Hybrid dendritic-linear polyester-ethers for in situ photopolymerization

Article abstract of DOI:10.1021/ja025576y

Novel first through fourth generation hybrid dendritic-linear copolymers, composed entirely of building blocks known to be biocompatible or degradable to natural metabolites in vivo, are described. Specifically, these copolymers are composed of poly(ethyl

Full text of DOI:10.1021/ja025576y

Published on Web 04/18/2002
Hybrid Dendritic-Linear Polyester-Ethers for in Situ Photopolymerization
Michael A. Carnahan, Crystan Middleton, Jitek Kim,† Terry Kim,† and Mark W. Grinstaff*
Departments of Chemistry, Ophthalmology, and Biomedical Engineering, Duke UniVersity and Duke UniVersity
Medical Center, Durham, North Carolina 27708
Received January 12, 2002
Scheme 1. Divergent Synthesis of G1, G2, and G3 Dendrimers
Dendritic macromolecules possess unique chemical and physical
properties, and are of interest for a wide-range of industrial and
pharmaceutical applications.1-17 These monodisperse macromol-
ecules possess a highly branched repeating structure, a high surface
area-to-volume ratio, numerous terminal ends and adopt globular
structures in solution.2-7,10,12 Hybrid dendritic-linear copolymers
are less explored, but offer additional opportunities for material
property optimization through discrete changes at the molecular
level. We are designing, synthesizing, and investigating novel
dendritic macromolecules composed entirely of building blocks
known to be biocompatible or degradable in vivo to natural
metabolites. Such polymers may provide unique solutions to
challenges in the medical and tissue engineering fields,18-25 where
spatial orientation and/or high ligand density are desired on the
macromolecular scale. Herein, we report the divergent synthesis
and characterization of novel first through fourth generation (G0-
G4) hybrid dendritic-linear polyester-ethers composed of poly-
(ethylene glycol), glycerol, and succinic acid, and the use of these
polymers to seal corneal lacerations.
Composed of Poly(ethylene glycol), Glycerol, and Succinic Acid3
Schemes 1 and 2 show the synthesis of new poly(glycerol-
succinic acid)-poly(ethylene glycol) hybrid dendritic-linear co-
polymers ([Gn]-PGLSA)2-PEG. This divergent synthetic strat-
egy15,16,26,27 builds upon our recent success in preparing polyether-
ester and polyester dendrimers28,29 using the benzylidene acetal
(bzld) protecting group, although using a different esterification
method. The molecular weight ratio of the linear to dendritic block
in these macromolecules is small as to maintain dendritic character
in the macromolecule while imparting greater aqueous solubility.
This is a departure from the majority of previous hybrid linear-
dendritic copolymers reported, which possess a large linear block
compared to the dendritic block.
Several esterification strategies were explored to optimize the
reaction yields and simplify purification, including DCC/DPTS,
NHS-activated ester in the presence of TEA, and anhydride coupling
in the presence of DMAP. The first two approaches met with limited
success. The G0 and G1 dendrimers were prepared, but further
reactions yielded incomplete dendrimer generations as determined
by NMR and MALDI-MS. Using an anhydride coupling approach,
as described by Fre´chet,30 we successfully synthesized a series of
([Gn]-PGLSA)2-PEG hybrid dendritic-linear copolymers. The
activated monomer was prepared in two steps by first treating cis-
1,3-O-benzylidene glycerol with succinic anhydride in pyridine to
obtain succinic acid mono(2-phenyl-[1,3]dioxan-5-yl) ester. The
monoester was then treated with 0.6 equiv of DCC in dichlo-
romethane (DCM) to afford the anhydride of succinic acid mono-
(2-phenyl-[1,3]dioxan-5-yl) ester, 2, after precipitation in hexanes
(95% yield). The tetrafunctional core, ([G0]-PGLSA-OH)2-PEG,
4, was synthesized in two steps by treating PEG (3400 Mw) with 6
a Reagents and Conditions (a) DMAP, CH2Cl2, 25 °C, 14 h. (b) H2,
Pd(OH)2/C, CH2Cl2/CH3OH (2/1) 25 °C, 8 h. (c) methacrylic anhydride,
CH2Cl2, DMAP, 25 °C, 14 h; (d) methacrylic anhydride, THF, DMAP, 25
°C, 14 h.
