Silicon-containing aromatic poly(esters) derived from bis(4-carboxyphenyl) methyl-r-silane and bis(4-(hydroxymethyl) phenyl)methyl-r-silane. synthesis, characterization and thermal studies

Article abstract of DOI:10.4067/S0717-97072012000200023

This work describes the synthesis, characterization and thermal studies of poly(esters) (PEs) containing two Si atoms in the main chain, derived from aromatic diacids and aliphatic dialcohols in which the Si atom is bonded to methyl and/or ethyl groups. P

Full text of DOI:10.4067/S0717-97072012000200023

J. Chil. Chem. Soc., 57, Nº 2 (2012)
SILICON-CONTAINING AROMATIC POLY(ESTERS) DERIVED FROM BIS(4-CARBOXYPHENYL)METHYL-R-
SILANE AND BIS(4-(HYDROXYMETHYL) PHENYL)METHYL-R-SILANE. SYNTHESIS, CHARACTERIZATION
AND THERMAL STUDIES
LUIS H. TAGLE, CLAUDIO A. TERRAZA, LEONARDO I. PINO
Facultad de Química, Pontificia Universidad Católica de Chile, P.O. Box 306, Santiago, Chile
(Received: November 16, 2011 - Accepted: March 2, 2012)
ABSTRACT
This work describes the synthesis, characterization and thermal studies of poly(esters) (PEs) containing two Si atoms in the main chain, derived from aromatic
diacids and aliphatic dialcohols in which the Si atom is bonded to methyl and/or ethyl groups. PEs were characterized by FT-IR and 1H, 13C and 29Si spectroscopy
and the results were in agreement with the proposed structures. Poly(esters) were soluble in polar aprotic solvents, such as DMSO and DMF, and acetone, and
partially soluble in ethanol. The inherent viscosity and glass transition and thermal decomposition temperatures, hinh, Tg and TDT respectively, were determined.
These results showed that PE-2 and PE-4, derived from the dialcohol with the –Si(CH )(CH2CH )- moiety as central element, had the larger values of hinh, Tg, TDT
and the residual weight after heating at 900 °C, indicating that the specific structure o3f aliphatic3dialcohols control the reaction.
Keywords: Silicon-containing aliphatic poly(esters), glass transition temperature, thermal degradation temperature
min-1 under N2 flow) after the second heating run. Thermogravimetric analyses
were carried out in a Mettler (Switzerland) TA-3000 calorimetric system
INTRODUCTION
equipped with a TC-10A processor, and a TG-50 thermobalance with a Mettler
MT5 microbalance. Samples of 6-10 mg were placed in a platinum sample
holder and the thermogravimetric measurements were carried out between 30
ºC and 800 ºC with a heating rate of 20 ºC min-1 under N flow. Elemental
analyses (EA) were made on a Fison EA 1108-CHNS-O equi2pment.
Precursors and monomers synthesis
The ditolyl derivatives, bis(4-methylphenyl)dimethylsilane (1a) and bis(4-
methylphenyl)ethylmethylsilane (1b), were obtained from p-bromotoluene and
dichlorodimethylsilane or dichloroethylmethylsilane, respectively, according
to a described procedure (Scheme 1).12, 19 The diacids, bis(4-carboxyphenyl)
dimethylsilane (2a) and bis(4-carboxyphenyl)ethylmethylsilane (2b), were
obtained by oxidation of the ditolyl compounds, according to a described
procedure.15 These diacids were reduced with LiAlH4 in THF in order to
obtain the dialcohols bis(4-hydroxymethylphenyl)dimethylsilane (3a) or bis(4-
hydroxymethylphenyl)ethylmethylsilane (3b), according to the procedure
described by Mingal et al.15 All compounds were characterized by FT-IR and
1H, 13C and 29Si NMR spectroscopy, and the results were in agreement with the
proposed structures.
