Accelerated growth of dendrimers via thiol-ene and esterification reactions

Article abstract of DOI:10.1021/ma1009935

By taking advantage of the orthogonal nature of thiol-ene coupling and anhydride based esterification reactions, a facile and chemoselective strategy to dendritic macromolecules has been developed. The ability to interchange growth steps based on thiol-ene and anhydride chemistry allows the synthesis of fifth-generation dendrimers in only five steps and under benign reaction conditions. In addition, the presented coupling chemistries eliminate the traditional need for protection/deprotection steps and afford dendrimers in high yield and purity. The modularity of this strategy coupled with the latent reactivity of the alkene/hydroxyl chain ends was demonstrated by using different cores (alkene and hydroxyl functional), various AB₂ and CD ₂ monomers and a range of chain end groups. As a result, three dendritic libraries were prepared which exhibited tunability of both the chemical functionality and physical properties including the fabrication of PEG hydrogels.

Full text of DOI:10.1021/ma1009935

6004 Macromolecules 2010, 43, 6004–6013
DOI: 10.1021/ma1009935
Accelerated Growth of Dendrimers via Thiol-Ene and Esterification
Reactions
Maria I. Montanez,†,§ Luis M. Campos,‡ Per Antoni,‡ Yvonne Hed,† Marie V. Walter,†
Brandon T. Krull,‡ Anzar Khan,‡ Anders Hult,† Craig J. Hawker,*,‡ and Michael Malkoch*,†
~
†Fiber and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm,
Sweden, ‡Materials Research Laboratory, Materials Department, and Department of Chemistry and
Biochemistry, University of California, Santa Barbara, California 93106, and §F-IMABIS-Carlos Haya
Hospital, Research Laboratory, Malaga, Spain
Received May 4, 2010; Revised Manuscript Received June 15, 2010
ABSTRACT: By taking advantage of the orthogonal nature of thiol-ene coupling and anhydride based
esterification reactions, a facile and chemoselective strategy to dendritic macromolecules has been developed.
The ability to interchange growth steps based on thiol-ene and anhydride chemistry allows the synthesis of
fifth-generation dendrimers in only five steps and under benign reaction conditions. In addition, the
presented coupling chemistries eliminate the traditional need for protection/deprotection steps and afford
dendrimers in high yield and purity. The modularity of this strategy coupled with the latent reactivity of the
alkene/hydroxyl chain ends was demonstrated by using different cores (alkene and hydroxyl functional),
various AB2 and CD2 monomers and a range of chain end groups. As a result, three dendritic libraries were
prepared which exhibited tunability of both the chemical functionality and physical properties including the
fabrication of PEG hydrogels.
Introduction
techniques were applied.21 While a modified version22 of this
procedure allowed the synthesis of the bis-MPA dendrimers
The development of robust and efficient strategies for the
synthesis of macromolecules with well-defined architecture
and chemical functionality is a continuing theme in polymer
chemistry.1-3 Recent success in the identification of orthogonal
reaction pairs has facilitated the large scale synthesis and sub-
sequent introduction of dendrimers to numerous applications.4,5
No longer an academic curiosity, the increased availability of
dendrimers has allowed their unique physical properties; globu-
lar structure, tunable periphery,6,7 and interior8 to be exploited in
drug delivery and microelectronic applications.4,9 In turn, this
has spurred a need for greater structural versatility and overall
synthetic ease in the construction of dendritic libraries.10-16
Traditional strategies for dendrimer construction has relied on
amidation, etherification, esterification and Michael reactions
resulting in dendritic macromolecules based upon aryl ethers,3
poly(amido amine) (PAMAM),17 propyleneimines,18 triazoles,19
and 2,2-bis(hydroxylmethyl)propionic acid (bis-MPA) repeat
units.20 These systems display interesting physical properties that
are derived from both their unique macromolecular architecture
as well as their chemical composition, for example thermal
stability (aryl ethers) and biodegradability (bis-MPA polyesters).
Dendrimers based on bis-MPA have been widely studied since
their introduction by Hult et al in 1996,20 due to their biocompati-
bility and pH-induced degradation. Initially, the bis-MPA den-
drimers were synthesized by a convergent growth approach
leading to high purity materials. However, the procedure requires
10 steps for a fourth generation material and involves a series of
through mild acidic conditions, it still involves multiple reaction
steps and purification protocols.
On the basis of the philosophy of click chemistry23-26 and the
demonstrated ability to prepare dendrimers through the copper-
catalyzed azide-alkyne cycloaddition (CuAAC) reaction,19,27,28
thiol-ene coupling has been successfully employed to synthesize
dendrimers divergently.7 In demonstrating the applicability
of click chemistry to accelerate dendrimer strategies, aryl ether-
based and bis-MPA-based dendrimers have been prepared by
alternate coupling of AB2 and CD2 monomers via CuAAC and
etherification/esterification reactions, increasing the dendrimer
generation at each iterative step.10 While a valuable demonstra-
tion, for biomaterial applications the use of copper is a major
drawback for biological systems. Given that the metal-free
thiol-ene coupling reaction has been shown to be a powerful
synthetic tool, especially for the preparation of functional bio-
materials,29-31 herein we describe an accelerated growth strategy
for the synthesis of a series of bis-MPA based dendrimers using
a library of interchangeable modular AB2/CD2 monomers
(Scheme 1). This allows the divergent synthesis of fifth generation
dendrimers in only 5 steps with minor purification protocols,
greatly decreasing the synthetic efforts toward multifunctional
dendritic macromolecules (Scheme 2).
Experimental Section
Materials. All materials were obtained from Sigma-Aldrich,
unless otherwise noted. Bis-MPA was kindly donated by Per-
storp AB. F-moc protected cysteine was purchased from Iris
Biotech GmbH. Flash chromatography was performed using
Acros silica gel pore size 60 A, 40-60 μm.
ꢀ
tedious purification steps. To address this challenge, Frechet and
co-workers presented an effective divergent approach to prepare
bis-MPA dendrimers through the esterification of an alcohol and
an anhydride under mild conditions, for which facile purification
Instrumental. NMR experiments were performed on a Bruker
Avance 400 MHz NMR instrument. The MALDI-TOF MS
analyses were conducted on a Bruker UltraFlex MALDI-TOF
*Corrsponding authors. E-mail: (C.J.H.) hawker@mrl.ucsb.edu;
(M.M.) malkoch@kth.se.
pubs.acs.org/Macromolecules
Published on Web 07/01/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 14, 2010 6005
Scheme 1. Range of Momoners and Cores Employed for the Synthesis
of Dendritic Libraries Using Thiol-Ene and Esterification Reactions
Rg values for the dendrimers were calculated by the OmniSec
4.5 software using the Flory-Fox equation. Ultraviolet
(UV) light irradiation of the samples was carried out with a
Hamamatsu L5662 equipped with a L 6722 Hg-Xe lamp. The
intensity was 20-60 mW cm-2, measured with a Hamamatsu
C6080-03 light power meter, calibrated for 365 nm. Differencial
scanning calorimetry (DSC) was performed on a Mettler Toledo
DSC 820, equipped with a sample robot and a cryocooler. The
heating and the cooling rates were 10 °C min-1 in the tempera-
ture range of -80 to þ150 °C. The DSC runs were carried out
with two heating and onecoolingcycles under nitrogen (flow rate
10 mL/min). The glass transition temperature (Tg) was deter-
mined from the second heating scan cycle and taken at the
midpoint of the transition. Hydrogel UV curing was induced
using a Fusion UV Curing System Model F300, equipped with
Fusion electrodeless bulbs standard type BF9 (Lamp power
300W/inch, 1800 W total). A Perkin-Elmer Spectrum 2000 NIR
FT-Raman instrument was employed on dehydrated hydrogel
subtrates using 32 scans at 1500 mW laser power.
