Synthesis and thermal behavior of Janus dendrimers, part 1

Article abstract of DOI:10.1016/j.tca.2009.08.018

Eight Janus-type dendrimers up to the second generation were synthesized, and their thermal properties were evaluated. Compounds consist of the dendritic bisMPA based polyester moieties, and either 3,4-dihexyloxybenzoic acid or 3,4-dihexadecyloxybenzoic a

Full text of DOI:10.1016/j.tca.2009.08.018

Thermochimica Acta 497 (2010) 101–108
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Synthesis and thermal behavior of Janus dendrimers, part 1ଝ
Tero Tuuttila, Manu Lahtinen, Noora Kuuloja, Juhani Huuskonen, Kari Rissanen∗
Nanoscience Center, Department of Chemistry, University of Jyväskylä, P.O. Box 35, FIN-40014, Jyväskylä, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Eight Janus-type dendrimers up to the second generation were synthesized, and their thermal properties
were evaluated. Compounds consist of the dendritic bisMPA based polyester moieties, and either 3,4-
dihexyloxybenzoic acid or 3,4-dihexadecyloxybenzoic acid moieties, attached to opposite sides of the
pentaerythritol core. The structures of the molecules were verified with 1H NMR, 13C NMR, ESI TOF mass
spectrometry and elemental analysis. The thermal stability was evaluated by thermogravimetric analysis,
displaying onset decomposition temperatures (Td) ranging from 241 to 308 ◦C. Phase transitions were
studied by differential scanning calorimetry. Based on the performed studies it was confirmed that OH
terminated dendrimers 2, 4, 6 and 8 exhibited liquid crystalline phases. Also, the X-ray powder diffraction
measurements were accomplished for the dendrimers having terminal hydroxyl groups.
© 2009 Elsevier B.V. All rights reserved.
Received 12 November 2008
Received in revised form 25 August 2009
Accepted 26 August 2009
Available online 3 September 2009
Keywords:
Polyester dendrimers
Thermal properties
TGA
DSC
1. Introduction
and co-workers [33] that alteration of the polarity of the ter-
minal groups has an influence on glass transition temperatures.
Dendrimers are highly branched macromolecules having well-
defined structures, which consist of a multivalent core, layers of ABx
branching monomers (generations), and defined number of termi-
nal groups [1–3]. Two common stepwise synthetic approaches have
been utilized to build up dendrimers in order to control the shape,
size, functionality, and, as a result, properties of the dendrimers.
In the divergent method [4–6] compounds are constructed genera-
tion by generation from the core to the periphery. In the convergent
manner [7,8] dendrimers are constructed from the periphery to the
core, by preparing initially dendrons of appropriate size, which are
attached to the core. Due to the versatile properties of dendrimers,
arising from the structural features, they are potential candidates
for various applications such as drug delivery systems [9,10], light
harvesting [11–13], and catalysis [14–17].
Since then, several groups have reported the influence of the end
group functionality [34–36] as well as other structural modifica-
tions [37,38] on the thermal properties of the dendritic molecules.
Percec et al. have extensively studied the self-assembling dendritic
molecules that exhibit thermotropic liquid-crystal phases [39–44].
Herein we report the synthesis of small amphiphilic Janus den-
drimers up to the second generation. Our interest was to study
the thermal behavior of Janus dendrimers having opposite ter-
minal groups of significantly different polarity. These dendrimers,
emanating from the pentaerythritol core, consist of bisMPA based
polyester branches, having either acetonide groups or hydroxyl
groups in the periphery, and opposite 3,4-dialkyloxybenzoic acids
containing alkyl chain ends of different length (C6 or C16).
Janus dendrimers, also called bow-tie or block co-dendrimers,
characterized by the diverse peripheral groups on opposite sides,
are relatively new class of dendrimers [18–27]. The interest
towards the dendritic Janus molecules stem from their bisfunc-
tional character, applicable in various purposes. Recent studies on
Janus dendrimers have, for example, demonstrated self-assembling
properties [21,28–30] as well as potential use in medical applica-
tions [31,32].
2. Experimental
2.1. Materials and instrumentations
All the starting materials were purchased from major suppliers
and used without any further purification. Dichloromethane (DCM)
was dried over 4 Å sieves. The pentaerythritol based core molecule,
(OH)2-PE-[G1]-acetonide, was synthesized according to published
procedure [27]. Isopropylidene-2,2-bis(hydroxymethyl)propionic
acid anhydride was prepared according to procedure described
by Malkoch et al. [45]. 3,4-Dihexyloxybenzoic acid and 3,4-
dihexadecyloxybenzoic acid was prepared by using a modified
literature procedure [46]. Column chromatography was performed
with Merck 60 F254 silica gel, particle size 0.040–0.063 mm. 1H
and 13C NMR spectra were recorded on a Bruker Avance DRX 500
Peripheral groups are primarily responsible for the thermal
properties of the dendrimers. It was first reported by Fréchet
ଝ
Janus dendrimers are C6 and C16 alkyl chain based polyester dendrimers.
Corresponding author. Fax: +358 14 260 2501.
E-mail address: kari.t.rissanen@jyu.fi (K. Rissanen).
∗
0040-6031/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2009.08.018

102
T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
J = 8.5, 1.9 Hz). 13C NMR (CDCl3, 126 MHz): ıppm = 13.97 (CH3), 22.57
(CH2CH3), 25.64 and 25.78 (CH2CH2CH2OAr), 29.05 and 29.15
(CH2CH2OAr), 31.55 and 31.58 (CH2CH2CH3), 69.01 and 69.23
(CH2OAr), 111.89 (ArC position 5), 114.59 (ArC position 2), 122.37
(ArC position 1), 124.24 (ArC position 6), 148.49 (ArC position 3),
153.57 (ArC position 4), 171.67 (CO). ESI TOF MS: m/z calcd. for
NMR (500.13 and 125.76 MHz) spectrometer in CDCl3 or DMSO-d6
solution. The solvent signal was used as an internal standard. Mass
spectral data was obtained with Micromass LCT Electronspray ion-
ization time-of-flight (ESI TOF) instrument with either positive-ion
or negative-ion mode. Thermal behavior of the compounds was
determined on power compensation type PerkinElmer PYRIS DIA-
MOND DSC. The measurements were carried out under nitrogen
atmosphere (flow rate 50 mL min−1) using 50 ␮L sealed aluminum
sample pans. The sealing was made by using a 30 ␮L aluminum
pan with capillary holes to ascertain good thermal contact between
a sample and pan, and to minimize free volume inside the pan.
