Synthesis of new AB2 monomers for polymerization to hyperbranched polymers by 1,3-dipolar cycloaddition

Article abstract of DOI:10.1135/cccc20041097

A number of AB₂ monomers containing either one azide and two acetylene functions or vice versa have been prepared. Upon polymerization, these compounds were found to give rise to hyperbranched oligomers as detected by NMR spectroscopy. However, the formation of high-molecular-weight polymers could not be proven by GPC analysis. Crosslinking was found to be a major drawback of this approach to hyperbranched polymers. Two examples of the interesting situation in which the polymerization already starts during the introduction of the polymerizable group are described.

Full text of DOI:10.1135/cccc20041097

New AB2 Monomers for Polymerization to Hyperbranched Polymers
1097
SYNTHESIS OF NEW AB2 MONOMERS FOR POLYMERIZATION TO
HYPERBRANCHED POLYMERS BY 1,3-DIPOLAR CYCLOADDITION
1,
2
3
Mario SMET *, Kris METTEN and Wim DEHAEN
Laboratory for Organic Synthesis, Department of Chemistry, University of Leuven,
Celestijnenlaan 200F, B-3001 Heverlee (Leuven), Belgium;
1
2
e-mail: mario.smet@chem.kuleuven.ac.be, kris.metten@chem.kuleuven.ac.be,
3
wim.dehaen@chem.kuleuven.ac.be
Received Decem ber 15, 2003
Accepted February 20, 2004
Dedicated to Professor Ivan Stibor on the occasion of his 60th birthday in recognition of his
outstanding contributions to the area of supramolecular chemistry.
A n um ber of AB2 m on om ers con tain in g eith er on e azide an d two acetylen e fun ction s or vice
versa h ave been prepared. Upon polym erization , th ese com poun ds were foun d to give rise
to h yperbran ch ed oligom ers as detected by NMR spectroscopy. However, th e form ation of
h igh -m olecular-weigh t polym ers could n ot be proven by GPC an alysis. Crosslin kin g was
foun d to be a m ajor drawback of th is approach to h yperbran ch ed polym ers. Two exam ples
of th e in terestin g situation in wh ich th e polym erization already starts durin g th e in troduc-
tion of th e polym erizable group are described.
Keywords: Hyperbran ch ed polym ers; Den drim ers; Alkyn es; Azides; 1,2,3-Triazoles; 1,3-Dipolar
cycloaddition s.
Hyperbran ch ed polym ers are of great in terest as m ore readily available sub-
stitutes for den drim ers1. Wh ile th e latter are very costly to prepare, th e for-
m er can be obtain ed in a sin gle polym erization step. Con sequen tly, h yper-
bran ch ed polym ers are ch aracterized by a broad m olecular weigh t distribu-
tion an d a degree of bran ch in g wh ich is typically aroun d 0.5 (in stead of 1
in den drim ers). A large n um ber of m on om ers h ave been polym erized to
yield h yperbran ch ed polym ers1. In particular polycon den sation of ABn
m on om ers h as been very popular but also th e use of th e so-called in im ers,
m olecules bearin g a polym erizable an d an in itiatin g fun ction at th e sam e
tim e, h as been studied in detail2. Our group h as been studyin g 1,3-dipolar
cycloaddition reaction s an d h as applied th ese reaction s to th e syn th esis of
m olecules wh ich are of particular in terest in supram olecular ch em istry3. In
th is paper, we presen t a prelim in ary study on th e use of th e 1,3-dipolar
Collect. Czech. Chem. Commun. (Vol. 69) (2004)
doi:10.1135/cccc20041097

