrangement of optically active absorption bands - - S - - of chromophores in PPS and - - S - - S chromophores in PPDS [5, 15]. For PPS and PPDS values of tc are within the range of 203-209 and 245-258 rim, respectively, which is in satisfactory agreement with UV spectroscopic data: for PPDS a clear band of the disulphide bond is observed at 255 n m with ~=-400-450, which is absent in PPS samples [5, 15]. Dispersion coefficients [~]480/[a]D which are within the range of 2.13-2.17 for PPS and 2.36-2.40 for PPDS also differ noticeably. With a combination of these structures in the polymer, values of Ic and [a]430/[a]D have intermediate values and with polysulphide bonds, higher values, since po]ysulphides show an even greater bathochrome shift of UV bands and therefore have higher rotation values [19]. The Table indicates t h a t in the initial stage of the process (in 5 hr) an increase in x (experiments I-IV) noticeably increases the conversion of PS and the intrinsic viscosity of the polymers obtained. This is not due to the pal% of LiOR* as active centres of polymerization of PS since PMR spectra of polymers (experiments I - I V ) in the strong field only contain proton signals of n-C6H13S groups and not R 0 groups of the catalytic system. The addition of LiOR* to the catalytic system apparently increases the probability of decomposition of less stable associates of (C6H13SLi)n and thus the proportion of active centres of the t y p e of RSLi.LiOR* in view of the strong tendency of LiOR* to associate. Although the accurate stoichiometry, the degree of association and the equilibrium of RSLi.xLiOR* complexes are unknown it m a y be assumed t h a t t h e y consist of associates of mixed structure (RSLi)n; (RSLi)n" (LiOR*)m and (LiOR*)m in dynamic equilibrium, the activity of which decreases during polymerization of PS considering the increased ability of lithium alcoholates to associate and the inertia of LiOR* itself in desulphuration and polymerization o f PS in bulk and in apolar solvents [3, 5, 15]. Nevertheless, assuming t h a t the mix-
Formation of PPDS by anionic polymerization of PS
207
ture of these complexes has an equilibrium composition and considering tha~ P P D S is basically formed with an equimolecular composition of C6H13SLi and LiOR* (x----l), it m a y be assumed that R S L i . L i O R * particles have a higher activity during the formation of P P D S than associates of RSLi resulting in PPS. This is confirmed b y tabulated results which show that formation kinetics of - - S - - S - - bonds exist in polymers both with the conversion of PS and with increase in x. Thus, a slight increase in sulphur content with the conversion of PS, observed when x z 0 (experiment 1 ) does not involve the formation of -- S-- S - bonds in polymers (propylene is not separated) b u t is caused b y an increase in molecular weight at which the proportion of terminal n-C6H13S groups decreases, which is in agreement with results of viscosity and P M R spectroscopy. However, when x > 0 propylene is separated and P P D S formed in every case (experiments l I - I V ) . The marked effect of monomer concentration on the formation of P P D S is here clearly apparent. Thus, with a molar ratio of PS/RSLi ----50 (experiments II and ]II) P P S is mainly formed at the beginning up to about 20% conversion of PS and only on reaching a given ratio of PS/RSLi (_~ 40) does selective formation of P P D S become possible. With a ratio of PS/RSLi-~35 (experiment VII), P P D S is formed with different degrees of conversion of PS. With an increase of this ratio up to 100 P P S is mainly formed and only with advanced conversion of PS is some separation of propylene observed, which is sho~vn b y a slight change in the type of PMI~ spectra of sample 2, experiment V ( - - S - - S - bonds are present in a proportion of minimum 5(~b). This behaviour may be explained by the basicity of PS which is relatively high and comparable with the basicity of some ethers with a fairly high solvaring power: a shift in the band of the OD-deuteromethano] in the Ii~ spectrum (in relation to the band of OD in CCI4) z~YODis 90, 99, 112 and 117 cm -1 for PS, propylene oxide, l-menthyl ethyl ester and T H F , respectively. Therefore, with an increase of the molar ratio of PS/RSLi the contact of the ionic pair decreases and the L i - - S - - bond is solvated, which is covalent in apolar solvents and determines the association of lithium thiolates [20]. Solvation of the L i - - S - - bond does not assist the preliminary coordination of R- or S-antipode of PS either at the stage of polymerization or desu]phuration of PS and thus explains the negligible optical activity of polymers obtained with low degrees of conversion of PS (experiment II, samples 1 and 2, experiments III, IV, sample 1). During polymerization of PS in T H F all organo-lithium compolmds, including lithium alcoholates only produce P P S (experiment VI) with low optical activity. LiOR* has an equally strong effect on the formation of PPDS. Thus, on increasing x to 3 (experiment IV) the rate of polymerization increases noticeably, conversion of PS increases and so does the viscosity of polymers, which contain - - S - - S in the main bond, in spite of the high ratio of P S / R S L i = 5 0 . These results suggest that with a~ increase of x not only the proportion of active eentres increases, producing PPDS, b u t the structure of active centres
~08
ALIYEV v$ al.