Scheme 2. Accelerated Synthesis of I-[G5]-(OH)96, a Fifth Generation
Dendrimer Using Thiol-Ene/Esterification Chemistry
MALDI-TOF Characterization. MALDI MS analysis was
conducted using either 9-nitroantracene, 2,5-dihydroxybenzoic
acid (DHB) or 2-(4-hydroxyphenylazo)benzoic acid (HABA) as
matrix materials. Matrix solutions were prepared as 0.1 M
solutions in THF and either sodium trifluoroacetate or silver
trifluoroacetate (5 mg) was added to each solution. Sample
preparation for MALDI: First, 5 mg of sample was dissolved
in 1 mL of THF. Then, 5 μL of the sample solution was added to
20 μL of the matrix solution. Finally, 0.05 μL of the sample-
matrix solution were added to the MALDI target plate, and the
solution was left to crystallize at room temperature. Spectra
were acquired using a reflector-positive method with an accel-
eration voltage of 25 kV and a reflector voltage of 26.3 kV. The
detector mass range was set to 500-10000 Da in order to exclude
high intensity peaks from the lower mass range. For the highest
molecular weight dendrimers, a linear positive method was
employed with the laser intensity set to the lowest value possible
to acquire high resolution spectra. The spectra were analyzed
with FlexAnalysis Bruker Daltonics, Bremen, version 2.2.
Synthesis of Bis[2-(acetonide-2,2-bis(methoxy)propanoate)-1-
ethyl] Disulfide (9). The esterification reaction was carried out
by the previously published procedures32 using the following
compounds (amounts): compound 8 (128 g, 0.39 mol), bis(2-
hydroxyethyl)disulfide 7 (20 g, 0.13 mol), DMAP (3 g, 0.03 mol),
in CH2Cl2 (180 mL) and pyridine (63 mL). Compound 9 was
obtained a viscous, yellowish product (55 g, 91%). 1H NMR (400
MHz, CDCl3), δ, ppm: 4.39 (t, J = 6.4 Hz, 4H, -SCH2CH2O-),
4.17 (d, J = 12.0 Hz, 4H,-CH2O-), 3.63 (d, J = 12.0 Hz, 4H,
-CH2O-), 2.93 (t, J = 6.4 Hz, 4H, -SCH2CH2O-), 1.41 (s, 6H,
CH3), 1.37 (s, 6H, CH3), 1.18 (s, 6H, -CH3). 13C{1H} NMR (100
MHz, CDCl3), δ, ppm: 174.2, 98.3, 66.1, 62.6, 42.1, 37.2, 24.9,
22.7, 18.8. Calculated: [M]þ m/z = 466.6. Found: MALDI-TOF
MS, [M þ Na]þ m/z = 489.2.
Synthesis of Bis[2-(2,2-bis(hydroxymethyl)propanoate)-1-ethyl]
Disulfide (10). The deprotection reaction was carried out by the
previously published procedures32 using the following compouds
(amounts): compound 9 (37 g, 0.08 mol), Dowex (30 g), and
MeOH (0.5 L). The pure product was obtained as a colorless
viscous oil (28 g, 91%). 1H NMR (400 MHz, CD3OD), δ, ppm:
4.31 (t, 4H, J = 6.8 Hz, -CH2O-), 3.64 (d, 4H, J = 10.8 Hz,
-CH2O-), 3.57 (d, 4H, J = 10.8 Hz, -CH2O-), 2.93 (t, 4H, J =
6.8 Hz, -SCH2-), 1.11 (s,6H, -CH3). 13C{1H} NMR (100 MHz,
CD3OD), δ, ppm: 176.2, 65.6, 63.3, 51.5, 37.8, 17.1. Calcu-
lated: [M]þ m/z=386.5. Found, MALDI-TOF MS: [M þ Na] þ
m/z = 409.1.
Synthesis of 2,2-Bis(hydroxymethyl)-2-mercaptoethylpropa-
noate (2). Thereductionreactionwas carriedoutby the previously
published procedures33 using the following compounds (amounts):
10 (33 g, 0.09 mol), DTT (26 g, 0.17 mol), triethylamine (47 mL,
0.69 mol), and MeOH (0.5 L). The mixture was purified by flash
chromatography eluting the product with EtOAc/heptane (20:80)
MS with SCOUT-MTP Ion Source (Bruker Daltonics, Bremen)
equipped with a nitrogen laser (337 nm), a gridless ion source,
and reflector design. Size exclusion chromatography (SEC)
analysis was performed on a TDA 301 Viscotek instrument
equipped with two GMHHR-M columns with TSK-gel. Mea-
surements were carried out at 35 °C using THF (1.0 mL min-1
)
as mobile phase. A calibration method was created using narrow
linear polystyrene standards. Corrections for the flow rate
fluctuations were made using toluene as an internal standard.

~
Montanez et al.
6006 Macromolecules, Vol. 43, No. 14, 2010
to yield 28.2 g (85%) of yellowish oil. 1H NMR (400 MHz,
CD3OD), δ, ppm: 4.21 (t, 2H, J = 6.8 Hz, HSCH2CH2O-),
3.70 (d, 2H, J = 11.2 Hz, -CH2OH), 3.62 (d, 2H, J = 11.2 Hz,
-CH2OH), 2.73 (2H, t, J = 6.8 Hz, -CH2SH), 1.18 (3H, s,
-CH3). 13C{1H} NMR (100 MHz, CD3OD), δ, ppm: 176.4, 67.3,
65.9, 51.8, 23.8, 17.5.
(100 MHz, CD3OD), δ, ppm: 176.6, 151.0, 66.0, 64.9, 51.7,
43.3, 31.2, 30.4, 28.8, 17.5. Calcd: [M]þ (C99H153N3O33S3)
m/z = 831.30. Found, MALDI-TOF MS: [M þ Na] þ m/z =
854.66, [M þ K] þ m/z = 870.65. Tg: -19 °C.
Synthesis of Dendrimer I-[G2]-(ene)12. Dendrimer I-[G1]-(OH)6
(1.25 g, 1.5 mmol), anhydride 3 (4.8 g, 110 mmol), and DMAP
(220 mg, 1.80 mmol) were reacted in pyridine (3.6 mL, 45 mmol)
and CH2Cl2 (5 mL) as described in the general procedure for
[Gn]-(ene)x, toyield1.8g (62%) ofa clearviscousliquid. 1HNMR
(400 MHz, CDCl3), δ, ppm: 5.84 (m, 12H, -CHdCH2), 5.24
(dd, J = 17.3 Hz, J = 1.6 Hz, 12H, dCHH), 5.14 (dd, J =
10.7 Hz, J=1.6 Hz, 12H, dCHH), 4.26 (d, J=3.7 Hz, 12H,
-CCH2OCO-), 4.23 (t, J = 7.2 Hz, 6H, -SCH2CH2O-),
3.97 (t, J = 6.9 Hz, 6H, -NCH2-), 3.95 (d, J = 5.3 Hz,
24H, -CH2CHdCH2), 3.52 (s, 24H, -CCH2O-), 2.74 (t,
J = 7.2 Hz, 6H, -SCH2CH2O-), 2.60 (t, J = 7.2 Hz, 6H,
-CH2CH2CH2S-), 1.93 (p, J=7.2 Hz, 6H, -CH2CH2CH2-),
1.24 (s, 9H, -CH3), 1.19 (s, 18H, -CH3). 13C{1H} NMR
(100 MHz, CDCl3), δ, ppm: 173.8, 172.5, 148.8, 134.7, 116.6,
72.2, 71.9, 65.1, 63.6, 48.5, 46.6, 42.1, 30.11, 29.4, 27.5, 17.9,
17.5. Calcd: [M]þ (C99H153N3Oþ33S3) m/z = 2007.95. Found,
MALDI-TOF MS: [M þ Na] m/z = 2031.60, [M þ Ag]þ
m/z=2115.12. Tg: -46 °C.
Synthesis of Bis(allyl propionic acid), BAPA (11). One equiva-
lent of bis-MPA (20 g, 0.15 mol) and 7 equiv of NaOH (58 g,
1.44 mol) were added to a round-bottomed flask containing
300 mL of toluene. The reaction vessel was heated to 110 °C, and
10 equiv of allyl bromide (125 g, 1.04 mol) was added. The
reaction mixture was refluxed overnight under vigorous stirring.