The temperature calibration was made using two standard mate-
rials (n-decane, In) and energy calibration by an indium standard
(28.45 J g−1). Typically, following temperature profile was used for
each sample: a sample was heated from −40 to desired end tem-
perature (from 120 to 200 ◦C) with a heating rate of 10 ◦C/min,
followed by 1 min hold at the end temperature, and cooled down to
−40 ◦C with a rate of 10 ◦C/min. The sample was held at −40 ◦C for
5 min and heated for a second time, respectively. Sample weights
of 3–6 mg were used on the measurements. Finally, the sample
weight was checked afterwards to monitor weight losses that may
have occurred during the scans. The uncertainty for measured tem-
peratures was less than 0.8 ◦C for all measurements. The thermal
decomposition was examined with PerkinElmer TGA7 thermo-
gravimetric analyzer. Measurements were carried out in platinum
pans under synthetic air atmosphere (flow rate of 50 mL/min) with
heating rate of 10 ◦C/min on temperature range of 25–700 ◦C. To
improve determination of decomposition onset by removing sol-
vent/moisture residues from the samples, an isothermal step at
150 ◦C for 3 h was introduced to the heating profile. The temper-
ature calibration of instrument was carried out using Curie-point
calibration technique (Alumel, Ni, Perkalloy, Fe). The weight bal-
ance was calibrated by measuring the standard weight of 50 mg
at room temperature. The sample weights used in the measure-
ments were about 5–9 mg. The decomposition onset was obtained
using step tangent method at which 5% decomposition is occurred
(taken account the initial weight loss occurred at isothermal step).
The X-ray powder diffraction data was measured with PANalyt-
ical X’Pert PRO diffractometer in Bragg–Brentano geometry using
Johansson monochromator (␣1 setup) to produce pure CuK␣1 radi-
ation (1.5406 Å; 45 kV, 30 mA) and step-scan technique in 2ꢀ range
of 3–70◦. The data was collected by X’Celerator detector in con-
tinuous scanning mode with a step size of 0.017 and using sample
dependently counting times of 100 s per step. Programmable diver-
gence slit (PDS) was used in automatic mode to set irradiated length
on sample to 15 mm together with 15 mm incident beam mask.
Soller slits of 0.02◦ rad were used on both incident and diffracted
beam sides together with anti-scatter 4◦ and 13 mm, respectively.
Before further analyses, the diffraction data were converted from
automatic slit mode (ADS) to a fixed slit mode (FDS) data by the
tools implemented in Highscore Plus v. 2.2c software package.
Hand-ground powder samples were prepared on a silicon-made
zero-background holder using petrolatum jelly as an adhesive. Pre-
liminary studies with polarizing optical microscope (POM) were
accomplished to confirm the presence of mesomorphic phases.
More detailed characterization of the mesomorphic phases will be
presented elsewhere.
C
19H30O4 312.21 [M−H]−, found 312.19 [M−H]−.
2.2.2. 3,4-Hexadecyloxybenzoic acid
4.49 g (92%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.88 (t, 6H, CH3, J = 7.0 Hz), 1.26–1.36 (m, 48H, (CH2)12CH3),
1.45–1.51 (m, 4H, OCH2CH2CH2), 1.80–1.87 (m, 4H, OCH2CH2),
4.03–4.07 (m, 4H, OCH2), 6.89 (d, 1H, ArH position 5, J = 8.5 Hz),
7.58 (d, 1H, ArH position 2, J = 2.0 Hz), 7.71 (dd, 1H, ArH position
6, J = 8.5, 2.0 Hz). 13C NMR (CDCl3, 126 MHz): ıppm = 14.10 (alkyl
CH3), 22.69 (CH2CH3), 25.97 and 26.02 (CH2CH2CH2OAr), 29.07
and 29.19 (CH2CH2OAr), 29.36–29.71 ((CH2)10CH2CH2CH3), 31.93
(CH2CH2CH3), 69.09 and 69.34 (CH2OAr), 111.99 (ArC position 5),
114.72 (ArC position 2), 121.27 (ArC position 1), 124.44 (ArC posi-
tion 6), 148.63 (ArC position 3), 153.95 (ArC position 4), 170.12 (CO).
ESI TOF MS: m/z calcd. for C39H70O4 601.52 [M−H]−, found 601.50
[M−H]−.
2.2.3. 3,4-Bis-hexyloxybenzoic ester-PE-[G1]-acetonide (1)
(OH)2-PE-[G1]-acetonide
(1.50 g,
3.34 mmol),
3,4-
hexyloxybenzoic acid (2.37 g, 7.36 mmol) and DPTS (0.98 g,
3.34 mmol) were dissolved in dry CH2Cl2 (60 mL) and dry THF
(15 mL). The mixture was flushed with nitrogen (30 min), and DCC
(1.38 g, 6.69 mmol) added in the flask. The mixture was stirred at
room temperature for 48 h under nitrogen atmosphere. DCC-urea
was filtered off and washed with small amount of CH2Cl2. Solvent
was evaporated, and the crude product was purified by column
chromatography (SiO2) eluting with hexane/ethyl acetate (4:1)
to give 1.97 g (56%) of viscous oil. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.90 (t, 6H, CH3, J = 7.0 Hz), 0.91 (t, 6H, CH3, J = 7.0 Hz), 1.13
(s, 6H, bisMPA-CH3), 1.31 (s, 6H, acetonide-CH3), 1.33–1.36 (m,
16H, (CH2)2CH3), 1.40 (s, 6H, acetonide-CH3), 1.45–1.51 (m, 8H,
CH2CH2CH2OAr), 1.79–1.86 (m, 8H, CH2CH2OAr), 3.62 (d, 4H,
bisMPA-CH2, J = 11.9 Hz), 4.02 (t, 4H, CH2OAr, J = 6.6 Hz), 4.04 (t, 4H,
CH2OAr, J = 6.6 Hz), 4.17 (d, 4H, bisMPA-CH2, J = 11.9 Hz), 4.40 (s, 4H,
PE-CH2), 4.48 (s, 4H, PE-CH2), 6.83 (d, 2H, ArH position 5, J = 8.5 Hz),
7.49 (d, 2H, ArH position 2, J = 2.0 Hz), 7.58 (dd, 2H, ArH position
6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR (CDCl3, 126 MHz): ıppm = 13.94
and 13.96 (alkyl CH3), 18.41 (bisMPA-CH3), 21.77 (acetonide-CH3),
22.54 and 22.56 (CH2CH3), 25.36 (acetonide-CH3), 25.60 and 25.64
(CH2CH2CH2OAr), 28.99 and 29.13 (CH2CH2OAr), 31.51 and 31.55
(CH2CH2CH3), 42.25 (C-bisMPA), 43.31 (PE-C), 62.50 and 62.52
(PE-CH2), 66.03 (bisMPA-CH2), 69.02 and 69.30 (CH2OAr), 98.15
(acetonide-C), 112.00 (ArC position 5), 114.34 (ArC position 2),
121.61 (ArC position 1), 123.63 (ArC position 6), 148.70 (ArC
position 3), 153.57 (ArC position 4), 165.75 (CO), 173.66 (CO). ESI
TOF MS: m/z calcd. for C59H92O16 1079.63 [M+Na]+, found 1079.57
[M+Na]+. Anal. Calcd for C59H92O16: C 67.02%, H 8.77%. Found: C
66.85%, H 8.83%.