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Smet, Metten, Dehaen:
cycloaddition of acetylen es an d azides to afford h yperbran ch ed polytri-
azoles. Depen din g on th e design of th e used AB2 m on om ers, h avin g eith er
on e acetylen e an d two azide m oieties or vice versa, h yperbran ch ed poly-
m ers with azide or acetylen e periph ery could be obtain ed. Th e periph ery of
th ese species could readily be m odified applyin g th e very rich acetylen e or
azide ch em istry. Th is is of h igh im portan ce as th e n ature of th e periph ery
dram atically in fluen ces ph ysical properties such as solubility an d glass tran -
sition tem perature of h yperbran ch ed polym ers1. Furth erm ore, th e h yper-
bran ch ed polytriazoles could be of in terest as cation -com plexin g agen ts.
In order to keep th e ch em istry as sim ple as possible, readily available an d
ch eap ch em icals were used an d h igh -yield reaction s applied. In th e first ap-
proach , but-2-yn e-1,4-diol (1) was esterified with ch loroacetyl ch loride in
th e presen ce of trieth ylam in e yieldin g bis(ch loroacetate) 2a, wh ich could
be con verted to th e diazide 3a by substitution with sodium azide at room
tem perature in DMF (Sch em e 1). First, th e polym erization of 3a was per-
SCHEME 1
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New AB2 Monomers for Polymerization to Hyperbranched Polymers
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form ed in 1,2-dich loroeth an e resultin g in a precipitation of oligom ers 4a.
Hen ce, th e solven t was ch an ged to aceton itrile in wh ich th e h yperbran ch ed
m olecules 4a proved soluble, stressin g th eir h igh polarity. To isolate th e
product, th e solven t was sim ply evaporated un der reduced pressure. Th e
1
residue obtain ed was an alyzed by H NMR spectroscopy in DMSO-d6. A set
of sin glets aroun d 5.3 ppm is observed, wh ich is ch aracteristic of CH2
groups in differen t m icroen viron m en ts at N-1 of th e triazole un its. Sin glets
aroun d 5.5 ppm can be attributed to th e CH2 groups at C-4 an d C-5 of th e
triazole cores. Th ese observation s, togeth er with th e disappearan ce of th e
sign als of CH2 groups of th e m on om er (δ 4.85 an d 3.94 ppm ) clearly in di-
cate th at th e polym erization h as proceeded. However, m olecular weigh ts
sign ifican tly h igh er th an th ose of oligom ers could n ot be detected by GPC
or ES MS.
After a few days of storage, crosslin kin g of th e solid oligom er occurred re-
sultin g in a form ation of in soluble fraction . Th is crosslin kin g is likely to re-
sult from th e decom position of azide groups givin g rise to h igh ly reactive
n itren es. In order to avoid th is side reaction , we con verted th e oligom er 4a
with periph eral azide fun ction s in to th e correspon din g am in e 5 by treat-
m en t with triph en ylph osph in e in DMF with 5% of water (Sch em e 2). Th e
resultin g polyam in e was in situ acylated with stearoyl ch loride an d
trieth ylam in e. It was n ot possible to separate th e obtain ed h yperbran ch ed
oligom er 6 from th e stearic acid form ed upon h ydrolysis of th e reaction
m ixture. However, th e disappearan ce of th e azide group absorption at
2112 cm –1 (asym m etric stretch ) an d th e presen ce of th e am ide absorption
at 1671 cm –1 in th e IR spectrum clearly in dicated th at th e acylation was
successful. Th is was con firm ed by th e fact th at 6 turn ed out to be well solu-
ble in h alogen ated solven ts such as dich lorom eth an e an d ch loroform , in
con trast to th e in itial azide.
SCHEME 2
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Smet, Metten, Dehaen:
An alogously, bis(ch lorobutan oate) 2b was prepared (Sch em e 1). As th e
prim ary 3-ch loropropyl groups are far less reactive in th e substitution reac-
tion with azide, th is reaction h ad to be perform ed at elevated tem perature
(in refluxin g aceton itrile), obviously resultin g in a form ation of oligom ers
of type 4b. Th is could be of great in terest as such cases wh ere th e polym er-
ization occurs con com itan tly with th e in troduction of th e polym erizable
group h ave n ot been described before in th e literature con cern in g h yper-
bran ch ed polym ers. In order to prepare diazide 3b with out an in terferen ce
of th e cycloaddition , we prepared th e diiodide 2c un der Fin kelstein con di-
tion s. However, substitution of diiodide 2c with azide turn ed out to require
elevated tem peratures as well an d, in con sequen ce, also resulted in a con co-
m itan t polym erization .
Th e presen ce of a set of sin glets at 5.4 ppm in th e 1H NMR spectrum ,
wh ich can be attributed to CH2 groups at N-1 of th e triazole un its, clearly
sh ows th at polym erization h as occurred. Oligom ers up to th e h exam er
could be detected by ES MS. Un fortun ately, th e GPC data sh owed on ly low-
m olecular-weigh t species, wh ich is again con tradictory to th e NMR data.
In th e cases of th e two oligom ers described above, isolation by precipita-