Aromatic and aliphatic poly(esters) have been condensation polymers
widely used, which have many important applications. The introduction in the
polymeric chain of heteroatoms such as Si or Ge can change some of their
properties such as to improve the solubility and maintain or increase the thermal
stability due to the polarity of the C-Si bond, derived from the difference in
electronegativity of these atoms.1-9 Also, glass transition temperature (Tg) of
the materials can be diminished due to the longer C-Si or C-Ge bond relative
to that of C-C. This fact, would allow higher rotation freedom of the polymeric
chain. Also, the incorporation of other functional groups or aliphatic moieties
in the repetitive unit can cause similar effects.
Several types of aromatic poly(esters) containing one or two silicon atoms
in the main chain have been synthesized and characterized.2, 10-13 Likewise, it
has been reported the preparation of polycondensation materials that include the
ester and other functional groups.4, 6, 14 On the other hand, Migdal et al.15 reported
the synthesis of a series of dimethylsilylen-containing aliphatic poly(esters).
These polymers are based on the reactions of bis(p-carboxymethylphenyl)
dimethylsilane diacid and the dialcohols bis(p-hydroxymethylphenyl)
dimethylsilane and bis(p-b-hydroxyethylphenyl)dimethylsilane. Moreover,
similar silicon-containing compounds have been prepared by Rotman et
al.16 and Kim et al.17 Recently, our group has also reported the synthesis and
characterization of some difunctional silicon-containing aliphatic precursors
and monomers.18
Continuing our works on the synthesis, characterization and thermal
studies of condensation polymers containing one or two Si atoms in the main
chain, in this work, we describe the synthesis of four new poly(esters) derived
from aromatic dicarboxylic acids and aliphatic dialcohols, all them containing
two Si atoms in their structure and bonded to methyl and/or ethyl groups. In all
cases, the thermal properties (Tg and thermal decomposition temperature) were
related to the specific structure of the repetitive unit.
EXPERIMENTAL PART
Materials
Lithium, CH3RSiCl2 (R= CH or CH2CH3), CrO3, LiAlH4, tosyl chloride
(TsCl) and pyridine were obtaine3d from Aldrich Chemical (Milwaukee, WI)
and used without further purification. Solvents were purchased commercially
as analytical-grade and used without further purification.
Instrumentation
The FT-IR spectra (KBr pellets) were recorded on a Perkin-Elmer
(Fremont CA) 1310 spectrophotometer over the 450-4000 cm-1 range. 1H, 13
C
and 29Si NMR spectra were carried out on a 400 MHz instrument (Bruker AC-
200) using DMSO-d , CDCl3 or acetone-d6 as solvents and TMS as internal
standard. Viscosimet6ric measurements were made in a Desreux-Bischof type
dilution viscosimeter at 25 °C (c = 0.3 g/dL). Tg values were obtained with a
Mettler-Toledo (Greifensee, Switzerland) DSC 821 calorimetric system (20 ºC
Scheme 1. Synthetic route for the preparation of the aliphatic dialcohols;
bis(4-hydroxymethyl)phenyl)methyl-R-silane (3).
e-mail: ltagle@uc.cl
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J. Chil. Chem. Soc., 57, Nº 2 (2012)
1
Polymer synthesis
(arom. p-subst.). H NMR (DMSO-d ) (d) (ppm): 0.49 (s, 6H, Si-CH ), 0.88
(m, 6H, CH -CH3), 1.04 (m, 4H, CH62-CH3), 5.29 (s, 4H, OCH2), 7.0-37.9 (m,
16H, arom.)2. 13C NMR (DMSO-d6) (d) (ppm): -2.4 (Si-CH3), 5.3 (CH2-CH3),
7.9 (CH2-CH3), 66.5 (OCH2), 126.1, 126.6, 127.7, 128.8, 134.1, 134.5, 135.0,
138.2 (8C arom.), 165.6 (C=O). 29Si NMR (DMSO-d ) (d) (ppm): -4.84 (acid
moiety), 5,01 (alcohol moiety). Elem. Anal. Calcd. for6[C34H36O4Si2]n; (564.52)
n: C: 72.33 %; H: 6.38 %. Found: C: 71.88 %; H: 5.71 %.