2.2.4. 3,4-Bis-hexyloxybenzoic ester-PE-[G1]-(OH)4 (2)
1 (1.75 g, 1.65 mmol) was dissolved in CH2Cl2 (15 mL) and
diluted with MeOH (15 mL). 2 teaspoons of Dowex 50W resin was
added, and the mixture stirred at 50 ◦C for 19 h. Resin was filtered
off, and washed with small amount of CH2Cl2. Filtrate was concen-
trated to give 1.55 g (96%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.90 (t, 6H, CH3, J = 7.0 Hz), 0.90 (t, 6H, CH3, J = 7.0 Hz), 1.08
(s, 6H, bisMPA-CH3), 1.33–1.35 (m, 16H, (CH2)2CH3), 1.46–1.51 (m,
8H, CH2CH2CH2OAr), 1.78–1.86 (m, 8H, CH2CH2OAr), 3.13 (s, 4H,
OH), 3.74 (d, 4H, bisMPA-CH2, J = 11.3 Hz), 3.86 (d, 4H, bisMPA-
CH2, J = 11.3 Hz), 4.01 (t, 4H, CH2OAr, J = 6.5 Hz), 4.03 (t, 4H, CH2OAr,
2.2. Synthesis
2.2.1. 3,4-Hexyloxybenzoic acid
4.64 g (97%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.88–0.93 (m, 6H, CH3), 1.33–1.35 (m, 8H, (CH2)2CH3),
1.41–1.50 (m, 4H, O(CH2)2CH2), 1.76–1.89 (m, 4H, OCH2CH2),
3.99–4.06 (m, OCH2), 6.84 (d, 1H, ArH position 5, J = 8.5 Hz), 7.56
(d, 1H, ArH position 2, J = 1.9 Hz), 7.67 (dd, 1H, ArH position 6,

T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
103
J = 6.5 Hz), 4.36 (s, 4H, PE-CH2), 4.50 (s, 4H, PE-CH2), 6.83 (d, 2H, ArH
position 5, J = 8.5 Hz), 7.49 (d, 2H, ArH position 2, J = 2.0 Hz), 7.59 (dd,
2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR (CDCl3, 63 MHz):
ıppm = 13.95 and 13.96 (alkyl CH3), 17.15 (bisMPA-CH3), 22.55 and
22.56 (CH2CH3), 25.61 and 25.66 (CH2CH2CH2OAr), 28.99 and 29.14
(CH2CH2OAr), 31.51 and 31.55 (CH2CH2CH3), 43.55 (PE-C), 49.92
(C-bisMPA), 61.77 and 62.06 (CH2-PE), 68.01 (bisMPA-CH2), 69.05
and 69.37 (CH2OAr), 111.99 (ArC position 5), 114.44 (ArC position
2), 121.37 (ArC position 1), 123.75 (ArC position 6), 148.74 (ArC
position 3), 153.79 (ArC position 4), 166.00 (CO), 175.15 (CO). ESI
TOF MS: m/z calcd. for C53H84O16 999.57 [M+Na]+, found 999.53
[M+Na]+. Anal. Calcd for C53H84O16: C, 65.14%, H 8.66%. Found: C
65.19%, H, 8.69%.
and 69.38 (CH2OAr), 111.99 (ArC position 5), 114.41 (ArC position
2), 121.23 (ArC position 1), 123.70 (ArC position 6), 148.74 (ArC
position 3), 153.84 (ArC position 4), 165.94 (CO), 172.57 (G1-CO),
175.01 (G2-CO). ESI TOF MS: m/z calcd. for C73H116O28 1463.76
[M+Na]+, found 1463.93 [M+Na]+. Anal. Calcd for C73H116O28: C
60.82%, H 8.12%. Found: C 60.73%, H 8.03%.
2.2.7. 3,4-Bis-hexadecyloxybenzoic ester-PE-[G1]-acetonide (5)
The procedure is the same as the synthesis of 1. (OH)2-PE-
[G1]-acetonide (1.00 g, 2.23 mmol), 3,4-hexadecyloxybenzoic acid
(2.96 g, 4.91 mmol), DPTS (0.66 g, 2.23 mmol), and DCC (1.20 g,
5.80 mmol) in CH2Cl2 (90 mL) was used to give 2.23 g (62%) of
white solid. 1H NMR (CDCl3, 500 MHz): ıppm = 0.87 (t, 12H, CH3,
J = 7.0 Hz), 1.13 (s, 6H, bisMPA-CH3), 1.26 (overlapped peaks, 96H,
(CH2)12CH3), 1.31 (s, 6H, acetonide-CH3), 1.41 (s, 6H, acetonide-
CH3), 1.44–1.50 (m, 8H, CH2CH2CH2OAr), 1.79–1.86 (m, 8H,
CH2CH2OAr), 3.63 (d, 4H, bisMPA-CH2, J = 11.9 Hz), 4.01 (t, 4H,
CH2OAr, J = 6.6 Hz), 4.03 (t, 4H, CH2OAr, J = 6.6 Hz), 4.17 (d, 4H,
bisMPA-CH2, J = 11.9 Hz), 4.40 (s, 4H, PE-CH2), 4.48 (s, 4H, PE-CH2),
6.83 (d, 2H, ArH position 5, J = 8.5 Hz), 7.49 (d, 2H, ArH position 2,
J = 2.0 Hz), 7.58 (dd, 2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR
(CDCl3, 126 MHz): ıppm = 14.08 (alkyl CH3), 18.44 (bisMPA-CH3),
21.80 (acetonide-CH3), 22.67 (CH2CH3), 25.37 (acetonide-CH3),
25.98 and 26.04 (CH2CH2CH2OAr), 29.08 and 29.23 (CH2CH2OAr),
29.35–29.70 ((CH2)10CH2CH2CH3), 31.92 (CH2CH2CH3), 42.27 (C-
bisMPA), 43.33 (PE-C), 62.53 (PE-CH2), 66.05 (bisMPA-CH2), 69.06
and 69.34 (CH2OAr), 98.17 (acetonide-C), 112.02 (ArC position 5),
114.38 (ArC position 2), 121.63 (ArC position 1), 123.65 (ArC posi-
tion 6), 148.73 (ArC position 3), 153.60 (ArC position 4), 165.77
(CO), 173.67 (CO). ESI TOF MS: m/z calcd. for C99H172O16 1641.26
[M+Na]+, found 1641.14 [M+Na]+. Anal. Calcd for C99H172O16: C
73.47%, H 10.71%. Found: C 73.20%, H 10.64%.