After the reaction slurry was acidified using concentrated
HCl (pH ca. 1-2), the mixture was washed using 200 mL of
water. The organic phase was dried using MgSO4 followed by
removal of the solvents under vacuum. The product was isolated
1
as slightly viscous and colorless liquid (32 g, 92%). H NMR
(400 MHz, CDCl3), δ, ppm: 10.23 (s, 1H, OH), 5.85 (m, 2H,
-CHdCH2), 5.24 (d, J = 17.2 Hz, 2H, dCHH), 5.15 (d, J =
10.3 Hz, 2H, dCHH), 3.99 (d, J = 5.5 Hz, 4H, -OCHH2CH-),
3.56 (d, J = 4.2 Hz, 4H, -CCH2O-), 1.23 (s, 3H, -CH3).
13C{1H} NMR (100 MHz, CDCl3), δ, ppm: 180.3, 134.5, 116.8,
72.3, 71.7, 48.1, 17.9.
Synthesis of Bis(allyl propionic acid anhydride), BAPA Anhy-
dride (3). Compound 3 was synthesized by the previously
published procedures21 using the following amounts: BAPA
11 (8 g, 37.0 mmol), DCC (3.8 g, 18.5 mmol) and CH2Cl2
(20 mL). The pure anhydride was obtained as colorless viscous
Synthesis of Dendrimer I-[G3]-(OH)24. To a vial were added
dendrimer I-[G2]-(ene)12 (1.0 g, 0.50 mmol), 2 (1.5 g, 7.77 mmol),
DMPA (30 mg, 119 mmol) and MeOH (1.3 mL). See the general
procedure for [Gn]-(OH)x reaction. The mixture was dissolved
in a small amount of MeOH and precipitated twice into
1
1
liquid (8 g, 92%). H NMR (400 MHz, CDCl3), δ, ppm: 5.87
diethyl ether, to yield 1.7 g (79%) of a clear viscous liquid. H
(m, 4H, -CHdCH2), 5.26 (d, J = 17.2 Hz, 2H, dCHH),
5.17 (d, J = 12.1 Hz, 2H, dCHH), 4.00 (d, J = 5.6 Hz, 4H,
-OCHH2CH-), 3.56 (d, J = 2.8 Hz, 4H, -CCH2O-), 1.26 (s,
3H, -CH3). 13C{1H} NMR (100 MHz, CDCl3), δ, ppm: 169.7,
134.7, 116.9, 72.5, 71.6, 50.0, 17.4.
General Procedure for the Synthesis of [Gn]-(OH)x Dendri-
mers. To a vial were added the alkene functional dendrimer,
1.3-5.0 equiv of thiol monomer (per alkene unit) and 0.02 equiv
of DMPA (per alkene unit). The mixture was dissolved in the
corresponding solvent and purged with argon and then irra-
diated with a 365 nm UV lamp for 20-30 min at room
temperature. The products were subsequently purified.
General Procedure for the Synthesis of [Gn]-(ene)x Dendrimers.
A solution of hydroxyl functional dendrimer, 0.2 equiv of
DMAP (per hydroxyl group), and 5-10 equiv of pyridine (per
hydroxyl group) in CH2Cl2 was prepared in a round-bottom
flask and placed in a cold bath. Then, 1.2-3.0 equiv of alkene
functional anhydride monomer (per hydroxyl group) dissolved
in CH2Cl2 was added dropwise to the solution and the reaction
stirred overnight at room temperature. The reaction mixture
was then quenched with water, diluted with CH2Cl2 and
extracted with 10% aqueous NaHSO4 solution (ꢀ5), 10%
aqueous NaHCO3 solution (ꢀ5), and brine. The organic phase
was dried over MgSO4, filtered, and evaporated to dryness. In
the case of III-[Gn]-(ene)x dendrimers, the extraction with
NaHCO3 was not needed and the products were purified by
flash chromatography, eluting the acid with heptane/diethyl
ether (90:10) containing 1% of AcOH, increasing the polarity to
EtOAc to elute the product.
Synthesis of Dendrimer I-[G1]-(OH)6. To a vial were added
triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione 1 (920 mg, 3.69
mmol), 2 (2.8 g, 14.40 mmol), DMPA (57 mg, 0.22 mmol) and
MeOH (2 mL). See general procedure for [Gn]-(OH)x reaction.
The mixture was purified by flash chromatography eluting the
product with EtOAc/heptane to yield 2.2 g (71%) of a clear
viscous liquid. 1H NMR (400 MHz, CD3OD), δ, ppm: 4.24 (t,
J = 6.8 Hz, 6H, -CH2O-), 3.98 (t, J = 7.1 Hz, 6H, -CH2N-),
3.69 (d, J = 10.8 Hz, 6H, -HCHOH), 3.62 (d, J = 10.8 Hz, 6H,
-HCHOH), 2.79 (t, J = 6.8 Hz, 6H, -SCH2CH2O-), 2.64 (t,
J = 7.1 Hz, 6H, -CH2CH2CH2S-), 1.95 (p, J = 7.1 Hz, 6H,
-CH2CH2CH2-), 1.16 (s, 9H, -CH3). 13C{1H} NMR
NMR (400 MHz, CD3OD), δ, ppm: 4.31 (m, 6H, -CH2O-),
4.25 (t, J=6.9 Hz, 36H, -CH2O-), 4.02 (t, J=7.1 Hz, 6H,
-CH2N-), 3.70 (d, J=11.1 Hz, 24H, -HCHOH), 3.64 (d, J=
11.1 Hz, 24H, -HCHOH), 3.53 (m, 48H, -CCH2OCH2-), 2.83
(t, J=6.0 Hz, 6H, -SCH2CH2O-), 2.79 (t, J=6.9 Hz, 24H,
-SCH2CH2O-), 2.65 (m, 30H, -CH2CH2CH2S-), 1.97 (m,
6H, -NCH2CH2CH2-), 1.84 (p, J=6.6 Hz, 24H, -OCHH2-
CH2CH2-), 1.19 (s, 18H, -CH3), 1.17 (s, 45H, -CH3). 13C{1H}
NMR (100 MHz, CD3OD), δ, ppm: 177.7, 176.2, 174.9, 151.6,
74.7, 71.6, 67.3, 66.7, 66.1, 65.6, 52.4, 48.9, 44.1, 32.0, 32.1, 31.3,
31.7, 30.5, 29.6, 19.4, 19.2, 18.3. Calcd: [M]þ (C183H321N3-
O81S15) m/z=4336.69 Found, MALDI-TOF MS: [M þ Na]þ
m/z=4362.97. Tg: -20 °C.
Synthesis of Dendrimer I-[G4]-(ene)48. Dendrimer I-[G3]-(OH)24
(1.4 g, 0.32 mmol), anhydride 3 (3.5 g, 10 mmol) and DMAP
(188 mg, 1.50 mmol) were reacted in pyridine (6.2 mL, 360
mmol) and CH2Cl2 (10 mL) as described in general procedure
for [Gn]-(ene)x reaction, to yield 1.3 g (45%) of a clear viscous
1
liquid. H NMR (400 MHz, CDCl3), δ, ppm: 5.82 (m, 48H,
-CHdCH2), 5.22 (dd, J=15.7 Hz, J=1.6 Hz, 48H, dCHH),
5.12 (dd, J=8.8 Hz, J=1.6 Hz, 48H, dCHH), 4.23-4.18 (m,
90H, -CH2OCO-), 3.92 (d, J = 5.3 Hz, 102H, -CH2CHd
CH2 þ -NCH2-), 3.54-3.40 (m, 144H, -CH2O-), 2.69 (m,
30H, -SCH2CH2O-), 2.56, (t, 30H, -CH2CH2CH2S-), 1.91
(m, 6H, -CH2CH2CH2-), 1.78 (q, J=6.6 Hz, 24H, -OCHH2-
CH2CH2-), 1.23 (s, 9H, -CH3), 1.22 (s, 36H, -CH3), 1.17 (s,
72H, -CH3), 1.13 (s, 18H, -CH3). 13C{1H} NMR (100 MHz,
CDCl3), δ, ppm: 173.8, 173.6, 172.4, 148.8, 134.7, 116.5, 72.2,
71.9, 69.6, 65.1, 63.7, 48.5, 48.4, 46.7, 46.6, 42.1, 30.3, 30.3,
30.1, 29.6, 28.9, 28.8, 17.9, 17.8, 17.7, 17.5. Calcd: [M]þ
(C447H705N3O153S15) m/z = 9043.33. Found, MALDI-TOF
MS: [M þ Na]þ m/z=9065.00. Tg: -52 °C
Synthesis of Dendrimer I-[G5]-(OH)96. To a vial were added
dendrimer I-[G4]-(ene)48 (40 mg, 4.4 ꢀ 10-3 mmol), 2 (53 mg,
0.28 mmol), DMPA (11 mg, 4 ꢀ 10-3 mmol), and MeOH
(0.5 mL). See the general procedure for the [Gn]-(OH)x reaction.