Poly(esters) were obtained according to the following general procedure20
(Scheme 2): 0.635 mmol of the diacid (2) was mixed with 2 mL of pyridine,
and then 1.3 mmol of TsCl was added, and the mixture stirred by 30 min.
To this solution N,N-dimehtylformamide (DMF) was added and the stirring
continued for 20 min. After this time, a solution of 0.42 mmol of the dialcohol
(3) in DMF was added dropwise and the mixture refluxed at 120 °C for one
hour. After this time, the mixture was poured into methanol and the poly(ester)
was filtered, washed with cold methanol, dried under vacuum at 40 °C and
characterized.
RESULTS AND DISCUSION
Diacid derivatives were obtained from p-bromotoluene as starting material
following four steps (Scheme 1). In step I, an organo-lithium salt was obtained
in nitrogen atmosphere, which was reacted with dichlorodimethylsilane or
dichloroethylmethylsilane (step II). The heterogeneous system was stirred and
heated for 16 h and then the salts remainders were filtered. The acid hydrolysis
yielded the ditolyl derivatives, which were oxidized in acid medium, to the
19
respective diacid, according to a described procedure (step III).15, The
dialcohols were obtained by reduction of the corresponding diacid with LiAlH
in N2 atmosphere (step IV), also according to a described procedure.16 Al4l
1
the precursors and monomers were characterized by FT-IR and H, 13C and
29Si NMR spectroscopy, and the results were in agreement with the proposed
structures.
Poly(esters) were obtained in two steps according to the method described
by Mallakpour et al.20 (Scheme 2) in which the diacid is activated by reaction
with tosyl chloride in pyridine at room temperature, in order to obtain the
ditosylate derivatives (step I). Then, the dialcohol reacts as a nucleophile with
the carbonyl carbon of tosylate intermediate to obtain the diester (step II) and
finally the poly(ester). The specific activating role of the tosylate is to convert
the hydroxyl group of the diacid in a good leaving group. Poly(esters) were
precipitated in cold methanol, washed and dried.
PE-2 and PE-4 samples, both polymers derived from the dialcohol
containing an ethyl group bonded to the silicon atom, presented a solid aspect,
in contrast to PE-1 and PE-3 which showed a semi-solid consistency.
Table I shows the yields for all PEs. This parameter is over 95 % in all
cases ratifying the activating role of TsCl in the step I of polymerization. This
table also shows the inherent viscosity values obtained from DMSO solutions.
Except PE-1, whose repetitive unit presents the lowest value, all samples
showed high hinh indicating a moderate molecular weight. Poly(esters) showed
good solubility in polar aprotic solvents such as DMF and DMSO, and also
in common organic solvents such as acetone, although a partial solubility in
ethanol.
Scheme 2. Synthesis of silicon-containing aliphatic poly(esters).
PE-1. IR (KBr) (cm-1) 3015 (C-H arom.), 2921 (C-H aliph.), 1715 (C=O),
1602, 1488 (C=C arom.), 1391 (Si-Ph), 1458, 1253 (Si-CH3), 1107 (C-O), 821
(arom. p-subst.). 1H NMR (DMSO-d6) (d) (ppm): 0.56-0.89 (m, 12H, Si-CH3),
5.3 (s, 4H ,CH2), 7.1-8.0 (m, 16H, arom.). 13C NMR (DMSO-d6) (d) (ppm): -4.7
(Si-CH3 alcohol moiety) -2.5 (Si-CH3 acid moiety), 63.5 (CH2), 126.4, 127.8,
128.9, 134.4, 135.5, 136.2, 137.5, 144.2 (8C arom.), 151.6 (C=O). 29Si NMR
(DMSO-d ) (d) (ppm): -6.29 (acid moiety), -5.73 (alcohol moiety). Elem. Anal.
