CC BY
19
UDC 541.64:547.315.2
M. I. Abdullin, А. B. Glazyrin, E. R. Gazizova, V. A. Dokichev, R. M. Sultanova, G. E. Zaikov, Kh. S. Abzaldinov
CYCLOPROPANE DERIVATIVES OF SYNDIOTACTIC 1,2-POLYBUTADIENE
Keywords: ^emical modification, 1,2-polybutadien, catalytic cyclopropanation, degrees of cyclopropanation, physicochemical
characteristics.
Polymer product containing methoxycarbonil-substituted cyclopropane groups in macromolecules with degrees of functionalization up to 40% have been obtained through the interaction of polybutadiene with methyl diasoacetate in an organic solvent in the presence of an organometallic catalyst. The comparative catalyst activity in the reaction of cyclopropanation ofpolybutadiene has been estimated. The incorporation of methoxycarbonil-substituted cyclopropane groups into polydiene units results in substantial changes in the polymer properties: solution viscosity, melt flow, glasstransition temperature, and thermal stability.
Ключевые слова: химическая модификация, 1,2-полибутадиен, каталитическое циклопропанирование, степени циклопропанирования, физико-химические характеристики.
Взаимодействием полибутадиена с метилдиазоацетатом в органическом растворителе в присутствии металлоорганического катализатора получен полимерный продукт, содержащий метоксикарбонил-замещенные циклопропановые группы в макромолекулах со степенями функционализации до 40%. Рассчитана сравнительная активность катализатора в реакции циклопропанирования полибутадиена. Введение метоксикарбонил-замещенных циклопропановых групп в полидиеновые звенья приводит к существенному изменению свойств полимера: вязкости раствора, текучести расплава, температуры стеклования и термической стабильности.
Introduction
Polymers containing cyclopropane units with polar substituents in the cycle are of great interest owing to the balanced complex of strength and elastic properties and can be used to produce polymer materials and various products, including adhesive compositions and paint and varnish materials [1].
The chemical modification of macromolecules containing reactive functional groups including -CH=CH2 is one of methods to produce polymers with cyclopropane units. The catalytic addition of carbenes to the C=C double bond is a suitable method to incorporate cyclopropane groups into unsaturated polymer chains [2, 3]. Hence, polybutadienes are convenient objects for polymer modification [4, 5].
The aim of this study was to investigate the synthesis of polymers containing methoxy-carbonyl-substituted cyclopropane groups via catalytic cyclopropanation of polybutadiene with methyl diazoacetate as well as some physicochemical characteristics of the modified polymers.
Experimental part
For chemical modification, polybutadiene samples (polymers were supplied by OAO Efremov Zavod SK) varying in molecular mass and 1,2-and 1,4-unit contents were used: (I) PB SN-8000 with Mn= 8.3 x 103, Mw/Mn = 1.98, and a 1,2-unit content of 68.3 mol %; (II) PB SKD-SR with Mn = 60.6 x 103, Mw/Mn = 2.98, and a 1,2-unit content of 61.5 mol %; (III) PB with Mn = 52.6 x 103, Mw/Mn = 2.22, a 1,2-unit content of 85 mol %, a degree of syndiotacticity of 53 mol %, and a crystallinity of 18%; and (IV) PB JSR RB-830 (Japan Synthetic Rubber) with Mn = 65x103, Mw/Mn = 1.78, a 1,2-unit content of 84 mol %, a degree of syndiotacticity of 86 mol %, and a crystallinity of 26%.
As cyclopropanation catalysts, copper (II) triflate Cu(CF3S03)2 (Acros); copper(I) triflate in a
complex with benzene, [Cu CF3S03] • 0.5C6H6 (Acros); and rhodium tetraacetate Rh2(OAc)4 prepared as described in [6] were used.
The cyclopropanation of PB with methyl diazoacetate was conducted in methylene chloride (the polymer concentration in the solution was below 3 wt %) at 40°C and stirred until cessation of gas evolution. The molar ratio PB : methyl diazoacetate : catalyst = 1.0 : 1.0 : 1.0 was used. Once the synthesis was completed, the polymer was precipitated from the reaction medium into ethanol, reprecipitated into a chloroform-methanol mixture, and dried under vacuum.