equiv of the anhydride and 0.6 equiv of DMAP in DCM. The
reaction was quenched with n-butanol and washed with saturated
sodium bicarbonate, and then the product, ([G0]-PGLSA-bzld)2-
PEG, 3, was precipitated from ethyl ether (91% yield). The
protected core and 20% palladium hydroxide on carbon, Pd(OH)2/
C, were added to a 2:1 DCM/methanol solution and shaken at 60
psi of hydrogen to obtain 4 in 97% yield. ([G1]-PGLSA-OH)2-
PEG, 7, was synthesized using similar conditions except 4 equiv
of the anhydride were used for each alcohol (93% and 93% yields,
respectively). Reiteration of the esterification (4 equiv of 2 per
alcohol; 91% yield) and deprotection (98% yield) reactions afforded
([G2]-PGLSA-bzld)2-PEG, 9, and ([G2]-PGLSA-OH)2-PEG, 10,
respectively. The ([G2]-PGLSA-bzld)2-PEG was precipitated from
ethyl ether. The third generation block polymer, ([G3]-PGLSA-
OH)2-PEG, 13, was prepared following another round of esterifi-
* Corresponding author. E-mail: mwg@chem.duke.edu.
† Department of Opthalmology.
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J. AM. CHEM. SOC. 2002, 124, 5291-5293
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C O M M U N I C A T I O N S
Scheme 2. Divergent Synthesis of the G4 Dendrimers Composed
of Poly(ethylene glycol), Glycerol, and and Succinic Acid (Green )
PEG; Red ) Succinic Acid; Blue ) Glycerol)3
Table 1. MALDI-MS and SEC Data for Gn-PGLSA-PEG
Copolymers
a
b
no.
calcd M
MALD M
SEC M
n
PDI
w
n
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
3366
3891
3714
3957
4763
4411
4956
6509
5804
6893
10000
8591
10773
16983
14163
3351
3875
3696
3914
4749
4394
4897
6492
5788
6792
9985
8575
10498
16968
14146
3550
3750
3500
3520
3790
4440
3740
4680
5210
4390
6900
7820
6820
7950
8980
1.03
1.04
1.04
1.04
1.04
1.03
1.04
1.04
1.03
1.04
1.02
1.03
1.03
1.02
1.02
a All PDIs are 1.02. b Relative molecular weights by size exclusion
chromatography with PEG standards.
a Reagents and Conditions: (a) DMAP, THF, 25 °C, 14 h. (b) H2,
Pd(OH)2Cl2/CH3OH (2/1), 25 °C, 8 h.
cation and deprotection reactions in similarly high yields (92% and
98%, respectively). Again, ([G3]-PGLSA-bzld)2-PEG, 12, was
isolated by precipitation in ethyl ether. Finally ([G4]-PGLSA-
bzld)2-PEG, 15, was prepared by treating 13 with 128 equiv of 2
in the presence of DMAP. After quenching the reaction and washing
with saturated sodium bicarbonate, the hybrid dendritic-linear
polymer was isolated in ethyl ether (87% yield). The deprotected
macromolecule, ([G4]-PGLSA-OH)2-PEG, 16, was isolated after
hydrogenolysis (98% yield). The photocross-linkable derivatives
of the hybrid dendritic-linear copolymers 5, 8, 11, and 14 were
prepared by reacting the hydroxyl functionalized copolymers with
methacrylic anhydride in the presence of DMAP (yields 89-93%).
The PEG block of this aliphatic ABA triblock copolymer serves
as a diagnostic NMR tag to monitor the change in proton
integrations for each successive generation. The FTIR spectrum of
each deprotected copolymer contains the expected broad O-H
stretch at ∼3400 cm-1, not present before hydrogenolysis. Molec-
ular weight data determined by MALDI MS and SEC for the
benzylidene protected and deprotected G0 through G4 PGLSA-
PEG copolymers are summarized in Table 1. The protected and
deprotected G0 through G2 PGLSA-PEG copolymers are soluble
in common aqueous and organic solvents. The protected G3 and
G4 PGLSA-PEG copolymers are soluble in halogenated solvents,
THF, and ethyl acetate. The deprotected G3 and G4 PGLSA-PEG
copolymers are soluble in polar solvents such as in DMF, EtOH,
and H2O.