2.2.5. 3,4-Bis-hexyloxybenzoic ester-PE-[G2]-acetonide (3)
2 (0.93 g, 0.95 mmol), anhydride of bisMPA (1.63 g, 4.92 mmol)
and DMAP (69 mg, 0.75 mmol) were dissolved in pyridine (1.5 mL)
and CH2Cl2 (6 mL) and stirred at room temperature for 68 h. 1 mL
of water was added with vigorous stirring for 2 h to quench the
anhydride. The mixture was then diluted with 100 mL of CH2Cl2,
and washed with 10% NaHSO4 (3 × 50 mL), 10% Na2CO3 (3 × 50 mL),
and with brine (50 mL). Organic layer was dried over anhydrous
MgSO4 and solvent evaporated. The product was further purified by
column chromatography (SiO2) eluting with hexane/ethyl acetate
(3:2) to give 1.16 g (76%) of glassy solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.90 (t, 6H, CH3, J = 7.0 Hz), 0.91 (t, 6H, CH3, J = 7.0 Hz), 1.09
(s, 12H, G2-CH3), 1.28 (s, 6H, G1-CH3), 1.32 (s, 12H, acetonide-CH3),
1.33–1.38 (overlapped peaks, 28H, (CH2)2CH3 + acetonide-CH3),
1.45–1.51 (m, 8H, CH2CH2CH2OAr), 1.79–1.86 (m, 8H, CH2CH2OAr),
3.56 (dd, 8H, G2-CH2, J = 11.9 Hz, J = 2.1 Hz), 4.01 (t, 4H, CH2OAr,
J = 6.6 Hz), 4.03 (t, 4H, CH2OAr, J = 6.6 Hz), 4.11 (d, 8H, G2-CH2,
J = 11.8 Hz), 4.33 (ABq, 8H, G1-CH2, J = 11.1 Hz), 4.34 (s, 4H, PE-
CH2), 4.45 (s, 4H, PE-CH2), 6.83 (d, 2H, ArH position 5, J = 8.5 Hz),
7.48 (d, 2H, ArH position 2, J = 2.0 Hz), 7.56 (dd, 2H, ArH posi-
tion 6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR (CDCl3, 63 MHz): ıppm = 13.96
and 13.98 (alkyl CH3), 17.65 (G1-CH3), 18.45 (G2-CH3), 22.02
(acetonide-CH3), 22.55 and 22.57 (CH2CH3), 25.11 (acetonide-CH3),
25.62 and 25.67 (CH2CH2CH2OAr), 29.01 and 29.16 (CH2CH2OAr),
31.52 and 31.57 (CH2CH2CH3), 42.03 (G2-C), 43.11 (PE-C), 47.07
(G1-C), 62.21 and 63.04 (PE-CH2), 64.86 (G1-CH2), 65.90 and 65.94
(G2-CH2), 69.05 and 69.32 (CH2OAr), 98.07 (acetonide-C), 111.99
(ArC position 5), 114.36 (ArC position 2), 121.49 (ArC position
1), 123.59 (ArC position 6), 148.75 (ArC position 3), 153.65 (ArC
position 4), 165.59 (CO), 172.01 (G1-CO), 173.44 (G2-CO). ESI TOF
MS: m/z calcd. for C85H132O28 1623.88 [M+Na]+, found 1624.02
[M+Na]+. Anal. Calcd for C85H132O28: C 63.73%, H 8.31%. Found: C
63.73%, H 8.17%.
2.2.8. 3,4-Bis-hexadecyloxybenzoic ester-PE-[G1]-(OH)4 (6)
The procedure is the same as the synthesis of 2. Compound 5
(2.03 g, 1.25 mmol) and teaspoons of Dowex 50W resin was used
to give 1.86 g (97%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.86 (t, 12H, CH3, J = 7.0 Hz), 1.08 (s, 6H, bisMPA-CH3),
1.26 (overlapped peaks, 96H, (CH2)12CH3), 1.44–1.50 (m, 8H,
CH2CH2CH2OAr), 1.79–1–86 (m, 8H, CH2CH2OAr), 3.07 (s, 4H,
OH), 3.75 (d, 4H, bisMPA-CH2, J = 11.3 Hz), 3.87 (d, 4H, bisMPA-
CH2, J = 11.3 Hz), 4.01 (t, 4H, CH2OAr, J = 6.6 Hz), 4.03 (t, 4H,
CH2OAr, J = 6.6 Hz), 4.36 (s, 4H, PE-CH2), 4.50 (s, 4H, PE-CH2), 6.83
(d, 2H, ArH position 5, J = 8.5 Hz), 7.49 (d, 2H, ArH position 2,
J = 2.0 Hz), 7.59 (dd, 2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz). 13C
NMR (CDCl3, 126 MHz): ıppm = 14.08 (alkyl CH3), 17.16 (bisMPA-
CH3), 22.67 (CH2CH3), 25.98 and 26.04 (CH2CH2CH2OAr), 29.07
and 29.23 (CH2CH2OAr), 29.35–29.70 ((CH2)10CH2CH2CH3), 31.92
(CH2CH2CH3), 43.58 (PE-C), 49.91 (C-bisMPA), 61.76 and 62.04
(PE-CH2), 68.13 (bisMPA-CH2), 69.07 and 69.40 (CH2OAr), 111.99
(ArC position 5), 114.46 (ArC position 2), 121.37 (ArC position
1), 123.75 (ArC position 6), 148.76 (ArC position 3), 153.81 (ArC
position 4), 166.00 (CO), 175.16 (CO). ESI TOF MS: m/z calcd. for
C93H164O16 1561.20 [M+Na]+, found 1561.26 [M+Na]+. Anal. Calcd
for C93H164O16·1/2 H2O: C 72.19%, H 10.75%. Found: C 72.29%. H
10.75%.