The mixture was dissolved in a small amount of MeOH and
precipitated twice into diethyl ether, to yield 60 mg (74%)
of a clear viscous liquid. 1H NMR (400 MHz, CD3OD), δ,
ppm: 4.23 (m, 186H, -CH2OCO-), 4.02 (m, 6H, -CH2N-),
3.66 (d, J=10.8 Hz, 96H, -HCHOH), 3.60 (d, J=10.8 Hz, 96H,

Article
Macromolecules, Vol. 43, No. 14, 2010 6007
-HCHOH), 3.49 (m, 240H, -CCH2OCH2-), 2.75 (bt, 136H,
-SCH2CH2O-), 2.71 (bt, 136H, -CH2CH2CH2S-), 1.99 (m,
6H, -NCH2CH2CH2-), 1.80 (m, 120H, -OCHH2CH2CH2-),
1.16 and 1.13 (bs, 279H, -CH3). 13C{1H} NMR (100 MHz,
CD3OD), δ, ppm: 177.3, 177.0, 176.2, 173.3, 151.0, 74.8, 71.8,
69.7, 67.4, 66.8, 66.2, 65.7, 52.9, 52.8, 52.5, 49.0, 46.5, 32.3, 32.2,
31.8, 30.8, 30.7, 27.5, 19.8, 19.6, 19.4, 18.5, 18.4. Tg: -10 °C.
Synthesis of Dendrimer II-[G1]-(OH)6. The first generation
dendrimer was synthesized by the previously published procedure.7
Synthesis of Dendrimer II-[G2]-(ene)12. II-[G1]-OH6 (800 mg,
1.4 mmol), 3 (4.8 g, 11.8 mmol), DMAP (205 mg, 1.68 mmol),
and pyridine (3.3 g, 42 mmol) were reacted. See the general
18H, -CCH3), 0.94-0.84 (m, 12H, -CH2CH3). 13C{1H}
NMR (100 MHz, CD3OD), δ, ppm: 176.5, 174.0, 173.7, 71.9,
70.8, 66.4, 66.0, 65.3, 64.9, 51.7, 43.7, 42.3, 31.3, 31.0, 30.2, 30.1,
29.9, 24.2, 17.6, 8.2, 8.0. Calcd: [M]þ (C96H170O42S6) m/z =
2186.95. Found, MALDI-TOF MS: [M þ Na]þ m/z=2210.91,
[M þ K]þ m/z=2226.91. PDI: 1.03. Tg: -20 °C
Synthesis of III-[G3]-(ene)24. Dendrimer III-[G2]-OH12 (0,9 g,
0.41 mmol), anhydride 5 (3.6 g, 5.92 mmol) and DMAP (120 mg,
0.98 mmol) were reacted in pyridine (2 mL, 25 mmol) and CH2Cl2
(8 mL) as described in general procedure for [Gn]-(ene)x reaction.
The mixture was purified by flash chromatography eluting the
product with EtOAc, to yield 1.9 g (80%) of a clear viscous liquid.
1H NMR (400 MHz, CDCl3), δ, ppm: 5.91-5.80 (m, 24H,
-CHdCH2), 5.24 (dd, J=1.8 Hz, J=17.3 Hz, 24H, dCHH),
5.14 (dd, J=1.8 Hz, J=10.4 Hz, 24H, dCHH), 4.25 (m, 36H,
-CH2OCO-), 4.05 (bs, 30H, -COOCH2C-), 4.00 (s, 6H,
-COOCH2C-), 3.92 (d, J = 5.4 Hz, 48H, CH2CHdCH2),
3.43 (t, J=6.0 Hz, 12H, -OCHH2-), 3.30 (s, 48H, -CCH2O-),
3.26 (s, 12H, -CCH2O-), 2.72 (t, J=7.1 Hz, 12H, -SCH--),
2.63-2.56 (bs, 72H, -COCH2CH2CO- þ -SCH2-), 1.81 (m,
12H, -CH2CH2CH2-), 1.47-1.38 (m, 32H,-CH2CH3), 1.24 (s,
18H, -CCH3), 0.84 (m, 48H, -CH2CH3). 13C{1H} NMR (100
MHz, CDCl3), δ, ppm: 172.4, 172.0, 171.9, 171.7, 135.0, 116.4,
72.2, 70.5, 70.2, 69.5, 65.4, 65.3, 63.8, 46.3, 42.2, 30.14, 29.7, 29.0,
28.9, 22.9, 17.8, 7.6. Calcd: [M]þ (C288H458O102S6) m/z=5740.90.
Found, MALDI-TOF MS: [M þ Na]þ m/z=5768.39. PDI:1.03.
Tg: -46 °C
1
procedure for the reaction conditions. Yield: 68%. H NMR
(500 MHz, CDCl3), δ, ppm: 5.87 (m, 12H), 5.24 (dd, 24H), 5.11
(m, 3), 4.31 (ddd, 6H), 3.99 (d, 24 þ 6H), 3.55 (m, 24H), 2.71 (d,
6H), 2.60 (m, 6H), 1.91 (m, 6H), 1.21 (s, 18H). 13C{1H} NMR
(125 MHz, CDCl3), δ, ppm: 174.03, 173.76, 148.89, 134.80,
134.77, 116.62, 116.53, 72.28, 72.27, 72.25, 71.98, 71.93, 70.69,
63.36, 48.50, 42.12, 31.78, 29.77, 27.54, 17.97. SEC PDI: 1.02
Synthesis of Dendrimer II-[G3]-(OH)24. II-[G2]-ene12 (700 mg,
0.30 mmol), 4 (580 mg, 5.36 mmol) and DMPA (21 mg, 0.07
mmol) were used. See the general procedure for the reaction
conditions. Yield: 93%.1H NMR (500 MHz, DMSO), δ, ppm:
1.12 (s, 18H), 1.73 (m, 25H), 1.82 (m, 6H), 2.60-2.42 (m, 54H),
2.62 (d, 6H), 3.35 (m, 48H), 3.54 (m, 12H), 3.83 (b, 6H),
4.17-4.27 (m, 6H), 4.53 (t, 12H), 4.70 (dd, 12H), 5.03 (m, 3H).
13C{1H} NMR (125 MHz, CDCl3), δ, ppm: 173.77, 173.47,
149.48, 72.61, 71.82, 70.87, 69.80, 67.49, 65.01, 63.75, 48.53,
35.72, 29.75, 29.50, 29.16, 27.74, 25.59, 18.03. SEC PDI: 1.05.
Synthesis of Dendrimer II-[G4]-(ene)48. II-[G3]-OH24 (400 mg,
1.31 mmol), 3 (1.7 g, 4.05 mmol), DMAP (77 mg, 0.63 mmol) and
pyridine (1.2 g, 15.7 mmol) were used. See the general procedure
Synthesis of III-[G4]-(OH)48. To a vial were added dendrimer
III-[G3]-(ene)24 (1.0 g, 0.18 mmol), 2 (1.6 g, 8.52 mmol), DMPA
(0.2 g, 0.82 mmol), and DMF (3 mL). See the general procedure
for the [Gn]-(OH)x reaction. The mixture was dissolved in a small
amount of DMF and precipitated twice into diethyl ether, to yield
1.1 g (60%) of a clear viscous liquid. 1H NMR (400 MHz,
CD3OD), δ, ppm: 4.30-4.21 (m, 84H, -CH2COO-), 4.08 (s,
12H, -COOCH2-), 4.02 (s, 12H, -COOCH2-), 3.69 (s, d, J=
10.8 Hz, 48H, -CHHOH), 3.63 (s, d, J = 10.8 Hz, 48H,
-CHHOH), 3.50 (t, J=5.9 Hz, 60H, -CH2OCH2-), 3.30 (bs,
60H, -CCH2O-), 2.77 (t, J=6.9 Hz, 60H, -SCH2-), 2.67-2.63
(m, 120H, -COCH2CH2CO- þ -SCH2-), 1.87-1.80 (m, 60H,
-CH2CH2CH2-), 1.46-1.40 (m, 32H, -CH2CH3), 1.16 (s, 90H,
-CCH3), 0.88 (bt, 48H, -CH2CH3). 13C{1H} NMR (100 MHz,
CD3OD), δ, ppm: 176.5, 173.8, 173.5, 173.2, 72.0, 71.4, 70.8, 67.0,
66.4, 66.0, 64.9, 51.7, 47.8, 43.7, 31.3, 31.0, 30.1, 29.9, 24.3, 18.6,
17.6, 8.5, 8.4. Calcd: [M]þ (C456H794O198S30) m/z = 10398.37.