The modified PB was analyzed via 1H and 13CNMR spectroscopy on a Bruker AM-300 spectrometer. 13C NMR spectra were recorded under conditions of broadband proton decoupling and JMOD at 25 and 60°C with the use of 3-10% polymer solutions in CDCl3 and TMS as an internal standard. The quantitative analysis of 13C NMR spectra was performed with the use of broadband proton decoupling and a delay between pulses of 12 s. The 13C NMR spectra of cyclopropanated 1,2-PBs were analyzed via comparison with the spectra of the pristine polymers that was based on the spectral data on the PB microstructure obtained in [7]. The degree of cyclopropanation of the polymer (the content of cyclopropane units), a, was determined from the ratio of the integral intensities of carbon atom signals in cyclopropane and carboxyl groups of the modified polymer and signals of the overall residual double bonds in the polymer.
IR spectra of polymers were recorded on a Shimadzu IR Prestige-21 FTIR spectrometer.
The viscosities of polymer solutions (in chloroform) were measured on an Ubbelode viscometer at 25±0.1°C. Intrinsic viscosities [n] were found through the common method [8].
Glass-transition temperatures of polymers were determined via DSC on a Mettler Toledo DSC-1
calorimeter at a heating rate of 10 K/min. The TGA of polymers was performed on a Mettler Toledo TGA-DSC instrument under dynamic operating conditions (a heating rate of 5 K/min, in air).
The melt flow indexes of polymers were measured on an IIRT-AM instrument at 140°C and a load of 49 N (with a capillary 8 mm in length and 2.09 mm in diameter).
Results and Discussion
Modified polydienes with methoxycarbonyl-substituted cyclopropane groups were prepared via the interaction of PB with carbomethoxycarbene obtained from methyl diazoacetate in the presence of a catalyst in an organic solvent.
N2CHCOOMe; [cat]
-N2
COOMe
COOMe
Note that the cyclopropanation reaction, as a rule, occurs without polymer-chain scission [9, 10].
The formation of cyclopropane fragments in macromolecules of modified PB was corroborated via IR and NMR spectroscopy measurements.
IR spectra of modified polymers display characteristic bands due to cyclopropane groups (844, 881 cm-1 and 1060, 1083 cm-1) and bands due to ester groups at 1728 cm-1 (Fig. 1).
Fig.1 - IR spectra of (a) initial and (b) cyclopropanated PB IV
1H NMR spectra exhibit high-field multiplets of cyclopropane group protons at 0.75-0.91 and 1.25— 1.48 ppm, and 13C NMR spectra display high-field broadened triplets at 13—15 ppm and doublets at 17-20 and 26-31 ppm due to carbon atoms of cyclopropane groups (Fig. 2, Table 1). The NMR signals were assigned via two-dimensional correlation spectroscopy (COSY). The COSY(C—H) spectra demonstrate crosspeaks of the interaction of high-field protons at 0.75-0.91 ppm as well as protons at 1.25-1.48 ppm with
C atoms of cyclopropane groups: 14.8 (t), 17.06 (d) and 26.63 (d), 28.83 (d), respectively. The presence of groups of doublet (d) and triplet (t) signals in 13C NMR spectra is related to the configurational, stereoisomeric, and structural differences in molecular chains of cyclopropanated PBs [11].
Fig. 2 - High-field region of 13C NMR (JMOD) spectra of (a) initial and (b) cyclopropanated PB IV
The chemical shifts of cyclopropane groups in modified PBs are close to the values given in [12] for the catalytic cyclopolymerization of butadiene. In the low-field region of 13C NMR spectra, two singlets at 174.86 and 173.21 ppm and a quartet at 52.22ppm (Fig. 2, Table 1) are due to carbon atoms of ester substituents in cyclopropane groups of 1,2 and 1,4 units, respectively.
13C NMR spectra of the reaction mixture after cyclopropanation display singlets at 164.0 and 132.7 ppm and a quartet at 52.1 ppm due to carbon atoms of fumaric acid ester (the product of carbene dimerization) [2]. The above signals are absent from the spectra of modified polymers isolated from the reaction mixture.