2.2.6. 3,4-Bis-hexyloxybenzoic ester-PE-[G2]-(OH)8 (4)
The procedure is the same as the synthesis of 2. Compound
3 (0.99 g, 0.62 mmol) and 2 teaspoons of Dowex 50W were used
to give 0.89 g (98%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.90 (t, 6H, CH3, J = 7.0 Hz), 0.91 (t, 6H, CH3, J = 7.0 Hz),
1.04 (s, 12H, G2-CH3), 1.28 (s, 6H, G1-CH3), 1.33–1.36 (m, 16H,
(CH2)2CH3), 1.44–1.50 (m, 8H, CH2CH2CH2OAr), 1.78–1.86 (m, 8H,
CH2CH2OAr), 3.26 (s, 8H, OH), 3.67 (dd, 8H, bisMPA-CH2, J = 11.3 Hz,
J = 4.8 Hz), 3.77 (dd, 8H, bisMPA-CH2, J = 11.2 Hz, J = 3.5 Hz), 4.00 (t,
4H, CH2OAr, J = 6.6 Hz), 4.03 (t, 4H, CH2OAr, J = 6.6 Hz), 4.34 (ABq,
8H, G1-CH2, J = 11.1 Hz), 4.37 (s, 4H, PE-CH2), 4.47 (s, 4H, PE-CH2),
6.83 (d, 2H, ArH position 5, J = 8.5 Hz), 7.48 (d, 2H, ArH position 2,
J = 2.0 Hz), 7.56 (dd, 2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR
(CDCl3, 63 MHz): ıppm = 13.94 and 13.96 (alkyl CH3), 17.08 (G2-
CH3), 17.96 (G2-CH3), 22.53 and 22.55 (CH2CH3), 25.60 and 25.65
(CH2CH2CH2OAr), 28.99 and 29.14 (CH2CH2OAr), 31.50 and 31.54
(CH2CH2CH3), 42.88 (PE-C), 46.83 (G1-C), 49.80 (G2-C), 62.60 and
63.62 (PE-CH2), 64.85 (G1-CH2), 67.22 and 67.34 (G2-CH2), 69.05
2.2.9. 3,4-Bis-hexadecyloxybenzoic ester-PE-[G2]-acetonide (7)
The procedure is the same as the synthesis of 3. Compound
6 (1.23 g, 0.80 mmol), anhydride of bisMPA (1.38 g, 4.17 mmol),
and DMAP (59 mg, 0.48 mmol) in pyridine (1.5 mL) and CH2Cl2
(6 mL) were used. Purified by column chromatography (SiO2) elut-
ing with hexane/ethyl acetate (2:1) to give 1.53 g (89%) of white
solid. 1H NMR (CDCl3, 500 MHz): ıppm = 0.88 (t, 12H, CH3, J = 7.0 Hz),
1.09 (s, 12H, G2-CH3), 1.26 (overlapped peaks, 96H, (CH2)12CH3)),
1.28 (s, 6H, G1-CH3), 1.32 (s, 12H, acetonide-CH3), 1.36 (s, 12H,

104
T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
13C NMR (CDCl3, 126 MHz): ıppm = 14.09 (alkyl CH3), 17.11 (G2-
CH3), 22.68 (CH2CH3), 26.00 and 26.07 (CH2CH2CH2OAr), 29.09
and 29.26 (CH2CH2OAr), 29.36–29.71 ((CH2)10CH2CH2CH3), 31.92
(CH2CH2CH3), 42.88 (PE-C), 46.89 (G1-C), 49.78 (G2-C), 62.66 and
63.72 (PE-CH2), 64.93 (G1-CH2), 67.65 and 67.78 (G2-CH2), 69.09
and 69.45 (CH2OAr), 112.00 (ArC position 5), 114.46 (ArC position
2), 121.26 (ArC position 1), 123.72 (ArC position 6), 148.79 (ArC
position 3), 153.88 (ArC position 4), 165.97 (CO), 172.59 (G1-CO),
175.06 (G2-CO). ESI TOF MS: m/z calcd. for C113H196O28 2025.38
[M+Na]+, found 2025.45 [M+Na]+. Anal. Calcd for C113H196O28·3/2
H2O: C 66.87%, H 9.88%. Found: C 66.90, H 9.76.
acetonide-CH3), 1.43–1.50 (m, 8H, CH2CH2CH2OAr), 1.78–1.86 (m,
8H, CH2CH2OAr), 3.56 (dd, 8H, G2-CH2, J = 11.9 Hz, J = 2.1 Hz), 4.01
(t, 4H, CH2OAr, J = 6.6 Hz), 4.03 (t, 4H, CH2OAr, J = 6.6 Hz), 4.11
(d, 8H, G2-CH2, J = 11.7 Hz), 4.34 (ABq, 8H, G1-CH2, J = 11.2 Hz),
4.34 (s, 4H, PE-CH2), 4.45 (s, 4H, PE-CH2), 6.82 (d, 2H, ArH posi-
tion 5, J = 8.5 Hz), 7.48 (d, 2H, ArH position 2, J = 2.0 Hz), 7.55
(dd, 2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz). 13C NMR (CDCl3,
63 MHz): ıppm = 14.08 (alkyl CH3), 17.66 (G1-CH3), 18.46 (G2-CH3),
22.05 (acetonide-CH3), 22.67 (CH2CH3), 25.10 (acetonide-CH3),
26.00 and 26.07 (CH2CH2CH2OAr), 29.10 and 29.26 (CH2CH2OAr),
29.34–29.72 ((CH2)10CH2CH2CH3), 31.92 (CH2CH2CH3), 42.03 (G2-
C), 43.12 (PE-C), 47.08 (G1-C), 62.29 and 63.05 (PE-CH2), 64.87
(G1-CH2), 65.91 and 65.95 (G2-CH2), 69.07 and 69.36 (CH2OAr),
98.07 (acetonide-C), 112.00 (ArC position 5), 114.40 (ArC position
2), 121.51 (ArC position 1), 123.59 (ArC position 6), 148.78 (ArC
position 3), 153.67 (ArC position 4), 165.59 (CO), 172.01 (G1-CO),
173.43 (G2-CO). ESI TOF MS: m/z calcd. for C125H212O28 2158.51
3. Results and discussion
3.1. Synthesis
The prepared first generation (G1) Janus dendrimers are shown
in Scheme 1. Dendrimers were built around pentaerythritol (PE)
by attaching initially the first layer of dendritic polyester moieties
on the other side, and then 3,4-dialkylbenzoic acid moieties on the
opposite side of the pentaerythritol.