Found, MALDI-TOF MS: [M þ Na]þ m/z=10448, [M þ K]þ
m/z=10509. PDI: 1.04. Tg: -18 °C
1
for the reaction conditions. Yield: 44%. H NMR (500 MHz,
CDCl3), δ, ppm: 1.23, 1.26 (s, s, 90H), 1.83 (b, 24H), 1.93 (b, 6H),
2.58-3.0 (b, m, 36H), 3.49 (b, 24H), 3.52-3.65 (m, 122H), 3.99
(b, 96H), 4.10-4.55 (b, m, 10H), 5.1-5.35 (m, 101H), 5.89 (m,
48H). 13C{1H} NMR (125 MHz, CDCl3), δ, ppm: 178.49, 155.96,
149.03, 144.05, 136.61, 134.52, 123.92, 116.59, 106.22, 76.71,
76.44, 74.79, 72.26, 71.97, 49.85, 48.44, 39.42, 32.38, 30.78,
29.49, 26.17, 25.53, 24.73, 18.21, 17.96. SEC PDI: 1.05.
Synthesis of dendrimer III-[G1]-(ene)6. Trimethylolpropane
13 (244 mg, 1.82 mmol), anhydride 5 (4.0 g, 6.55 mmol), and
DMAP (133 mg, 1.09 mmol) were reacted in pyridine (2 mL,
27 mmol) and CH2Cl2 (4 mL) as described in general procedure
for [Gn]-(ene)x reaction. The mixture was purified by flash chro-
matography eluting the product with heptane/EtOAc (80:20),
1
Synthesis of III-[G5]-(ene)96. Dendrimer III-[G4]-(OH)48
(0,9 g, 0.08 mmol), anhydride 5 (3.0 g, 4.98 mmol), and DMAP
(101 mg, 0.83 mmol) were reacted in pyridine (5 mL) and
CH2Cl2 (20 mL) as described in the general procedure for the
[Gn]-(ene)x reaction. The mixture was purified by flash chro-
matography eluting the product with EtOAc/MeOH (70:30),
to yield 1.5 g (80%) of a clear viscous liquid. H NMR (400
MHz, CDCl3), δ, ppm: 5.87 (m, 6H, -CH=CH2), 5.23 (dd, J=
1.7 Hz, J=17.4 Hz, 6H, dCHH), 5.14 (dd, J=1.7 Hz, J=10.3
Hz, 6H, dCHH), 4.06 (s, 6H, -COOCH2C-), 4.04 (s, 6H,
-COOCH2C-), 3.93 (d, J= 5.6 Hz, 12H, CH2CHdCH2), 3.31
(s, 12H, -CCH2O-), 2.62 (s, 12H, -COCH2CH2CO-), 1.45
(m, 8H,-CH2CH3), 0.85 (m, 12H, -CH2CH3). 13C{1H} NMR
(100 MHz, CDCl3), δ, ppm: 172.0, 171.9, 135.0, 116.4, 72.2,
70.2, 65.3, 63.9, 42.3, 40.7, 29.0, 22.9, 22.8, 7.6, 7.3. Calcd
[M]þ (C54H86O18) m/z=1022.58. Found, MALDI-TOF MS:
[M þ Na]þ m/z=1046.03. PDI: 1.02. Tg: -26 °C.
1
to yield 1.6 g (73%) of a yellowish viscous liquid. H NMR
(400 MHz, CDCl3), δ, ppm: 5.91-5.78 (m, 96H, -CHdCH2),
5.24 (dd, J=1.7 Hz, J=17.3 Hz, 96H, dCHH), 5.14 (dd, J=
1.7 Hz, J = 10.5 Hz, 96H, dCHH), 4.34-4.21 (m, 180H,
-CH2OCO-), 4.05 (s, 96H, -COOCH2C-), 3.99 (s, 36H,
-COOCH2C-), 3.92 (d, J = 5.4 Hz, 192H, CH2CHdCH2),
3.44 (t, J = 6.1 Hz, 60H, -OCHH2-), 3.31 (bs, 144H,
-CCH2O-), 3.26 (s, 12H, -CCH2O-), 2.84-2.69 (m, 60H,
-SCH2-), 2.61(bs, 312H, -COCH2CH2CO- þ -SCH2-),
Synthesis of Dendrimer III-[G2]-(OH)12. To a vial were
added dendrimer III-[G1]-(ene)6 (700 mg, 0.68 mmol), 2 (1.2 g,
6.15 mmol), DMPA (0.2 g, 0.82 mmol), and DMF (2.5 mL). See
the general procedure for the [Gn]-(OH)x reaction. The mixture
was dissolved in a small amount of DMF and precipitated twice
into diethyl ether, to yield 1 g (67%) of a clear viscous liquid.
1H NMR (400 MHz, CD3OD), δ, ppm: 4.24 (t, J = 7.1 Hz,
12H, -CH2COO-), 4.06 (s, 6H, -COOCH2C-), 4.02 (s, 6H,
-COOCH2C-), 3.69 (s, d, J = 10.8 Hz, 12H, -CHHOH),
3.62 (s, d, J=10.8 Hz, 12H, -CHHOH), 3.48 (t, J=6.8 Hz,
-CH2OCH2-), 2.77 (t, J=7.1 Hz, 12H, -SCH2-), 2.67-2.64
(m, 24H, -COCH2CH2CO- þ -SCH2-), 1.82 (m, 12H,
-CH2CH2CH2-), 1.45-1.40 (m, 8H,-CH2CH3), 1.16 (s,
1.81 (m, 60H, -CH2CH2CH2-), 1.47-1.38 (m, 128H,-CH2-
13
CH3), 1.10 (s, 90H, -CCH3), 0.84 (m, 192H, -CH2CH3).
-
C{1H} NMR (100 MHz, CDCl3), δ, ppm: 173.0, 172.4, 172.2,
171.9, 171.7, 135.0, 116.4, 72.2, 70.2, 65.3, 65.2, 65.1, 63.8, 62.6,
46.3, 42.3, 29.7, 28.9, 28.5, 28.1, 23.5, 22.9, 22.5, 21.7, 7.7, 7.6, 7.5.
Calcd:[M]þ (C1224H1946O438S30) m/z=24614.16. Found, MALDI-
TOF MS: [M þ Na]þ m/z=25032. PDI: 1.04. Tg: -43 °C
Synthesis of I-[G2]-11-mercapto-1-undecanol Adduct. To a
vial were added dendrimer I-[G2]-(ene)12 (60 mg, 0.03 mmol),

~
Montanez et al.