Calcd. for6[C32H32O4Si2]n; (536.50)n: C: 71.63 %; H: 5.96 %. Found: C: 71.03
%; H: 5.10 %.
TABLE I.- Yields, inherent viscosity, glass transition temperatures (Tg)
and thermal decomposition temperatures (TDT) of the PEs.
Yield (%)
hinh (dL/g)*
Tg (ºC)
TDT** (ºC) Residue (%)
(900 °C)
PE-1
PE-2
PE-3
PE-4
95
95
97
96
0.15
0.38
0.24
0.47
48
64
7
227
358
217
337
15
31
14
34
PE-2. IR (KBr) (cm-1) 3035 (C-H arom.), 2879 (C-H aliph.), 1719 (C=O),
1599, 1498 (C=C arom.), 1389 (Si-Ph), 1440, 1212 (Si-CH3), 1090 (C-O), 809
44
1
(arom. p-subst.). H NMR (DMSO-d6) (d) (ppm): 0.46-0.53 (m, 9H, Si-CH3),
* Inherent viscosity, in DMSO at 25 ºC (c = 0.3 g/dL)
** 10 % weight loss temperature
1.19 (t, 3H, CH2-CH3), 2.0 (q, 2H, CH2-CH3), 5.28 (s, 4H, OCH2), 7.3-7.9
(m,16H,arom.). 13C NMR (DMSO-d6) (d) (ppm): -2.7 (Si-CH3 acid moiety),
-2.2 (Si-CH3 alcohol moiety), 5.6 (CH2-CH3), 7.9 (CH2-CH3), 66.5 (OCH2),
126.6, 127.5, 128.8, 130.0, 131.0, 131.9, 134.5, 137.5 (8C arom.), 166.5
(C=O). 29Si NMR (DMSO-d6) (d) (ppm): -6.14 (acid moiety), -4.03 (alcohol
moiety). Elem. Anal. Calcd. for [C H34O4Si2]n; (550.51)n: C: 71.99 %; H: 6.18
%. Found: C: 71.22 %; H: 5.95 %.33
PEs were also characterized by spectroscopic methods. The results
are described in the Experimental Part and were in agreement with the
proposed repetitive units. The FT-IR spectra showed the disappearance of the
characteristic O-H band of both, the dialcohol and the diacid (Figure 1). Also
the band associated to the C=O group was shifted to about 1715- 1719 cm-
1, corresponding to the ester group. All samples show the specific absorption
bands for the Si-C bond, according with the nature of the carbon atom: aliphatic
or aromatic.
PE-3. IR (KBr) (cm-1) 3023 (C-H arom.), 2957 (C-H aliph.), 1718 (C=O),
1601, 1488 (C=C arom.), 1389 (Si-Ph), 1390, 1275 (Si-CH3), 1107 (C-O),
1
809 (arom. p-subst.). H NMR (DMSO-d6) (d) (ppm): 0.45-0.55 (m, 9H, Si-
CH3), 0.94 (m, 3H, CH2-CH3), 2.2 (t, 2H, CH2-CH3), 5.81 (s, 4H, OCH2), 7.07-
8.13 (m,16H,arom.). 13C NMR (DMSO-d ) (d) (ppm): -5.31 (Si-CH3 alcohol
moiety), -3.4 (Si-CH3 acid moiety), 5.76 (CH2-CH3), 7.9 (CH -CH ), 63.5
(OCH ), 125.9, 126.5, 127.0, 127.7, 128.0, 129.1, 134.4, 135.32 (8C3arom.),
159.6 2(C=O). 29Si NMR (DMSO-d ) (d) (ppm): -4.28 (acid moiety), -5.02
(alcohol moiety). Elem. Anal. Calcd.6for [C H34O4Si2]n; (550.51)n: C: 71.99 %;
H: 6.18 %. Found: C: 71.07 %; H: 5.71 %. 33
1H NMR analyses show, in the aliphatic zone, the protons for the methyl
and ethyl groups bonded to silicon atoms, whose integration corresponds to the
identity of the respective monomers used. The spectra of all polymers showed
the methyl groups from the diacid and dialcohol moieties as a broad signal.