16 17 COOMe
n 12
The process of catalytic decomposition of alkyl diazoacetates involves the formation of an intermediate complex with the participation of alkyl diazoacetate and the catalyst [2, 13]. Alkoxycarbonylcarbene generated during the subsequent splitting off of nitrogen is stabilized by the catalyst to yield a carbene complex,
b
whose interaction with alkene cyclopropanation products [14].
results in
Table 1 - Assignment of signals in the C NMR spectrum of cyclopropanated 1,2-PB (CDCI3, TMS, 25°C)
Atom Signal, Mul Atom Signal, Mul
no. ppm tipli no. ppm tipli
city city
Ci 33.30 t C12 34.12 t
C- 37.80 d C13 17.33 d
C3 31.11 d C14 17.06 d
C4 26.63 d C15 28.83 d
C5 14.08 t C16 173.21 c
C6 174.86 c C17 52.22 k
C7 52.22 k C18 32.70 t
C8 41.76 t C19 27.40 t
cis- t
32.81
trans-
C9 38.50 d C20 128.07 d
cis- d
127.63
trans-
C10 142.85 d C2, 131.73 d
cis- d
130.61
trans-
C11 114.99 t C22 25.70 t
cis- t
33.12
trans-
The yield of the cyclopropanation products is determined by the reactivity of the C=C double bond in alkene along with the stability and reactivity of the carbene complex Ln M = CH(O)R1, which depend on the type of catalyst used [2, 13].
COR
R1(O)CHC=N2
LnM=CHC(O)R1
N-
M=Cu, Rh, Ru
The effect of the type of catalyst and the structure of the pristine PB on the degrees of cyclopropanation, a, and on the compositions of the modified polymers was studied.
The experimental results showed that rhodium tetraacetate has the highest activity of the studied catalysts of cyclopropanation. During cyclopropanation of oligomeric PB I with methyl diazoacetate in the presence of Rh2(OAc)4, the content of cyclopropane groups in the modified polymer was as high as 40 mol %, whereas, in the presence of the copper (I) triflate-
benzene complex and copper (II) triflate, the degrees of functionalization of the polymer were lower: 33 and 21%, respectively (Table 2).
Table 2 - Composition of pristine PBs I-IV and products of their catalytic cyclopropanation with methyl diazoacetate
PB Catalyst Content, mol % 5= 1,2/1,4
of cyclo pro pane groups of C=C bonds
1,2 units 1,4 units
I - 65.8 34.2 1.92
II - 63.5 36.5 1.74
III - 85.0 15.0 5.67
IV - 84.0 16.0 5.25
I 21.2 51.5 27.3 1.89
II Cu(OTf)2 25.1 47.7 27.2 1.75
III 22.2 65.7 12.1 5.43
IV 15.4 71.1 13.5 5.27
I [Cu OTf]x 0,5 C6H6 32.7 42.9 24.4 1.76
I Rh2(OAc)4 40.0 41.6 18.4 2.26
IV 20.4 76.6 3.0 25.53
Thus, by their activities in cyclopropanation of PB, the studied catalysts are arranged in the following
series: Rh2(OAc)4 > [Cu OTf] -OJCfH > Cu(OTf)2.
The analogous activity series of the mentioned catalysts was obtained previously during cyclopropanation of low-molecular-mass olefins with alkyl diazoacetates [4]. In addition, higher catalytic activities of rhodium compounds than those of copper salts were found during cyclopropanation of cis-1,4-polyisoprene with ethyl diazoacetate [3].
Thus, rhodium catalysts with the highest activities in the cyclopropanation reaction of almost every type of olefin show high activities in the catalytic cyclopropanation of PB with methyl diazoacetate.
With allowance for the fact that, in macromolecules of the studied polymers, double bonds of two types, namely, C=C bonds in 1,2- and 1,4-units, are present, the comparative estimation of the reactivities of these bonds in catalytic cyclopropanation was conducted on the basis of their residual contents in the modified polymers. The relative contents of double bonds were determined from the integral intensities of signals in 13C NMR spectra: 113.7-15.0 and 142.3-144.7 ppm for 1,2-units and 127.0-132.0 ppm for 1,4-units.
It follows from the experimental results that, in
the presence of Cu(OTf)2 and [CuOTf] • 0.5 CsH6, the cyclopropanation of PBs I-IV proceeds at double bonds
1
of both types: the ratios of unreacted double bonds in 1,2- and 1,4-units of modified PBs are close to their ratios in the pristine polymers (Table 2).