A.D.
~l~ l i
r r @,1
I r .~®
o
o, .m
0
I
0 ~D
i~'rl J
~
rll
Formation of PPDS by anionic polymerization of PS
71
~:
I
77
~7
I
III III Ill
I I I oO
,.&
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o6
o6
6
~
~
6
[ ~
© 0
©
o
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°
209
210
A.D. ALIY~.v et al.
changes since a more highly associated complex of the type Li..O--R* : : --CH--S... Li I
:
:
CHa Li..0--R* m a y form. Steric hindrance increases and the polarizability of the growing thiolate anion decreases (three lithium atoms which do not favour nucleophilie opening of the PS ring in view of the possible reduction of the polarizability of Li--S bond, but can promote desulphuration of PS). After desulphuration an alkyldisulphide anion is formed which is more active in polymerization of PS in view of the a~ffect of the hetero-atom [17] and a reduction of steric hindrance for the opening of PS at the CH~--S-- bond. The sequence of these reactions may account for the selective formation of PPDS. Thus, the formation of disulphide bonds in polymers is considerably influenced by solvation and lithium alcoholates, which promote association and thus clearly change the activity of growing thiolate anions in desulphuration of PS and opening of the episulphide ring. I$ follows from results that PPDS is formed by a "relay mechanism" as a consequence of a strict alternation of desulphuration of PS and nucleophilic opening of the episulphide ring of PS by the alkyldisulphide anion formed CHa
CH3
I
PS d e s u l p h u r a t i o n
,~CH--S...Li "
"
propylen¢
I
, ~ CH--S--S...Li *
:
Li.. ()--R*
PS
opening
~-.>
:
I~i..O--R*
CH3 CH, I I --, ~ C H S S C H ~ C t t S . . . • Li.
