Photocurable hard and porous biomaterials from ROMP precursors cross-linked with diyl radicals

Article abstract of DOI:10.1016/j.bmcl.2005.08.060

A combination of (ROMP) ring-opening metathesis polymerization and diradical (diyl) cross-linking provides a new access to hard biomaterials and potential artificial bone replacements. ROMP was used to construct soft and pliable linear polymers bearing photolabile diazene functions. After treatment with light, a nitrogen aerosol is released throughout the polymer to create desirable porosity, cross-linking, and hardening in a single step. Nonpolymeric mechanistic work supporting these studies was also examined.

Full text of DOI:10.1016/j.bmcl.2005.08.060

Bioorganic & Medicinal Chemistry Letters 15 (2005) 5262–5265
Photocurable hard and porous biomaterials from ROMP
precursors cross-linked with diyl radicals
Eric Enholm,a,* Aarti Joshib and Dennis L. Wrightb
aDepartment of Chemistry, University of Florida, Gainesville, FL 32611, USA
b
Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA
Received 13 July 2005; revised 15 August 2005; accepted 15 August 2005
Available online 3 October 2005
Abstract—A combination of (ROMP) ring-opening metathesis polymerization and diradical (diyl) cross-linking provides a new
access to hard biomaterials and potential artificial bone replacements. ROMP was used to construct soft and pliable linear polymers
bearing photolabile diazene functions. After treatment with light, a nitrogen aerosol is released throughout the polymer to create
desirable porosity, cross-linking, and hardening in a single step. Nonpolymeric mechanistic work supporting these studies was also
examined.
Ó 2005 Elsevier Ltd. All rights reserved.
Olefin metathesis with well-defined ruthenium catalysts
can provide a wide range of possibilities for hard mate-
rialsÕ synthesis.1,2 Increasing knowledge about these rea-
sonably stable catalysts, promulgated by Grubbs and
co-workers, has led to an improved understanding of
ring-opening metathesis polymerization (ROMP) of
substituted norbornyl compounds with a high toleration
for a variety of functional groups and substrates.3–5 We
recently began a study of combining ROMP chemistry
with novel cross-linking methods to create hard bioma-
terials.6 A useful application of porous biomaterials is
for the sustained delivery of biologically active agents
such as drug molecules, nutrients, growth factors, and
hard tissue replacements.7–9 Many current hard tissue
substitutes are nonporous and not ideal for irregularly
shaped or hard to reach defects such as those found in
many bone injuries.
N2
hυ
N2
N = N
N=N
N=N
N2
release
N2
= Diyl Intermediate
N=N = Photoreactive
Diazene Species
Cross
Linking
Scheme 1. Release of N2 and cross-linking.
During the photocuring process, we hoped that nitro-
gen would be released and escape throughout the
polymer as it hardened. There are two important pro-
cesses taking place here. First, as the N2 aerosol es-
capes, it creates the porosity we needed by boring
holes through the soft polymer. Second, the diradicals
(diyls) cause the linear ROMP chains to cross-link and
solidify. The diyls could react with other diyl radicals
in the same or an adjacent polymer chain and/or could
react with carbon–hydrogen bonds in the backbones.
Some mechanistic studies discussed below, were car-
ried out to shed some light on this event. Once
cross-linked, the polymer becomes intractable and
insoluble in solvents; hence, mechanistic data had to
be obtained in an indirect fashion.
In this paper, we investigated the chemical synthesis of
new materials using ring-opening metathesis polymeriza-
tion (ROMP) to construct linear polymers bearing photo-
labile diazene substituents as shown in Scheme 1.10 It was
not clear from the outset if a diazo function would survive
the polymerization or GrubbsÕ ruthenium catalyst. These
polymers would need to be soft and pliable enough to
mold into any shape needed (i.e., like toothpaste), yet
maintain this form until treated with light.
Keywords: Metathesis; ROMP; Polymer; Cross-linking; 1,3-Diyl.
*
The light-induced hardening process was selected be-
cause we wanted to utilize conditions that specifically
Corresponding author. Tel.: +1 352 392 9700; fax: +1 352 392 8758;
e-mail: enholm@chem.ufl.edu
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.08.060