6008 Macromolecules, Vol. 43, No. 14, 2010
14 (150 mg, 0.72 mmol), and DMPA (18 mg, 0.07 mmol). The
reactants were dissolved in THF and purged with argon for
10 min. Irradiation with a 365 nm UV lamp was carried out for
20 min. The solvent was evaporated and the resulting reaction
mixture was dissolved in the minimal amount of CH2Cl2 and
precipitated three times in cold heptanes and one in cold diethyl
ether to give 60 mg (45%) of a white solid. 1H NMR (400
MHz, CDCl3þCD3OD), δ, ppm: 4.29 (m, 18H, -CH2OCO-),
3.97 (m, 6H, -CH2N-), 3.56 (m, 72H, -CCH2OCH2-
þ CH2-OH), 2.81 (m, 6H, -CH2S-), 2.67 (m, 6H, -CH2S-),
2.56 (t, J=7.3 Hz, 24H, -CH2S-), 2.51 (t, J=7.3 Hz, 24H,
-CH2S-), 1.97 (m, 6H, -NCH2CH2CH2-), 1.81 (q, J=7.3 Hz,
24H, -OCHH2CH2CH2-), 1.53 (m, 48H, -CH2CH2OH þ
-CH2CH2S), 1.31 (m, 168H, -CH2CH2CH2-), 1,23 (s, 9H,
-CH3), 1.19 (s, 18H, -CH3). 13C{1H} NMR (100 MHz, CDCl3
þ CD3OD), δ, ppm: 173.2, 171.8, 148.5, 71.7, 68.9, 64.4,
63.1, 61.0, 50.9, 45.9, 41.2, 37.8, 31.6, 31.0, 28.8, 28.7, 28.6,
28.5, 28.4, 28.1, 28.0, 27.9, 27.6, 27.4, 24.9, 16.7, 16.5. Calcd:
[M]þ (C231H441N3O45S15) m/z=4457.81 Found, MALDI-TOF
MS: [M þ Na]þ m/z=4498.94, [M þ Na þ K]þ m/z=4519.08.
Tg: 5 °C.
Synthesis of I-[G2]-(monoethyl ether triethylenglycol)-3-
mercaptopropionate) Adduct. To a vial were added dendrimer
I-[G2]-(ene)12 (60 mg, 0.03 mmol), 15 (858 mg, 3.23 mmol), and
DMPA (18 mg, 0.07 mmol). The reactants were purged with
argon for 10 min. Irradiation with a 365 nm UV lamp was
carried out for 20 min. The product was purified by flash
chromatography eluting with EtOAc/MeOH (80:20) to yield
102 mg (65%) of a yellowish viscous liquid. 1H NMR (400 MHz,
CDCl3), δ, ppm: 4.27-4.16 (m, 42H, -CH2OCO-), 3.94 (bt,
6H, -CH2N-), 3.67 (t, J=6.9 Hz, 24H, -CH2O-), 3.64-3.58
(m, 72H, -CH2O-), 3.58-3.53 (m, 24H, -CH2O-), 3.49 (q, J=
6.9 Hz, -OCHH2CH3), 3.45-3.42 (m, 24H, -CCH2OCH2-),
2.88 (bt, 6H, -SCH2-), 2.80-2.70 (m, 30H, -SCH2-), 2,59 (t,
J=7.2 Hz, 24H, -SCH2CH2CO-), 2.53 (t, J=7.2 Hz, 24H,
-SCH2CH2CO-), 1.89 (q, J= 7.2 Hz, 6H, -NCH2CH2CH2-),
1.76 (q, J=6.8 Hz, 24H,-OCHH2CH2CH2-), 1.21 (s, 18H),
1.20 (s, 9H, -CH3), 1.17 (t, J=6.9 Hz, -CH2CH3). 13C{1H}
NMR (100 MHz, CDCl3), δ, ppm: 173.6, 172.3, 171.7, 148.7,
72.2, 70.6, 70.5, 70.4, 69.7, 69.6, 68.9, 66.5, 65.0, 63.8, 63.7, 53.4,
48.4, 43.9, 34.6, 29.3, 28.6, 26.9, 26.7, 23.8, 17.8, 17.4, 15.06. Calcd:
[M]þ (C231H417N3O93S15) m/z = 5201.38. Found, MALDI-
TOF MS: [M þ Na þ K]þ m/z=5261.19. Tg: -55 °C.
3.69 (s, 96H, -CH2-Ar), 3.43 (m, 240H, -CCH2OCH2-), 2.71
(m, 30H, -SCH2CH2O-), 2.58 (m, 30H, -CH2CH2CH2S-),
2.45 (t, 96H, -CH2SCH2Ar), 1.93 (m, 6H, -NCH2CH2-),
1.76 (m, 120H, -OCHH2CH2CH2-), 1.25 (s, 9H, -CH3), 1.22
(s, 36H, -CH3), 1.13 (s, 72H, -CH3), 1.10 (s, 18H, -CH3).
13C{1H} NMR (100 MHz, CDCl3), δ, ppm: 173.8, 173.7, 172.4,
172.3, 148.7, 138.5, 128.8, 128.4, 126.8, 72.5, 72.3, 69.8, 69.7,
65.01, 63.69, 48.4, 46.7, 36.2, 30.33, 30.14, 29.6, 29.2, 28.8, 28.0,
27.6, 18.2, 17.9, 17.7, 17.5. Tg: -27 °C
Synthesis of I-[G4]-N- r -(9-Fluorenylmethyloxycarbonyl)-L-
cysteine Adduct. To a vial were added dendrimer I-[G4]-(ene)48
(57 mg, 6.3 ꢀ 10-3 mmol), 18 (436 mg, 1.21 mmol), and DMPA
(15 mg, 0.06 mmol). The reactants were dissolved in the minimal
amount of DMF and purged with argon for 10 min. Irradia-
tion with a 365 nm UV lamp was carried out for 1 h. The mixture
was precipitated into cold EtOAc to give 102 mg (63%) of a
yellowish solid. 1H NMR (400 MHz, DMSO-d6), δ, ppm:
7.94 (s, 48H, NH), 7.87 (bd, 96H, Ar), 7.81 (bd, 96H, Ar),
7.35 (bd, 96H, Ar), 7.26 (bd, 96H, Ar), 4.42-3.91 (m, 282H,
-CH2OCO- þ -CHNH- þ -CCHC-), 3.32 (m, -CCH2S-,
96H), 2.89 (m, 30H, -SCH2CH2O-), 2.78-2.58 (m, 126H,
-CH2CH2CH2S-), 1.81 (m, 6H, -NCH2CH2-), 1.64 (m,
120H, -OCHH2CH2CH2-), 1.17-0.94 (m, 135H, -CH3).
13C{1H}-NMR (100 MHz, DMSO-d6), δ, ppm: 173.1, 172.4,
172.3, 170.5, 162.5, 156.1, 143.9, 140.8, 127.7, 125.3, 124.1,
120.1, 72.9, 72.3, 69.3, 65.9, 59.9, 46.8, 35.9, 30.8, 30.0, 29.1,
28.3, 20.8, 19.7, 19.0, 17.5. Tg: 30 °C.
Fabrication of III-[G3]-(ene)24 Cross-Linked PEG Hydrogel.
The III-[G3]-(ene)24 dendrimer (40 mg, 0.07 mmol), dithiol
functionalized 3k-PEG (234 mg, 0.08 mmol), and the photo-
initiator 2,2-dimethoxy-1,2-diphenylacetophenone (DMPA)
(5 mg, 2 wt %) were dissolved in THF (500 mg). The solution
was drop cast in a Teflon mold, covered with a glass plate, and
exposed to UV irradiation at a dose of 1.66 J/cm2 for a duration
of 10 s using a Hg-lamp. All fabricated hydrogels were soaked at
room temperature in distilled water. A total of five cycles were
efficient for solvent exchange and resulted in a clear hydrogel.
Results and Discussion
The AB2 and CD2 monomers shown in Scheme 1 were selected
based on commercial availability and their facile synthesis from
readily available starting materials. Three basic building blocks
were examined to illustrate the modular nature of this accelerated
approach to dendrimers; thioglycerol (TG, 4),7 bis-MPA, and
trimethylolpropane diallyl ether (TMPDE). The chemical versa-
tility of these building blocks allowed for a range of synthetic
approaches and dendritic structures to be developed.
For the AB2 monomers based on a single thiol group and
2 hydroxyl groups, two starting materials were chosen; thiogly-
cerol (TG), 4, which is commercially available and the bis-MPA
derivative, 2. As shown in Scheme 3A, the parent bis-MPA build-
ing block could be converted into the acetonide protected
anhydride 8 through previously published procedures.21,22 Esteri-
fication of 2-hydroxyethyl disulfide (HED, 7) with 8 using
catalytic amounts of 2-(dimethylamino)pyridine (DMAP) and
excess pyridine afforded the diester, 9 in good yield. When the
carboxylic acid 6 was directly coupled to 7 using N,N0-dicyclo-
hexylcarbodiimide34,35 (DCC), the resulting product 9 was obtained
in lower yields and column chromatography was required to iso-
late the product. Deprotection of the acetonide using acidic
DOWEX gave the alkoxy-functional disulfide 1032 and reduction
with DL-dithioerythritol (DTT)33 afforded the desired monomer,
20-mercaptoethyl 2,2-bis(hydroxymethyl)propionate (bis-MEP,
2). It should be noted that four out of the five steps in the
synthesis of 2 did not involve any chromatography and each of
the five steps resulted in yields greater than 85%.
