Dendrimer-like Star Polymers
J. Am. Chem. Soc., Vol. 120, No. 19, 1998 4645
105, 106 Å) were used with THF as solvent at 25 °C. Poly(styrene)
been used to simplify the purification of intermediate generations
of dendrimers.12 Dendritic macromolecules have also been
constructed by successive grafting of polymeric building blocks.
These so-called comb-burst polymers utilize narrow polydis-
persity linear polymers as building blocks in the construction
of highly and randomly branched structures.13 For example,
Gnanou et al.14 prepared a branched poly(ethylene oxide) by
iterative AB2 functionalization and polymerization from an
hydroxy-functional polymer. With this method it was possible
to prepare polymers with a more or less exact size and
functionality in up to three generations and six end-groups.
standard samples were used for calibration.
Calculation of the Degree of Polymerization by the Use of H
1
NMR. The 1H NMR spectra of G-1(6 OH), G-2(12 OH), and G-3(24
OH) were used to calculate the degree of polymerization (DP) of the
different generations of the synthesized polymers and hence also the
molecular weights of these polymers. The DP of G-1(6 OH) was
calculated as the ratio of the integrated area of the peak corresponding
to one of the repeating units of PCL (C, Figure 1) to the integrated
area of the peak originating from the chain ends (a, Figure 1). The
DP of the second generation (DP2) and the third generation (DP3) were
then calculated from the following equation
A new type of well-defined dendritic macromolecules is
reported here. The new polymers, Scheme 1, are first, second,
and third generation dendrimer-like poly(ꢀ-caprolactone) syn-
thesized by a divergent approach using repetitive living ring-
opening polymerization (ROP), followed by functionalization
and deprotection of an AB2 branching-point, Scheme 2. The
concept was developed in order to obtain structures that combine
classical properties of linear polymers such as entanglements
and crystallinity with those of dendritic macromolecules, in
particular, the high functionality resulting from the many end-
groups.3 The novel polymers reported here are different from
classical dendrimers because a high molecular weight material
is obtained in only a few steps. In addition, the new approach
is flexible since the degree of polymerization (DP) as well as
the monomer selection allows one to tailor-design a wide variety
of properties. Poly(ꢀ-caprolactone) was used as the polymeric
building block, a concept that has been reported earlier for the
generation of hyperbranched polymers.7h The “living” ROP
produces polymers with accurate control of molecular weight,
molecular weight distribution, and end-group functionality. The
choice of 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) as
the branching point was based both on its aliphatic structure as
well as its documented use as building block in the synthesis
of dendritic macromolecules.4e,7c,h,15 Furthermore, the possibility
exists of developing semicrystalline morphologies.
x ) aDP1 + bDP2 + cDP3...
(1)
where x is the summed integrated area of any specific repeating unit
of the monomer for all generations in the specific polymer, and a, b,
and c are the integrated areas of the chain ends of the first, second,
and third generations, respectively. To calculate the DP of the second
and third generation, it was assumed that the DP of the inner generations
were unchanged and that all chain ends had initiated polymerization.
These assumptions lead to the relation, a ) b/2 ) c/4. In addition, it
was assumed that the protons of the chain ends and the protons of the
repeating units have equivalent relaxation times (τ). The results of
these calculations are listed in Table 1.
Synthesis. 2,2-Bis(phenyldioxymethyl)propionic Acid (2). 2,2-
Bis(hydroxymethyl)propionic acid (bis-MPA) (25.0 g, 187 mmol),
benzaldehyde dimethyl acetal (42.6 g, 280 mmol), and p-toluenesulfonic
acid (p-TSA) (0.69 g) were all dissolved and stirred in acetone (100
mL) at ambient temperature. After 2 h, a few drops of NH4OH (aq,
30%)/EtOH (1:1) solution was added to neutralize the p-TSA. The
reaction mixture was then diluted with 400 mL of CH2Cl2 and extracted
once with H2O (25 mL). The organic phase was separated, filtered,
and concentrated. The residue was recrystallized from CH2Cl2 to give
the product as a white powder. Yield: 37.3 g (90%). Mp: 197-198
°C. 1H NMR (CDCl3) δ 1.09 (s, 3H, -CH3), 3.66-4.64 (2d, 4H,
-(CH2O)2CH-, J ) 9.0 Hz), 5.46 (s, 1H, -CHPh), 7.31-7.47 (m,
5H, -Ph). 13C NMR (DMSO-d6) δ 22.75, 46.73, 77.81, 105.52, 131.25,
133.17, 133.85, 143.56, 180.71.