E. Enholm et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5262–5265
5263
avoid physiologically incompatible acid, base, and high-
DIC/DMAP
37 %
6
HO
ly exothermic methods. The diyl chemistry was adopted
from extensive studies on trimethylene methane trap-
ping reactions.11 For our chemistry to be successful, it
was modified to allow a diazo moiety to be attached
to a ROMP polymer backbone. The work herein, dis-
cusses the first example of an internal nitrogen aerosol
to prepare a porous architecture in a hard polymer.
7
n
N
N
Mes
Mes
Ph
Cl
10
8
Ru
O
O
Cl
9
O
O
PCy3
Thus, a norbornyl ring system was envisioned that could
be constructed bearing a light sensitive diazene bridge.
Another norbornyl residue would be appended by an es-
ter linkage to expedite ROMP reactions with GrubbsÕ cat-
alyst. The synthesis was initiated in Scheme 2.
Commercially available d-valerolactone 1 was ring-
opened to an acyclic methyl ester with a catalytic amount
of acidic Dowex resin in methanol and subsequently oxi-
dized with pyridinium chlorochromate to aldehyde 2 in
66% yield. The standard method for the construction of
fulvenes was adopted when 2 was treated with cyclopent-
adiene and a base.12 The desired fulvene 3 was obtained in
51% yield.
10 mole %
CH2Cl2
Mn= 3000
N
N
N
N
H
H
Scheme 3. ROMP reaction of norbornene 8.
the worrisome degradation of the diazene function (vide
infra). Upon addition of ethylvinyl ether to the reaction
mixture, the polymerization was halted. Some precipi-
tate was formed in the flask after the ether was added.
The mixture was poured into rapidly stirring methanol
at room temperature to obtain all of the remaining pre-
cipitate. The precipitate was washed several times with
excess methanol to remove unwanted byproducts. A
GPC analysis was obtained to elucidate the extent of
polymerization. The analysis indicated an average
molecular weight of 3000, which indicated short poly-
mer chains, and a polydispersity of 1.19 for 10.
We chose the chemistry of 4-(S,S-dimethylsulfoximi-
no)-1,2,4-triazoline-3,5-dione (S-TAD), as described
by Meehan and Little,13 as an ideal method that could
be adopted for the construction of a diazene bridge.
The Diels–Alder cycloadduct 5 was prepared via the
reaction of S-TAD 4 with a fulvene 3 in 90% yield, fol-
lowed by a subsequent in situ reduction of the p-bond
of the resulting Diels–Alder adduct using diimide at
low temperature. Conversion of 5 to the corresponding
diazene–acid 6 involved treating it under mildly basic
conditions with hot water, in 77% yield.
Polymer 10 was then photocross-linked using a 400 W
Hanovia lamp with a Pyrex filter for 3 h at a distance
of 2 in. from the lamp. The polymer was observed to
undergo gel–solid transformation. A cross-section of
the polymer using scanning electron microscopy
(SEM) is shown in Figure 1. This view shows the porous
nature of the interior of the polymer.14 The cross-section
images clearly indicate that the nitrogen aerosol creates
several holes (some being 40 lm) throughout the poly-
The synthesis was continued in Scheme 3 with a N0N
diisopropylcarbodiimide-mediated (DIC) esterification
of diazene–acid 6 and norbornene alcohol 7 to give the
desired ROMP monomer 8 in 37% yield after purifica-
tion. The ROMP of 8 was then examined using GrubbsÕ
second-generation catalyst 9. We were pleased to ob-
serve that the catalyst led to ROMP polymer 10 without
1. H+ Dowex
O
H
O
CH OH
3
pyrrolidine
O
OCH
O
3
2. PCC, CH Cl
CH OH
2
2
3
1
2 (66%)
O
O
O
Diels-Alder
N
N N S CH
OCH
3 KO CN=NCO K
3
2
2
2
N
AcOH, CH Cl
CH
2
3
O
4
3 (51%)
S-TAD
H
O
H
CO H
OCH
O
3
2
0.1 N K CO
2
3
N
O
1.5 h, 75
C
N
N
N
N
O
N S
CH
6 (77%)
5 (90%)
3
O
CH
3
Figure 1. Cross-section of porous cross-linked material 10 (10 lm per
division on lower scale).
Scheme 2. Synthesis of the diazene–acid.