Synthesis of I-[G4]-(3-mercaptopropionic acid) Adduct. To a
vial were added dendrimer I-[G4]-(ene)48 (80 mg, 8.8 ꢀ 10-3
mmol), 3-mercaptopropionic acid 16 (71 μL, 0.849 mmol), and
DMPA (22 mg, 0.08 mmol). The reactants were dissolved in the
minimal amount of DMF and purged with argon for 10 min.
Irradiation with a 365 nm UV lamp was carried out for 20 min.
The resulting reaction mixture was precipitated once into diethyl
ether to give 75 mg (60%) of clear viscous liquid. 1H NMR
(400 MHz, CD3OD), δ, ppm: 4.28 (m, 90H, -CH2OCO-), 4.02
(m, 6H, -CH2N-), 3.53 (m, 240H, -CCH2OCH2-), 2.90-2.75
(m, 126H, -SCH2CH2O- þ -SCH2CH2COOH), 2.70-2.59
(m, 222H, -CH2CH2CH2S-), 2.02 (m, 6H, -NCH2CH2-),
1.83 (m, 120H, -OCHH2CH2CH2-), 1.30 (bs, 45H, -CH3),
1.20 (bs, 90H, -CH3). 13C{1H} NMR (100 MHz, CD3OD), δ,
ppm: 175.8, 175.5, 175.4, 174.3, 174.2, 151.0, 74.1, 71.1, 66.7,
65.5, 52.6, 48.3, 39.7, 35.9, 31.5, 31.2, 30.9, 30.4, 30.02, 29.7,
28.1, 27.1, 19.0, 18.8, 18.7, 18.6. Tg: -30 °C
Synthesis of I-[G4]-benzyl mercaptan Adduct. To a vial were
added I-[G4]-(ene)48 (80 mg, 8.8 ꢀ 10-3 mmol), 17 (105 mg,
0.85 mmol), and DMPA (22 mg, 0.08 mmol). The reactants were
dissolved in THF and purged with argon for 10 min. Irradiation
with a 365 nm UV lamp was carried out for 20 min. The solvent
was evaporated and the resulting reaction mixture was dissolved
in the minimal amount of chloroform and precipitated once into
methanol to give 80 mg (60%) of clear viscous liquid. 1H NMR
(400 MHz, CDCl3), δ, ppm: 7.30 (m, 192H, Ar), 7.24 (m,
48H, Ar), 4.23 (m, 90H, -CH2OCO-), 3.97 (m, 6H, -CH2N-),
For the corresponding AB2 and CD2 monomers based on 2
alkene units and an anhydride group, 3 and 5, high yielding and

Article
Macromolecules, Vol. 43, No. 14, 2010 6009
Scheme 3. Synthetic Path To Obtain the AB2 and CD2 Monomers
divergent growth strategies that were efficient in terms of materi-
als used and purifications required, the final result being a
chemoselective approach for the accelerated synthesis of I-[G5]-
(OH)96 in gram quantities.
The facile growth of the I-[Gx] dendrimers and the efficient
conversion of the chain end functional groups is illustrated in
Figure 1. 1H NMR analysis reveals the complete substitution of
the end-functional groups at each step with essentially quantita-
tive conversion being observed at generation 4, which contains
48 chain end groups. In the case of the thiol-ene growth step, the
alkene decorated moieties (G0, G2, and G4) are completely
substituted with monomer 2, disappearance of the peaks at
5.25 and 5.80 ppm, while the resonances for the hydroxymethyl
groups are similarly shifted during esterification, 3.65 ppm to
3.95 ppm. This high level of efficiency and associated purity is
also illustrated by comparison of the size exclusion chromato-
graphy traces for the first four dendrimer generations (Figure 2A).
In all cases, the polydispersity indices (PDIs) varied from 1.02 to
1.04 and symmetrical peaks were observed. The purity of the third
generation dendrimer is evident from the mass spectrum in
Figure 2B. The higher generation dendrimers were analyzed using
linear mode and due to their high molecular weight and chemical
structure, the laser intensity was set to a relatively high level,
resulting in spectra with low isotopic resolution.37
Similar to the synthesis of the dendrimer in Scheme 2, other
dendrimers were fabricated using the AB2 and CD2 monomer
combinations shown in Scheme 1. In order to add architectural
variability and to use an alternate, commercially available mono-
mer, thioglycerol 4 was examined as an alternate AB2 thiol
functional monomer. While 4 is commercially available as a
racemic mixture, it is asymmetric and it contains a primary, and a
secondary alcohol which can be exploited for subsequent modi-
fication reactions. For the dendrimer family II-[Gx] prepared
from 1, 3, and 4, the first thiol-ene coupling was performed
under solvent-free conditions, and the product was obtained after
precipitation into diethyl ether.7 The sequential dendrimer
growth process involved similar conditions as the I-[Gx] family
of dendrimers which allows the fourth generation, alkene-func-
tional dendrimer (M.W.=7759 g/mol) to be prepared in only 4
steps, Figure 3. In addition, 4 is a low molecular weight, sterically
compact monomer, which allows dendrimers with different
density profiles to be prepared. Such difference can also lead to
a difference in the overall globular size of the dendrimers. In fact,
a direct comparison between I-[Gx] and II-[Gx] dendrimers in
THF revealed the impact of the chosen monomer on the density
profile, Table 1. The fourth generation I-[G4]-(ene)48 based on the
more extended bis-MEP 2 displayed a radius of gyration of 31 A
which was 7 A larger than the II-[G4]-(ene)48 counterpart (Rg 24
A) with the more dense thioglycerol monomer 4.
To demonstrate the versatility inherent with this chemoselective
strategy, the modular building blocks could be modified to prepare
dendrimers with complementary functionality at the periphery.
For example, the dendrimers I-[Gx] and II-[Gx] in Scheme 1, have
hydroxyl functionality at generations 1, 3, and 5, and alkenes at
generations 2 and 4. To switch the functionality sequence, den-
drimers III-[Gx] bearing a trimethylolpropane core were syn-
thesized by the initial esterification of 13 with anhydride 5.
Through this route, alkene-functional dendrimers can be obtained
at generations 1, 3, and 5, while hydroxyl groups are present at the
periphery of the even-number generations. In this reverse sequence
of generation growth steps, the reaction between 5 and 13
proceeded in a similar fashion to previous esterifications with 3,
the main difference being observed during the work-up process.
In this case, the resulting carboxylic acid byproduct was not easily
removed by a basic wash due to the formation of persistent
emulsions. Therefore, a lower excess of anhydride was employed
for the esterification and the acid byproduct was primarily
efficient chemistry was again employed. The synthesis of the ene-
functional monomer 3 was accomplished in two steps with an
overall yield of 85%, starting from bis-MPA, and again no
chromatography was required which allowed multigram quan-
tities of all monomers to be easily prepared (Scheme 3B).
The key to accelerated synthesis inherent in this AB2 and CD2
approach is a high degree of orthogonality for the coupling
reactions. To initially demonstrate the combination of thiol-ene
chemistry and esterification reactions, the synthesis of a fifth
generation dendrimer, I-[G5]-(OH)96, starting from the core
triallyl isocyanurate 1 is shown in Scheme 2. To a mixture of 1
and the monomer 2 in a 1.3 to 1, thiol to alkene ratio, was added
the radical initiator 2,2-dimethoxy-2-diphenylacetophenone
(DMPA), in a 0.02 of initiator to 1 of alkene ratio. Unlike the
thiol-ene-based dendrimer previously reported, in which the
core was miscible with the monomer,7 the immiscibility of 1 and 2
required the use of solvent. For this purpose, the reaction mix-
ture was deoxygenated by purging with nitrogen,36 followed
by irradiation with a 365 nm UV lamp (intensity of 20-60 mW
cm-2) for 20 min which resulted in quantitative conversion. The
first generation dendrimer was subsequently esterified with
anhydride 3 to yield I-[G2]-(ene)12. An advantage of thiol-ene
chemistry for these reactions is that only 1.3-5.0 equiv of thiol to
alkene is required for essentially quantitative conversion even at
high generations. This is in direct contrast to other divergently
grown dendrimers, such as PAMAM, which require the use of
increasingly larger excesses of monomer as the generation num-
ber of the dendrimer increases.17 At higher generations of the
I-[Gx] dendrimers, minimum amounts (<5%) of dendrimer
coupling were observed when using 1.3 equiv of monomer. In
order to prevent dendrimer couplings, the use of 5 equiv of thiol
per alkene were used, with the SEC traces displaying only
minimal higher molecular weight, coupled species. Similarly,
the esterification reaction requires only 3 equiv of anhydride
per -OH functionality at higher generations, again leading to

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Montanez et al.