Figure 1. Optical image of an eye with a sealed 4.1 mm linear laceration
using copolymer 8. The sealed laceration is the horizontal line between the
two arrowheads.
closure procedure since the use of current monofilament nylon
sutures inflicts additional trauma to the wound and can be associated
with a number of post-operative complications including microbial
keratitis, extended healing time, and astigmatism.37-39
An enucleated eye with the cornea facing upward was held under
a surgical microscope and a 4.1 mm laceration was made with a
keratome blade. This wound was then closed using either three
interrupted 10-0 nylon sutures in a standard 3-1-1 suturing
configuration or the photocross-linkable biodendritic copolymer (see
Scheme 1). Specifically, 4 µL of copolymer 5, 8, 11, or 14 was
applied to the laceration and argon ion laser irradiation produced
the dendritic gel sealing the wound (Figure 1; 200 mW, 1 s
exposures; 50 s total irradiation time; the polymer solution contained
ethyl eosin in 1-vinyl pyrrolidinone and TEA as photoinitiator and
cocatalyst). Next, saline was injected in the anterior chamber via a
syringe inserted through the scleral wall adjacent to an ocular muscle
until the repaired laceration leaked. A cardiac transducer probe
inserted approximately 1 cm through the optic nerve monitored the
leaking pressure for both the nylon suture (N ) 6) and biodendrimer
sealant (N ) 3; for each copolymer tested) treated eyes. For
reference, normal intraocular pressure in a human eye is between
15 and 20 mmHg. In this in vitro study, the mean leaking pressure
(LP) for the suture treated eyes was 90 ( 18 mmHg. The LP for
the eyes sealed with copolymer 8 was 171 ( 44 mmHg (range
142-222 mmHg). Copolymer 5 did not gel adequately to seal the
wound and leaked before measurements could be obtained.
Copolymer 11 polymerized too quickly under the operating
microscope to be delivered to the wound in a controlled fashion
Photocross-linkable hybrid dendritic-linear copolyester-ethers
are of interest for tissue engineering applications where in situ
photopolymerization of a polymer to afford a temporary cell scaffold
can aid tissue growth or repair.31 This new surgical modality is
being explored with linear polymers in cardiology,32 ophthalmol-
ogy,33,34 and orthopaedics.35,36 We envision that in situ photopo-
lymerization will be particularly useful when the damaged tissue
site is not easily accessible or removal of sutures at a postoperative
date is unfavorable. Full-thickness corneal lacerations are one
clinical indication that is likely to benefit from a sutureless wound
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C O M M U N I C A T I O N S
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slightly soluble in alcohols. Copolymer 14 precipitated slightly when
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The hybrid linear-dendritic copolymer, 8, seals the wound better
than conventional sutures and can withstand greater pressures and
stresses placed on or around the wound site. The procedure with 8
is approximately five times faster than suturing the wound,
potentially reducing surgical time and intervention. Moreover, the
cross-linked gel of 8 is transparent, elastic, and adhesivesfavorable
properties for an ophthalmic sealant. The tissue sealing mechanism
with 8 is likely one of physical entrapment where an interpenetrating
network (IPN) is formed between the cross-linked copolymer and
the tissue. This is substantiated by the observation that the
copolymer does not seal the wound without laser polymerization.
If the copolymer is too hydrophobic an IPN does not form, as with
copolymer 14. With these concepts in mind, we are currently
designing other dendritic structures that possess high end-group
functionality while balancing the hydrophilic/hydrophobic properties
of the macromolecule to favor interpenetrating network formation
for subsequent tissue sealing.
In summary, novel aliphatic polyester-ether hybrid dendritic-
linear polymers composed of poly(ethylene glycol), glycerol, and
succinic acid are synthesized using an efficient and high yield
divergent procedure. Once prepared, these dendritic macromolecules
can be further functionalized to contain photocross-linkable groups.
The photocross-linkable dendritic gels possess sufficient tissue
adhesive properties to seal corneal lacerations. These polyester-
ether ABA-triblock copolymers further expand the polymers
available for study. Tailoring the linear and dendritic blocks will
afford macromolecules with unique and interesting chemical,
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Acknowledgment. This work was supported by the Pew
Scholars Program in the Biomedical Sciences, the Johnson and
Johnson Focused Giving Program, and the NIH (ROI-EY 13881).
We thank the North Carolina Eye Bank. M.A.C. gratefully
acknowledges the NIH Biological Chemistry Training Grant
Program at Duke University. M.W.G. also thanks the Dreyfus
Foundation for a Camille Dreyfus Teacher-Scholar, the 3M
corporation for a Non-Tenured Faculty Award, and the Alfred P.
Sloan Foundation for a Research Fellowship.
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of charge via the Internet at http://pubs.acs.org.
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