5264
E. Enholm et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5262–5265
R
H
O
R
hυ
-N
10
9, (10 mole %)
No Reaction
OCH3
2
CH2Cl2
2h
12
11
N
18
N
R
R
R
path 1
path 2
cycloaddition
13
14
hυ
CH3CN
8
+
O
O
O
H-atom
abstraction
O
R
O
H
19
O
15
16
20
R
R
path 3
dimerization
2
O
O
12
R
17
hυ
CH3CN
(2) 8
Scheme 4. Cross-linking mechanisms.
21
O
O
mer, capable of carrying water, nutrients, and blood
cells through the hard, sponge-like material.15 Ideally,
the size of the pores should enable migration of water
and blood, with an average of 10 lm width per red
blood cell.16
Scheme 5. Metathesis studies.
related cases. Third, we noted that no olefin migration
was observed, a common occurrence with alkenes sub-
jected to GrubbsÕ catalysts.20 Because no chemical reac-
tion was observed, we conclude that diazenes are stable
to GrubbsÕ catalyst, a facet of their reactivity not clearly
delineated prior to these studies.
Several cross-linking mechanisms are proposed in
Scheme 4. Upon photochemical extrusion of nitrogen
from 10, diyl species 11 M 12 is formed, which can react
minimally via three pathways. These reactivity pathways
have been observed with other 1,3-diyls in solution.11a
Path 1 features a cycloaddition reaction with an alkene,
while path 2 shows how a diyl can insert in a C–H bond
(atom transfer). Both reactions can lead to cross-linked
polymer and with the diyl species. A third path 3 in-
volves direct dimerization to the tricyclic product 17
from two diradical precursors.
In a second study, monomer 8 was dissolved in spectral
grade acetonitrile and was photolyzed with 19 (Scheme
5) to obtain a cycloaddition reaction. The intermolecu-
lar cycloaddition product was obtained in 66% yield.
This shows that the 1,3-diyls formed by photoextrusion
can react with electron-deficient alkenes. Co-polymers
of 19 and 10 are possible with 19 acting a spacer in a
polymer chain.21
To further understand the photocross-linking step, three
mechanistic studies were conducted to confirm the
extrusion of nitrogen and diyl formation upon photoir-
radiation, as shown in Scheme 5. We synthesized methyl
ester 18 without the norbornene group to confirm that a
diazene moiety can survive a ROMP. This compound
was easily formed by the reaction of acid 6 with dia-
zomethane. Ester 18 was reacted with 0.1 equiv of
GrubbsÕ second-generation catalyst in CH2Cl2 for 2 h
and it was recovered unchanged. The lack of reactivity
by the diazene p-bond precludes any metathesis at this
site.17
In a third study, compound 8 was dissolved in acetoni-
trile and irradiated for 3 h; the resulting dimerization
product 21 was isolated in 95% yield. It is noteworthy
that in the absence of the olefin, the diyl simply reacted
with itself via the 1,3-diradical coupling.
In conclusion, ring-opening metathesis polymerization
(ROMP) and diradical (diyl) cross-linking provided
new access to hard biomaterials. ROMP was used to
construct soft and pliable linear polymers, that extruded
a nitrogen aerosol when photolyzed. The nitrogen gas
created porosity, cross-linking, and hardening.
It is also noteworthy that no cross-metathesis products
formed from the trisubstituted alkene in 18.18 Unsym-
metrically substituted alkenes usually readily undergo
cross-metathesis and can give up to three products.19
That this outcome was not observed can be rationalized
by considering that the alkene is located in an apical
position on a norbornyl ring system and very much hin-
dered; thus, the bulky catalyst 9 could not readily access
it. Second, the electron-rich diazene could coordinate to
the catalyst and deactivate it, as has been observed in
Acknowledgments
The authors thank Dr. R. Dan Little for helpful discus-
sions. The authors also gratefully acknowledge support
by the National Science Foundation (Grant CHE-
0111210) for this work.

E. Enholm et al. / Bioorg. Med. Chem. Lett. 15 (2005) 5262–5265
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