6010 Macromolecules, Vol. 43, No. 14, 2010
Figure 1. 1H NMR spectra of I-[Gx] in methanol-d4 (OH-functionalized) and CDCl3 (ene-functionalized). Generation growth is shown by the
disappearance of resonances for the chain end functional groups, i.e., alkene or methylol groups.
Figure 3. Fourth generation dendrimer, II-[G4]-(ene)48, based on bis-
MEP 2 and TG 4 monomers.
Table 1. Theoretical Molecular Weights and Radius of Gyration for
I-[Gx], II-[Gx], and III-[Gx] Dendrimers
I-[Gx]
II-[Gx]
III-[Gx]
generation Mcalc (Da) Rg (A) Mcalc (Da) Rg (A) Mcalc (Da) Rg (A)
Figure 2. (A) Size exclusion chromatography (SEC) traces of (i)
I-[G1]-(OH)6, (ii) I-[G2]-(ene)12, (iii) I-[G3]-(OH)24, and (iv) I-[G4]-
G1
G2
G3
G4
G5
831
2008
4337
9043
18 372
13
24
28
31
47
574
1751
3049
7759
7,0
8,0
17
1023
2187
5741
10 398
24 616
6.0
7.0
13
44
74
(ene)48. (B) MALDI-TOF spectrum of I-[G3]-(OH)24
.
24
removed by column chromatography. SEC and MALDI-TOF
traces for this family of dendrimers, III-[Gx], are shown in Figure 4.
The different building blocks led to a higher molecular weight
and Rg (24632 g/mol; 73 A) for the fifth generation derivative
when compared to the corresponding fifth generation dendrimer,
I-[G5]-(OH)96 (18372 g/mol; 47 A). Additionally, the alkene
functionality at the periphery of dendrimer III-[G5]-(ene)96
rendered it soluble in THF, which is in direct contrast to the

Article
Macromolecules, Vol. 43, No. 14, 2010 6011
Scheme 4. Chemical Modifications of I-[G2]-(ene)12 and I-[G4]-(ene)48
via Thiol-Ene Couplings
-10 °C respectively. It is interesting to note that these values are
much lower than for the corresponding hydroxyl-terminated
TMP-core, Bis-MPA dendrimers, which have Tg values in the
range of 15 to 25 °C.38 This difference can be attributed to the
nature of the molecular architecture with the thiol-ene dendri-
mers containing more flexible repeat units. As expected, the
change of the end group functionality from hydroxyl to alkene
resulted in a lowering of Tg values for all the dendrimers due to
the lack of secondary interactions (I-[G2]-(ene)12, I-[G4]-(ene)48,
III-[G1]-(ene)6, III-[G3]-(ene)24, and III-[G5]-(ene)96 exhibited
Tg values of -46, -52, -26, -46, and -43, °C respectively).
To vary the properties and functionality of the alkene-
functional dendrimers, coupling with a variety of mercaptans
under thiol-ene conditions was carried out with I-[G2]-(ene)12
and I-[G4]-(ene)48. The commercially available thiols 14-18 were
selected and the thiol-ene couplings were carried out under similar
conditions as described for the dendrimer growth (Scheme 4).7
In most examples, only 2 equiv of thiol per alkene unit were
used with the coupling of 15 to the dendrimer proceeding without
any solvent due to the miscibility of all starting materials. The
dendrimers were purified by simple trituration, with the exception
of the adduct resulting from I-[G2]-(ene)12 and 15, which needed
flash chromatography.
Additionally, to illustrate the ability to tailor physical pro-
perties, the thiols, 16-18, were reacted with I-[G4]-(ene)48. The
3-mercaptopropionic acid 16 was chosen for its ability to intro-
duce carboxylic groups at the periphery and, therefore, change
the solubility and pH responsive properties of the resulting
dendrimer. As expected, the carboxylic acid-terminated dendri-
mer was soluble in methanol, water, dimethyl sulfoxide (DMSO),
and N,N-dimethylformamide (DMF), and insoluble in chloro-
form and THF. In contrast, the use of benzylmercaptan 17
allowed aromatic moieties to be introduced at the periphery,
resulting in solubility in aprotic solvents such as chloroform,
benzene and THF and insolubility in protic solvents, such as
methanol and water. The protected amino acid 18 was selec-
ted as a model for the coupling of cysteine-functionalized peptide
fragments and in contrast to the reactions above, reaction
with 18 required a longer period of time to reach complete
conversion (1 h), as well as increased amounts of thiol and radical
initiator.
Figure 4. (A) Size exclusion chromatography traces and (B) MALDI-
TOF Spectra of (i) III-[G1]-(ene)6, III-[G2]-(OH)12, (ii) III-[G3]-
(ene)24, (iii) III-[G4]-(OH)48, and (iv) III-[G5]-(ene)96
.
solubility observed with I-[G5]-(OH)96, thus allowing for char-
acterization by SEC. As can be seen in Figure 4, a high molecular
weight shoulder (<5%) is observed which corresponds to double
the molecular weight of the fifth generation dendrimer. Given the
high density of alkenes at higher generations, and the fact that the
thiol-ene coupling occurs via a radical mechanism,29 the high
molecular weight shoulder may arise from dendrimer-dendrimer
coupling during the previous thiol-ene generation growth step.
The simple order exchangein the reaction sequence delivered fifth
generation dendrimers, I-[G5]-(OH)96 and III-[G5]-(ene)96, with
distinctively dissimilar solubility profiles. Consequently, by care-
fully designing the AB2 and CD2 monomers and their order of use
during growth, the switching approach becomes a valuable tool
for dendrimer chemists to predetermine the physical properties of
a desired dendrimer i.e. being soluble in polar or nonpolar
solvents.
In order to compare the effect of the alternating sequence of
chemical functionality and thermal properties between dendri-
mers I and III, the glass transition temperatures were analyzed by
differential scanning calorimetry (DSC). The hydroxyl functional
dendrimers exhibit the highest glass transition temperatures (Tg),
as compared to the alkene-functional dendrimers, due to the
ability to form hydrogen bonds, with the Tg of the dendrimers
I-[G1]-(OH)6, I-[G3]-(OH)24, III-[G2]-(OH)12, and III-[G4]-
(OH)48, and I-[G5]-(OH)96, being -19, -20, -20, -18, and
As expected, the thermal properties of the functionalized
dendrimers displayed significant differences. The Tg values of

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Montanez et al.
6012 Macromolecules, Vol. 43, No. 14, 2010
Figure 5. (A) Thiol-ene initiated PEG hydrogel based on dendritic cross-linker. (B) Raman spectrum displayed the efficient consumption of enes
and thiols.
the adducts between I-[G2]-(ene)12 and dodecanol, 14/methoxy-
(triethylene glycol), 15, were þ5 and -55, °C respectively,
yielding both higher and lower values when compared to the
starting material (Tg of -46 °C). The adducts obtained from
I-[G4]-(ene)48 and thiols 16, 17, and 18, showed higher Tg values
compared to the starting material (Tg of -30, -27, and þ30, °C
respectively).
Minnesfond (P.A.), Willhelm Beckers Jubileumsfond (Y.H.),
Swedish Research Council (VR) grant-2008/5609 (M.V.W),
Instituto de Salud Carlos III (M.I.M.), and VR Grant 2006-3617
(M.M) are greatly acknowledged.
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