The magnetic equivalence of these protons is similar; for this reason, it is not
possible obtain pure singlets. Similar results are observed for the protons from
the ethyl groups in PE-4. On the other hand, the 13C NMR spectra showed
clearly the effect of the silicon atom on the CH3 displacement. All signals
PE-4. IR (KBr) (cm-1): 3069 (C-H arom.), 2960 (C-H aliph.), 1719 (C=O),
1598, 1498 (C=C arom.), 1427 (Si-Ph), 1389, 1275 (Si-CH3), 1089 (C-O), 821
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J. Chil. Chem. Soc., 57, Nº 2 (2012)
appear at negative values due to the difference of electronegativity between
Si and C atoms. 29Si NMR spectra allow identifying the monomer from where
the signals come. Thus, the -Si(CH )2- signal from diacid moiety appears at
-6.2 ppm approximately (PE-1 and 3PE-2), while for the dialcohol, the same
signal is observed between -5.02 and -5.73 ppm (PE-1 and PE-3). Likewise,
the –Si(CH3)(CH2CH )- signal can be observed at -4.6 ppm approximately for
acid moiety (PE-1 an3d PE-4) and at -4.03 and -5.01 ppm (PE-2 and PE-4).
PE-3 (semi-solid polymers) showed lower but similar TDT values.
Regarding the highest hinh, Tg and TDT values, they correspond to PE-2
and PE-4, both polymers derived from the dialcohol containing an ethyl group
bonded to the Si atom. Thus, it is possible to conclude that the properties of
these polymers depend on the structure of the dialcohol instead of the diacid.
The quantity of residual weight after heating at 900 °C is shown in Table I.
The tendency points out that the PE-2 and PE-4 samples, both with a –Si(CH3)
(CH CH )- moiety from dialcohol, showed similar values and higher than those
of P2E-13and PE-3. In previous works these residues have always been high
assigned to silicon oxide principally.3-5, 21
Figure 1. FT-IR spectra of the PEs.
Figure 3. Thermogravimetric and differential curves for PEs (20 °C/min
in nitrogen).
Figure 2 shows the DSC curves for PEs and the results are summarized
in Table I. From these analyses is possible to see that PE-2 and PE-4, both
derived from the dialcohol with an ethyl group bonded to the Si atom, show the
highest Tg values. This fact is probably due to the high hinh values evidenced
for the samples, which are associated to the molecular weights. The exception
was PE-1, with the lowest hinh value, which showed a Tg value similar to that
of PE-4. On the other hand, PE-1 and PE-4 are symmetric, showing similar Tg
values, but different hinh values. In this family of poly(esters) there is no clear
relation between the Tg and the hinh values.
CONCLUSIONS
Four new aliphatic poly(esters) containing two silicon atoms in the main
chain were synthesized and characterized, obtaining good yields and, in two
cases, good inherent viscosity values. Polymers derived from the dialcohol
containing ethyl and methyl groups bonded to the silicon atom, presented a
solid aspect, and gave the best values of hinh, Tg and TDT, being the most
asymmetrical dialcohol moiety the most determinant part of the polymeric
system, in contest to other PEs, which showed a semi-solid consistency only.
Poly(esters) were soluble in polar aprotic solvents and in common solvents,
such as acetone, and were partially soluble in ethanol. PE-2 and PE-4 showed
a good thermal stability in spite of the aliphatic condition of the dialcohol used.
ACKNOWLEDGEMENTS
The authors acknowledge the financial assistance to this work by Fondo
Nacional de Investigación Científica y Tecnológica, FONDECYT, through
Project 1070778.
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