et¢
Li.. b-n* is due to steric factors and a Varying activity of sec. alkylthiolate- and sec. alkyldisulphide-anions. It should be noted that sulphur content found in every case in the polymers does not reach 60.3%--the calculated value for PPDS. This may be due to the effect of SH and C~H13S end groups, considering the low molecular weight of PPDS. This fact and the absence of polysulphide bonds from PPDS, according to elementary analysis, DOR, UV and PMR spectroscopy, may point to the formation of structurally homogeneous polymers. The relay mechanism of polymerization of PS does not exclude the possible formation of structurally regular PPDS. This is underlined by a slight change in the structure and chiroptical properties of PPDS (experiment VII), obtained with different conversions of S (--) PS on the achiral system of n-C~HlaSLi, xLiOR (LiOR was obtained from d and/-menthol). However, the possibility of two similar reactions should be considered which may cause structural irregularity in which
Formation of PPDS by anionic polymerization of PS
21I
P P D S : a) e x c h a n g e reactions o f t h e t y p e of t h i o l - d i s u l p h i d e a n d d i s u l p h i d e - d i sulphide [21], t h e effect o f w h i c h is insignificant in t h e presence of reactive P S in t h e system; b) nucleophilic s u b s t i t u t i o n [22] CH--SS-Li+ + Li+-S--CH -- --~ ~ CH--S--S--CH ~ -~ Li2S
I
CH3
i
CH3
I
CH3
I
CH~
H o w e v e r , this reaction which c o m p e t e s w i t h t h e r e a c t i o n of extension, is n o t v e r y likely since a significant r e d u c t i o n in t h e r a t e of p o l y m e r i z a t i o n a n d p o l y m e r molecular w e i g h t is f o u n d as a result o f t h e r u p t u r e of t w o g r o w i n g chains to f o r m Li2S inactive in p o l y m e r i z a t i o n of P S [8]. Translated by E. SEMERE
REFERENCES
1. R. M. FITCH and D. C. HELGESON, J. Polymer Sci. C22: 1101, 1968 2. E. M. FETTES, Organic Sulphur Compounds, Ed. N. Kharaseh, Oxford, 1961 3. A. D. ALIEV, B. A. KRENTSEL', G. M. MAMEDIAROV, I. P. SOLOMATINA and Ye. P. TIURINA, Kinetics and Mechanism of Polyreactions, Budapest, 1969 4. I. P. SOLOMATINA, Ye. P. TYURINA, A. D. AL1YEV and B. A. KRENTSEL', Sb. l~iziologicheski i opticheski aktivnye polimernye veshchestva (Physiologically and Optically Active Polymeric Substances). Riga, 1971 5. I. P. SOLOMATINA, Dissertation, 1971 6. A. D. ALIEV, I. P. SOLOMATINA and B. A. KRENTSEL, International Conference on Chemical Transformation of Polymers, Bratislava, 1971 7. I. P. SOLOMATINA, A. D. ALIYEV and B. A. KRENTSEL', Vysokomol. soyed. B13: 252, 1971 (Not traalslated in Polymer Sci. U.S.S.R.) 8. I. P. SOLOMATINA, Zh. ZHUMABIYEV, A. D. ALIYEV and B. A. KRENTSEL', Doklady I I I respublikanskoi nauchno-tekhnicheskoi konferentsii po neftekhimii, Guryev, 1974 9. A. D. ALIEV, I. P. SOLOMATINA and B. A. KRENTSEL, Macromolecules 6: 738, ]973 10. S. SEARLE, H. R. HAYS and E. F. LUTZ, J. Organ. Chem. 27: 2832, 1962 11. J. FURUKAWA, N. KAWABATA and A. KATO, J. Polymer Sei. B5: 1073, 1967 12. D. R. MORGAN and R. T. WRAGG, Makromolek. Chem. 125: 220, 1969 13. A. WEISBERGER, E. PROSCAUER, G. RIDDEY and E. TOOPS, Organicheskiye rastvoriteli (Organic Solvents). Izd. inostr, lit., 1958 14. R. CAMMERECK, C. J. FETTES and M. MORTON, Polymer Preprints 11: 72, 197{) 15. A. D. ALIEV, B. A. KRENTSEL, G. M. MAMEDIAROV, I. P. SOLOMATINA and E. P. TIURINA, Europ. Polymer J. 7: 1721, 1971 16. B. D. VINEYARD, J. Organ. Chem. 32: 3833, 1967 17. J. O. EDWARDS and R. G. PEARSON, J. Amer. Chem. Soc. 84: 16, 1962 18. D. GRANT and J. R. VAN WASER, J. Amer. Chem. Soc. 86: 3012, 1964 19. K. BALEHOVIC and B. GASPERT, Chem. Ind., 624, 1960 20. E. CAMPOS-LOPER, A. LEON-GROSS and M. A. PONCE-VELEZ, J. Polymer Sol., Polymer Chem. Ed. 11: 3021, 1973 21. Khimicheskiye reaktsii polimerov (Chemical Reactions of Polymers). Edited by E_ Fettes, Mir, 1967 22. W. A. PRYOR, Mechanisms of Sulphur Reactions, New York, 1962