Dissimilar results were obtained during the interaction of methyl diazoacetate with PB IV with a high content of syndiotactic sequences in the presence of a rhodium catalyst: The relative content of unreacted double bonds in 1,4-units of the modified polymer (a = 20.4%) was considerably lower than that of unreacted double bonds in 1,4 units of the initial poly diene. Parameter Ö, the molar ratio of double bonds in 1,2- and 1,4-units of polydiene, is ~4.8 times greater for the cyclopropanated polymer than it is for the initial PB (Table 2).
From the experimental results, it follows that the cyclopropanation of PB IV in the presence of Rh2(OAc)4, unlike the modification with Cu(OTf)2, occurs predominantly at double bonds in 1,4-units of macromolecules. Thus, the use of Rh2(OAc)4 makes it possible to obtain PB derivatives containing substituted cyclopropane groups predominantly in the back-bone.
In addition, the double bonds in 1,4-units are subjected to greater modification during cyclopropanation of PB I in the presence of Rh2(OAc)4. This resultis confirmed by a higher ö value for the modified polymer than that for the pristine polydiene (Table 2). Moreover, in modified PB I, the content of unreacted double bonds in 1,4 units remains high: ~18% or ~60% of their content in the initial polydiene.
The observed variations are related to the special features of the macromolecular structure of a polydiene. The content of 1,4-units in PB I, for example, is almost twice that in PB IV. Moreover, unlike PB I, polydiene IV features a high stereoregularity of the configuration sequences of 1,2-units. In the olefin region of the 13C NMR spectrum of PB IV, the doublet signals at 127.6 and 130.6 ppm due to C atoms of double bonds of cis- and £rans-1,4-units neighboring 1,2-units prevail. This result suggests that 1,4-units in macromolecules are distributed mainly between 1,2-units and are mainly single units (v-T-v and v-C-v). In the spectrum of PB I, in contrast, doublet signals at 129-131 ppm, as well as triplets at 27.4 and 32.7 ppm due to microblocks of cis- and /rans-1,4-units, dominate [11].
It is likely that, during cyclopropanation of PB I with the microblocks of 1,4-units in the chain, methoxycarbonyl-substituted cyclopropane groups formed in the microblocks are responsible for steric hindrances preventing the approach of bulky catalytic complexes and functionalization of adjacent double bonds in 1,4-units (the "neighbor effect" [15]). As a result, the degree of modification of double bonds in 1,4-units of PB I is much lower than that of PB IV.
Thus, the composition of a modified PB is determined both by the type of catalyst and by the microstructure of the pristine polydiene: the use of Rh2(OAc)4 and PB predominantly of the syndiotactic structure containing single 1,4-units makes it possible to obtain products with substituted cyclopropane groups mainly in the backbone. The synthesis with copper salts as catalysts and/or the presence of microblocks of 1,4-units in the polymer chain results in products containing
cyclopropane groups both inside groups and in the backbone (at comparable ratios).
Some properties of cyclopropanated PBs prepared via chemical modification of PBs III and IV in the presence of Cu(OTf)2 were studied.
It was found that the modification of PBs with methoxycarbonyl-substituted cyclopropane groups results in a significant reduction of intrinsic viscosities [Hi of modified-polymer solutions relative to those of the pristine-polymer solutions (Table 3). For cyclopropanated PB IV with a degree of functionalization of 27.6%, the [ni value in chloroform is approximately two times lower than that for the pristine polydiene. Along with the decrease in intrinsic viscosity, a gain in Huggins constant K is observed (Table 3), a circumstance that suggests the deterioration of the thermodynamic affinity of a modified polymer for chloroform [8].
The modification of PB macromolecules by cyclopropane groups with ester substituents (a = 1828%) leads to changes in the rheological properties of polymer melts. A substantial (more than tenfold) decrease in the flowability of a polymer melt, which is characterized by melt flow index (MFI), is observed (Table 3). This phenomenon may be related to the intensified intermolecular interaction and reduced segmental mobility of macromolecules. As a result, glass-transition temperature Т% of modified PB increases significantly. For example, Т% of cyclopropanated PB IV (a = 27.6%) exceeds that of the pristine polymer by ~32°С (Table 3).