G-1(6 OH) and a General Procedure for Poly(E-caprolactone)
Formation. The hexahydroxy-functional initiator 1 (3.50 g, 5.35 mmol)
was dissolved in dry THF and dried with MgSO4. The liquid was
filtered into the reaction flask where the solvent was evaporated under
a nitrogen atmosphere. Toluene (3 mL) was added, and the temperature
was raised to 90 °C under high vacuum to remove the toluene and
residual H2O. The reaction flask was then filled with N2(g) and toluene
(1.5 mL) to dissolve the initiator. ꢀ-Caprolactone (73.2 g, 642 mmol)
was added, and the temperature was raised to 110 °C before a catalytic
amount of Sn(Oct)2 (32 mg, 0.08 mmol) was added. The molar amount
of catalyst is 1/400 of the initiator. The reaction was stirred for 24 h,
diluted with THF, and precipitated into MeOH to give 70.0 g (yield:
91.2%) of a white crystalline powder. Mp: 49.9 °C. 1H NMR (CDCl3)
δ 1.30 (m, poly, -CH2-), 1.60 (m, poly, -CH2CH2CH2-), 2.26 (t,
poly, -COCH2-, J ) 5.2 Hz), 3.60 (t, 12H, -CH2OH, J ) 5.0 Hz),
4.01 (t, poly, -CH2O-, J ) 5.2 Hz), 4.31 (s, 12H, -CCH3(CH2O)2-
), 6.88 (dd, 6H, Ph-, J ) 6.9 Hz), 7.06 (dd, 6H, Ph-, J ) 6.9 Hz).
13C NMR (CDCl3) δ 17.66, 24.43, 25.39, 28.20, 32.20, 33.96, 46.64,
51.52, 62.23, 63.97, 65.07, 120.62, 129.57, 146.17, 148.56, 171.29,
172.69, 173.35.
G-1.5(0 OH) and a General Procedure for AB2 Functionalization.
To a stirred solution of G-1(6 OH) (Mn ) 14 300 g/mol) (14.3 g, 1.00
mmol), 2 (2.01 g, 9.00 mmol), triphenylphosphine (TPP) (3.17 g, 12.1
mmol), and THF (5 mL) at room temperature was slowly added
diisopropyl azodicarboxylate (DIAD) (2.44 g, 12.1 mmol). The reaction
mixture was precipitated into cold methanol after 24 h. The filtered
product was a white crystalline powder. Yield: 14.4 g (94%). Mp:
46.3 °C. 1H NMR (CDCl3) δ 0.96 (s, 18H, -CH3), 1.30 (m, poly,
-CH2-), 1.60 (m, poly, -CH2CH2CH2-), 2.26 (t, poly, -COCH2-,
J ) 6.0 Hz), 3.55-4.60 (2d, 24H, -(CH2O)2CHPh, J ) 9.2 Hz), 4.01
(t, poly, -CH2O-, J ) 5.3 Hz), 4.14 (t, 12H, -CH2OCO), 4.31 (s,
12H, -CCH3(CH2O)2-), 5.37 (s, 6H, -CHPh), 6.88 (dd, 6H, Ph-, J
Experimental Section
Materials. Stannous-2-ethylhexanoate (Sigma) and all other chemi-
cals (Aldrich) were purchased and used without any further purification
except the ꢀ-caprolactone which was dried over CaH2 for 24 h and
then distilled under high vacuum before use. The hexahydroxy-
functional initiator 1 was synthesized according to a procedure
developed by Ihre et al.15b,c
Techniques. 1H NMR were recorded in CDCl3 solution, on a Bruker
AM 250 (250 MHz) apparatus with the solvent proton signal for
reference. 13C NMR spectra were recorded at 62.9 MHz on the same
instrument using the solvent carbon signal as a reference. All polymer
13C NMR spectra were recorded on 250 mg of sample using 16384
scans. The number average molecular weights of the polymers were
calculated from the 1H NMR data. The molecular weight distributions
were determined by size exclusion chromatography (SEC) using a
Waters chromatograph connected to a Waters 410 differential refrac-
tometer and an UV-detector. Four 5 µm Waters columns (300 × 7.7
mm) connected in series in order of increasing pore size (100, 1000,
(12) Chapman, T. M.; Hillyer, G. L.; Mahan, E. J.; Schaffer, K. A. J.
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1991, 24, 1438. (b) Gauthier, M.; Mo¨ller, M. Macromolecules 1991, 24,
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1997, 30, 5602.
(14) Six, J.-L.; Gnanou, Y. Macromol. Symp. 1995, 95, 137.
(15) (a) Malmstro¨m, E.; Johansson, M.; Hult, A. Macromolecules 1995,
28, 1698. (b) Ihre, H.; Hult, A. Polym. Mater. Sci. Eng. 1997, 77, xxx. (c)
Trollsa˚s, M.; Hedrick, J.; Dubois, P.; Jerome, R.; Ihre, H.; Hult, A.
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