The preparation of the half protected first generation dendritic
core, (OH)2-PE-[G1]-acetonide, has been described previously by
our group [27]. Monodendrons, 3,4-dihexyloxybenzoic acid and
3,4-dihexadecyloxybenzoic acid, were obtained from etherifica-
tion of methyl 3,4-dihydroxybenzoate with corresponding n-alkyl
bromide in DMF, followed by the hydrolysis of ester group with
KOH in refluxing EtOH/THF solution [46]. Monodendrons were
attached to the dendritic core using DCC esterification in the pres-
ence of 4-(dimethylamino)pyridinium p-toluenesulfonate (DPTS)
[47] as a catalyst, giving compounds 1 and 5 in 51 and 62% yields,
respectively, after chromatographic purification. Deprotection of
the isopropylidene groups of the compounds 1 and 5 were done by
[M+Na]+, found 2185.71 [M+Na]+. Anal. Calcd for C125
69.41%, H 9.88%. Found: C 69.65%, H 9.90%.
H212O28: C
2.2.10. 3,4-Bis-hexadecyloxybenzoic ester-PE-[G2]-(OH)8 (8)
The procedure is the same as the synthesis of 2. 7 (1.13 g,
0.52 mmol) and
2 teaspoons of Dowex 50W were used to
give 0.76 g (73%) of white solid. 1H NMR (CDCl3, 500 MHz):
ıppm = 0.88 (t, 12H, CH3, J = 7.0 Hz), 1.04 (s, 12H, G2-CH3) (over-
lapped peaks, 96H, (CH2)12CH3)), 1.29 (s, 6H, G1-CH3), 1.44.1.50
(m, 8H, CH2CH2CH2OAr), 1.78–1.85 (m, 8H, CH2CH2OAr), 3.13 (s,
8H, OH), 3.68 (dd, 8H, bisMPA-CH2, J = 11.3 Hz, J = 4.8 Hz), 3.80
(dd, 8H, bisMPA-CH2, J = 11.2 Hz, J = 3.5 Hz), 4.00 (t, 4H, CH2OAr,
J = 6.6 Hz), 4.03 (t, 4H, CH2OAr, J = 6.6 Hz), 4.35 (ABq, 8H, G1-
CH2, J = 11.1 Hz), 4.37 (s, 4H, PE-CH2), 4.47 (s, 4H, PE-CH2), 6.83
(d, 2H, ArH position 5, J = 8.5 Hz), 7.48 (d, 2H, ArH position
2, J = 2.0 Hz), 7.56 (dd, 2H, ArH position 6, J = 8.5 Hz, J = 2.0 Hz).
Scheme 1. The first generation Janus dendrimers.

T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
105
Scheme 2. The second generation Janus dendrimers.
DOWEX 50W resin in the DCM/MeOH solution at 50 ◦C. Compounds
were stirred with the resin until deprotection was complete. The
resin was filtered, and the filtrate concentrated to give the first
generation hydroxyl terminated dendrimers 2 and 6 in 96 and 97%
yields, respectively.
were completely decomposed at around 600 ◦C. The Td-values of
6 dendrimers slightly increased with increasing molecular weight
C
by additional generation. In addition, hydroxyl terminated den-
drimers displayed higher Td-values than corresponding acetonide
protected dendrimers, probably on account of hydrogen bonding
interactions. These results correlate with the previously reported
results of thermal behavior of bisMPA based polyester dendrimers
[37]. On the other hand, all the Td-values of C16 dendrimers were
around 250 ◦C, showing little or no effect of the size or the ter-
minal groups of the polyester moieties on thermal stability of the
dendrimers. The higher Td-values of the C6 alkylated dendrimers
are most likely caused by more dense packing of the dendrimers
with shorter alkyl chains, allowing stronger bonding interactions
(hydrogen bonding and weak ␲–␲-interactions) of the dendritic
moieties. Packing in the case of C16 alkylated dendrimers is proba-
bly more disordered due to the entanglement of the alkyl chains.
The second generation (G2) dendrimers (Scheme 2) were pre-
pared by divergent method utilizing the anhydride coupling.
Hydroxyl terminated dendrimers 2 and 6 were allowed to react
with isopropylidene-2,2-bis(methoxy)propionic anhydride [45] in
DCM in the presence of 4-dimethylaminopyridine (DMAP) and
pyridine. The crude product was dissolved in DCM followed by
extraction with 10% NaHSO4, 10% Na2CO3 and brine. Finally, purifi-
cation by column chromatography gave compounds 3 and 7 in
76 and 89% yields, respectively. The preparation of the hydroxyl
terminated second generation dendrimers were performed in the
presence of DOWEX resin as described above, giving compounds 4
and 8 in 98 and 73% yields, respectively. Hydroxyl terminated com-
pounds 2, 4, 6, and 8 were further recrystallized from the solution
of ethyl acetate and hexane.
3.2. Thermogravimetric studies
Thermogravimetric analysis (TGA) was utilized to evaluate the
thermal stability of the dendrimers. The onset decomposition tem-
peratures (Td) were recorded at a heating rate of 10 ◦C/min. An
isothermal step at 150 ◦C for 3 h was included to the heating profile,
to remove the traces of solvent/water. Mass losses on the isother-
mal step ranged from 0.2 to 1.7%. Since the decomposition of the
dendrimers started relatively slowly, step tangent method at 5-wt%
mass loss was utilized, in order to avoid overestimating the ther-
mal stability. TGA curves are shown in Fig. 1, and decomposition
temperatures collected in Table 1.