Table 3 - Properties of cyclopropanated 1,2-PBs with different degrees of functionalization
а, fol, K MFI, Т g Т^ д
% dL/g g/1G 0С 0С m4oo,
<u min %
¿y.
CM
1 - 1.7G G.22 1G.9 -19.S 29G 2.4
2 17.7 1.24 G.65 G.6 S.6 1SG 15.1
21.S G.S4 G.61 G.4 1G.1 16S 17.5
3 - 1.2S G.75 4.2 -1G.1 336 2.2
4 27.6 G.61 1.1G G.4 22.3 224 25.G
* - 1 - PB (III), 2 - cyclopropanated PB (III), 3 - PB (IV), 4 - cyclopropanated PB IV
The modified PBs have lower thermal stabilitiesthan those of the initial polydienes. For PB III, the temperature of the onset of decomposition, Тd, which corresponds to the onset of a change (decrease) in the polymer mass during heating in air, exceeds that for cyclopropanated PB (a = 17.7%) by 110°C (Table 3). Parameter Am400, which characterizes the change in the polymer mass during heating to 400°C (i.e., to the temperature of the onset of the intense degradation related to the thermal scission of carbon-carbon bonds in macrochains), ranges from 15 to 25% for cyclopropanated PBs, whereas, it is as low as ~2% for the pristine PBs (Table 3). It may be assumed that lower
thermal stabilities of modified PBs result from changes in the mechanism of thermo-oxidative degradation. The results of studying this process will be presented in further publications.
Conclusions
The polymer products containing methoxycarbonyl-substituted cyclopropane groups in macromolecules with degrees of functionalization up to 40 mol % have been prepared via the interaction of PB with methyl diazoacetate in an organic solvent in the presence of an organometallic catalyst. It has been shown that the activities of the studied catalysts in the cyclopropanation of PB decrease in the following series:
Rh2(OAc)4-[Cu OTf] ■0.5C6H6-Cu(OTf)2.
The cyclopropanation of PB in the presence of Rh2(OAc)4 makes it possible to obtain modified polydienes with cyclopropane groups predominantly in the backbone, whereas, in the presence of copper (I) triflate and copper (II) triflate, double bonds in both 1,2-and 1,4-units of macromolecules are subjected to cyclopropanation.
The modification of PB with a high content of syndiotactic sequences via incorporation of methoxycarbonyl-substituted cyclopropane groups into the polymer chain results in changes in the physicochemical characteristics of the polymer: decreases in solution viscosity and melt flowability, an increase in the glass transition temperature, and a decrease in thermal stability.
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© M. I. Abdullin - Doctor of Chemistry, Full Professor, Head of Technical Chemistry Department, Bashkir State University, Ufa, Russia, A. B. Glazyrin - PhD (Engineering Sciences), Associate Professor of Technical Chemistry Department, Bashkir State University, Ufa, Russia, glaab@inbox.ru, E. R. Gazizova - PhD Student, Bashkir State University, Ufa, Russia; V. A. Dokichev -Doctor of Chemistry, Full Professor, Head of Organometallic compounds and catalytic synthesis laboratory, Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences, Ufa, Russia; R. M. Sultanova - Doctor of Chemistry, Associate Professor, Institute of Organic Chemistry, Ufa Research Center, Russian Academy of Sciences, Ufa, Russia, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, chembio@sky.chph.ras.ru; Kh. S. Abzaldinov - PhD (Chemical Sciences), Associate Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© М. И. Абдуллин - доктор химических наук, профессор, заведующий кафедрой Технической химии, Башкирский государственный университет, Уфа, Россия; А. Б. Глазырин - кандидат технических наук, доцент кафедры Технической химии, Башкирский государственный университет, Уфа, Россия, glaab@inbox.ru; Э. Р. Газизова - аспирант, Башкирский государственный университет, Уфа, Россия, В. А. Докичев - доктор химических наук, профессор, заведующий лабораторией Металлорганического синтеза и катализа, Институт органической химии УНЦ РАН, Уфа, Россия; Р.М. Султанова - доктор химических наук, доцент, Институт органической химии УНЦ РАН, Уфа, Россия; Г. Е. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия, chembio@sky.chph.ras.ru; Х. С. Абзальдинов - кандидат химических наук, доцент кафедры Технологии пластических масс КНИТУ, Казань, Россия.