The Td-values of C6 alkylated dendrimers ranged from 268 to
Fig. 1. TG curves of acetonide protected dendrimers 1, 3, 5, and 7 (upper), and
hydroxyl terminated dendrimers 2, 4, 6, and 8 (lower).
308 ◦C, indicating fairly high thermal stability, and compounds

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T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
Table 1
Thermal properties of compounds 1–8.
Compound Mw
Thermal transitions (◦C) and their corresponding enthalpy
DSC range of
scan (◦C)
TGA decomposition
temperature (◦C)b
and/or heat capacity changes (kJ mol−1) [kJ mol−1 ◦C−1
]
a
Heating scans
Cooling scans
1st: –
1
5
2
6
3
7
4
8
1057.38
1st: g −17.8 [0.40] i
2nd: g −17.6 [0.38] i
−40 to 160
−40 to 120
−40 to 160
−40 to 140
−40 to 150
−40 to 150
−40 to 200
−40 to 200
268 (0.25)c
250 (0.22)
295 (0.26)
254 (0.71)
289 (1.74)
241 (0.85)
308 (0.30)
253 (1.37)
1618.47
977.25
1st: k 52.0 (135.61) i
2nd: k 50.8 (107.58) i
1st: i 44.9 (−105.82) k
1st: k 71.7 (60.64) i
2nd: g 7.6 [0.28] LC 33.7 (0.81) i
1st: i 31.4 (−0.79) LC 7.4 [0.13] g
1st: i 113.1 (−2.43) LC 53.5 (−78.80) k
1st: –
1538.33
1601.98
2163.07
1441.72
2002.81
1st: k 65.6 (128.52) LC 114.1 (2.26) i
2nd: k 62.9 (82.11) LC 114.3 (2.37) i
1st: k 38.1 (52.61) i
2nd: g −8.6 [0.60] i
1st: k 44.5 (131.59) i
2nd: k 43.3 (106.31) i
1st: i 36.7 (−110.99) k
d
1st: k {72.9 (26.90), 104.9 (−3.71)} 115.0 (54.78) i
1st: i 62.5 (−4.63) LC 22.5 [0.20] g
2nd: g 26.3 [0.51], 46.8 (−30.15) k 109.7 (56.32) i
e
1st: k {67.9 (103.01), 71.3 (−7.93), 73.4 (25.91)} 110.3
1st: i 139.8 (−1.17) LC 62.3 (−71.43) k
2nd: i 139.6 (−1.18) LC 62.3 (−79.92) k
(62.81) LC 140.6 (1.06) i
f
2nd: k {67.4 (75.28), 73.0 (−68.32)} 111.0 (57.21) LC 140.4
(1.15) i
f
3rd: k {67.6 (75.20), 74.2 (−43.62)} 110.1 (57.00) LC 140.2
(1.07) i
Abbreviations: k, crystalline; i, isotropic liquid; g, glass; LC, liquid-crystal phase. Crystallization peak is prolonged close to the melting transition of a second polymorph at
111 ◦C; no weight loss was observed by TG/DTA at given temperature range.
a
The glass transition temperatures are taken as half ꢁCp extrapolated.
Decomposition temperatures determined from onset at 5 wt% mass loss.
ꢁwt% observed on isothermal step.
Two broad overlapping endothermic transitions followed by weak exothermic transition.
Overlapping transitions in order of endo-exo-endo; no weight loss was observed by TG/DTA at given temperature range.
Subsequent melting-crystallization transitions.
b
c
d
e
f
3.3. Differential scanning calorimetry studies
0.06–1.2 wt% due to removal of water/solvent correlate with the
mass losses observed in isothermal steps that was implemented in
TGA measurements.
The DSC curves of the hydroxyl terminated G1 dendrimers 2 and
6 are shown in Fig. 2. In the initial heating scan compound 2 shows
only melting transition (Tm) to isotropic liquid at 71.1 ◦C. However
during the cooling scan, compound 2 shows weak exothermic peak
at 31.4 ◦C before entering to a glassy state at about 8 ◦C. Further-
more, during the second heating scan Tg is observed again at 7.6 ◦C
Differential scanning calorimetry (DSC) studies were done
to determine the transition temperatures of the dendrimers
with heating and cooling rates of 10 ◦C/min. DSC curves of the
dendrimers are illustrated in Figs. 2–4, and results from DSC mea-
surements are summarized in Table 1. The weight of each sample
was measured after the last DSC scan, and the weight losses of
Fig. 2. DSC curves of the OH terminated G1 dendrimers. Black: first heating, red:
cooling, and green: second heating. To enhance visibility of e.g. glass transitions, the
y-axis scales are different for each compound (similarly in Figs. 3 and 4) 2 and 6. (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.)
Fig. 3. DSC curves of the OH terminated G2 dendrimers. Black: first heating, red:
cooling, and green: second heating. (For interpretation of the references to color in
this figure legend, the reader is referred to the web version of the article.)

T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
107
sitions. As seen in Table 1, the additional runs correlate well with
the previous runs, signifying thermally controllable phase transi-
tion behavior.
Melting transitions of the hydroxyl terminated G2 dendrimers,
4 and 8, were about 45 ◦C higher than those of the corresponding
G1 dendrimers 2 and 6. An increase of about 25 ◦C was observed in
isotropisation temperature of compound 8 in relative to compound
6. In addition, since enthalpy changes (ꢁH) of the compounds
exhibiting transitions from LC phase to isotropic liquid (compounds
2, 6 and 8) were about 0.8, 2.4, 1.8 kJ mol−1, respectively, it leads to
a conclusion that the LC phases could be nematic.
Thermal behavior of acetonide terminated dendrimers were
also studies (Fig. 4). During the first heating scans, endothermic
peaks corresponding to melting transition were observed for all the
protected dendrimers, except for 1, for which only Tg at −17.8 ◦C
was found. In the cooling run, Tcs were observed for C16 dendrimers
5 and 7. In the subsequent second heating run of dendrimers 5 and
7, corresponding melting transition were found again. In the case
of C6 dendrimers 1 and 3, glass transitions were the only detectable
phase transitions in the subsequent cooling and heating runs.
All the C6 alkylated dendrimers exhibited glass transitions in
the second heating run. The glass transition temperatures of the
C6 dendrimers increased with additional generation. In addition,
hydroxyl terminated C6 dendrimers showed higher Tg values than
the corresponding acetonide protected dendrimers. The result is in
good agreement with the previous studies of polyester dendrimers
[37] and polyether dendrimers [33], demonstrating that increasing
polarity of the end groups as well as branching increases glass tran-
sition temperature. However, none of the C16 dendrimers showed
Tg at the temperature range of the measurements, but distinct melt-
ing transitions due to the crystalline feature of the long alkyl chain.
The result is comparable with the study of Ihre et al. who reported
the impact of the linear hydrocarbon chain ends on the crystalliza-
tion of the otherwise amorphous polyester dendrimers [34].
Fig. 4. DSC curves of the acetonide protected G1 and G2 dendrimers. Black: first
heating, red: cooling, and green: second heating. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of the article.)
but instead of entering to isotropic state dendrimer turns to LC
phase (confirmed by POM), as sharp endothermic indicating clear-
ing point is observed again at ∼33.7 ◦C. It is rather unusual that
glass transition is followed by LC phase but few examples can be
found in the literature for example on monodendritic crown ethers
[48,49].
The initial heating scan of compound 6 exhibit melting transi-
tion at 65.6 ◦C, and weak endothermic peak at 114.1 ◦C signifying
isotropisation temperature (Ti). During the cooling scan two
exothermic peaks were observed—weak peak at 113.1 ◦C indicating
the transition from isotropic liquid phase to LC phase, followed by
crystallization transition (Tc) at 53.5 ◦C to a solid state. Reversible
thermal behavior is shown during the second heating scan, as tran-
sitions to LC phase and isotropic liquid were observed again.
The hydroxyl terminated G2 dendrimers 4 and 8 exhibited more
complex thermal behavior in the DSC measurements (Fig. 3). In
the initial heating scan compound 4 shows two broad overlapping
endothermic transitions starting at 72.9 ◦C, followed by a weak
exothermic transition at 104.9 ◦C, and finally, melting transition
at 115.0 ◦C. These endotherms are apparently from the melting
and crystallization events of different polymorph. In the cooling
sequence, compound exhibit exothermic peak at 62.5 ◦C above
glass transition. The second heating scan shows the glass transition,
followed by broad exothermic peak at 46.8 ◦C indicating rearrange-
ment into a crystalline phase (cold crystallization), and subsequent
melting transition to isotropic liquid at 109.7 ◦C. Additional scans
revealed that exothermic transition at around 62 ◦C on cooling,
regardless of cooling rate, was reproducible indicating monotropic
liquid crystal forming behavior. On the subsequent heating scans
LC phase is absent due to cold crystallization occurring on a same
temperature range.
3.4. X-ray powder diffraction studies
Since the hydroxyl terminated dendrimers (2, 4, 6, and 8)
showed phase transitions in DSC measurements, their crystallinity
was also evaluated by X-ray powder diffraction studies at room
temperature. The results are illustrated in Fig. 5. Even though the
DSC measurements show apparent melting transitions for all the
hydroxyl terminated dendrimers, the first generation C6 dendrimer
2 was the only compound having diffraction pattern of sharp peaks,
and thus, can be considered to be crystalline. However in the case
In the case of compound 8, initial heating scan shows over-
lapping sequence of endothermic and exothermic transitions
indicating melting-crystallizing behavior, followed by phase tran-
sition to LC phase at 110.3 ◦C, and clearing point at 140.6 ◦C. In the
cooling scan, two peaks were observed—transition from isotropic
liquid to LC phase, and subsequent crystallization at 62.3 ◦C. During
the second heating, two melting transition were found indicat-
ing the presence of polymorphism. The first melting transition at
67.4 ◦C was followed by exothermic peak at 73.0 ◦C representing
rearrangement to another crystalline phase (polymorph). Subse-
quent phase transitions – Tm at 111.0 ◦C and Ti at 140.2 ◦C – are
consistent with the first heating scan. Additional cooling-heating
runs were recorded for compound 8, in order to confirm phase tran-
Fig. 5. X-ray powder diffraction patterns of OH terminated dendrimers 2, 4, 6 and
8.

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T. Tuuttila et al. / Thermochimica Acta 497 (2010) 101–108
of compounds 4, 6, and 8, diffraction patterns show only a few
broad but intensive peaks. On the basis of the diffraction patterns
and sharp melting transitions in the DSC measurements, it seems
probable that the at least dendrimers 4 and 6 may assemble into
two-dimensional (layered) structures.
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Janus-type polyester dendrimers up to the second generation
were synthesized in high yields and their thermal behavior was
evaluated. Phase transitions of both acetonide and hydroxyl ter-
minated dendrimers were determined with DSC measurements.
All the compounds, except 1, had melting transition during the
first heating. All the C16 dendrimers (5–8) displayed both crys-
tallization and melting transitions on subsequent cooling-heating
scans, providing higher crystalline character over the dendrimers
of the C6 series. Both hydroxyl terminated C16 dendrimers (4
and 8) exhibited enantiotropic LC phases in subsequent heating-
cooling-heating scans. Upon first heating, dendrimers 2 and 6 of
the C6 series exhibited only melting transitions, while on sub-
sequent cooling both compounds showed LC phases below their
melting transitions. In case of the dendrimer 2 the LC phase was
reversible, thus obtained on both heating and cooling scans, while
compound 6 exhibits mesomorphic transition only on cooling.
Both melting and isotropisation temperatures increased along with
an increase hydroxyl groups introduced by the second genera-
tion. The decomposition temperatures varied between 241 and
308 ◦C. The C6 dendrimers had somewhat higher thermal stability
over the corresponding C16 dendrimers. In case of the C6 alky-
lated dendrimers, additional generation slightly increased their
thermal stability. In addition, inside this group OH terminated
dendrimers displayed slightly higher decomposition tempera-
tures over the corresponding acetonide terminated ones. This
is assumed to stem from hydrogen bonding interactions. Simi-
lar behavior was not observed for the C16 alkylated dendrimers,
as all the compounds started to decompose at about the same
temperature.
Acknowledgements
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We thank Mr. Reijo Kauppinen for his help in running the NMR
spectra, Ms. Mirja Lahtiperä for her assistance with the ESI TOF-MS
spectra, and Ms. Elina Hautakangas for the elemental analysis data.
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