tion proved to be very difficult as a sticky oil was obtain ed, presum ably be-
cause th e glass tran sition tem perature of th ese “polym ers” is far below
room tem perature. Th erefore, we turn ed our atten tion to m on om ers con -
tain in g ben zen e rin gs as we reason ed th at th e glass tran sition tem perature
could in crease dram atically upon in troduction of arom atic rin gs. Moreover,
we design ed a m on om er con tain in g on e azide an d two acetylen e groups
in order to prepare a polym er with acetylen e en d groups to avoid th e
aforem en tion ed crosslin kin g reaction s. Th erefore, m eth yl 3,5-dih ydroxy-
ben zoate (7) was allowed to react with but-3-yn -1-ol un der Mitsun obu con -
dition s resultin g in bisacetylen e 8 (Sch em e 3). After reduction with LiAlH4,
th e resultin g ben zyl alcoh ol 9 was brom in ated usin g CBr4 an d triph en yl-
ph osph in e. Th e obtain ed brom ide 10 easily un derwen t con version to azide
SCHEME 3
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New AB2 Monomers for Polymerization to Hyperbranched Polymers
1101
11 by treatm en t with NaN3 in DMF at room tem perature, allowin g th e
preparation of th is m on om er with out in terferen ce of cycloaddition reac-
tion s. Polym erization of m on om er 11 was carried out in 1,2-dich loroeth an e
1
un der reflux. Again , H NMR spectrum of th e resultin g product revealed th e
presen ce of CH2 groups con n ected to N-1 n itrogen s of triazole un its (set of
sin glets at 5.4 ppm ). Un fortun ately, th is could n ot be con firm ed by GPC
m easurem en ts.
Sin ce we foun d th at n on actived acetylen es are n ot reactive en ough to
yield h igh -m olecular-weigh t h yperbran ch ed polym ers, we turn ed our atten -
tion to acetylen edicarboxylates, wh ich are kn own to un dergo cycloaddition
with azides far m ore readily. As both azide an ion s an d organ ic azides
readily react with acetylen edicarboxylate, we reason ed th at protection of
th e acetylen e groupin g is n ecessary. Con version to dibrom ofum aric m oiety
was ch osen as th e protectin g reaction as th is m oiety can readily be con -
verted to th e acetylen edicarboxylate by treatm en t with zin c in THF in th e
presen ce of a catalytic am oun t of iodin e4. Sin ce th e substitution with azide
an ion s on a substrate con tain in g th e dibrom ofum aric m oiety was an tici-
pated to be problem atic, a n um ber of azidoalcoh ols was prepared in order
to couple th em with dibrom ofum aric acid un der Mitsun obu con dition s.
Th ree brom oalcoh ols were prepared. Th e first on e h ad a lon g aliph atic
spacer wh ich was expected to facilitate polym erization due to lowerin g
steric h in dran ces an d to yield rath er n on polar polym ers. Th erefore,
11-azidoun decan -1-ol (13) was prepared by sim ple treatm en t of th e corre-
spon din g com m ercial brom ide 12 with sodium azide in refluxin g aceton i-
trile (Sch em e 4). 4-(Azidom eth yl)ben zyl alcoh ol (16) was obtain ed by
SCHEME 4
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Smet, Metten, Dehaen:
m on obrom in ation of ben zen e-1,4-dim eth an ol 14 an d subsequen t substitu-
tion of th e resultin g brom ide 15 by azide in DMF at room tem perature.
Fin ally, azidoalcoh ol 20 was prepared by alkylation of 4-h ydroxyben zyl al-
coh ol (17) with 1,4-bis(brom om eth yl)ben zen e (18) followed by treatm en t
of th e resultin g m on obrom ide 19 with NaN3 un der sim ilar con dition s.
Dibrom ofum aric acid5 (21) could readily be esterified with each of th ese
azidoalcoh ols un der Mitsun obu con dition s resultin g in th e protected AB2
m on om ers 22–24 (Sch em e 5). To avoid form ation of th e im in oph osph o-
ran e, triph en ylph osph in e was added to an ice-cool solution of th e oth er re-
agen ts.
SCHEME 5
As th e deprotection of th e resultin g protected m on om ers 22–24 was car-
ried out in refluxin g THF, it was im possible to isolate acetylen edicarboxylic
esters 25 (active AB2 m on om ers), because a polym erization already started
durin g th e deprotection step (Sch em e 6). After 24 h reflux in th e presen ce
of zin c, th e solid was filtered off an d th e residue was con cen trated un der re-
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New AB2 Monomers for Polymerization to Hyperbranched Polymers
1103
duced pressure. However, we foun d th at th e m aterials obtain ed un derwen t
rapid crosslin kin g, so as on ly oligom ers of 24 could be in vestigated by NMR
spectroscopy. A broad sin glet at 6.1 ppm , ch aracteristic of th e ben zylic CH2
group attach ed to N-1 of triazole un its clearly in dicated th at polym eriza-
tion occurred. Un fortun ately, GPC m easurem en ts did n ot sh ow an y com -
pon en ts with sign ifican tly h igh m olecular weigh ts. Th e disagreem en t ob-
served in all cases between NMR an d GPC data can be explain ed by th e
sm aller h ydrodyn am ic volum e of h yperbran ch ed as com pared to lin ear
polym ers.
SCHEME 6
In con clusion , we foun d th at it is possible to syn th esize AB2 m on om ers
bearin g on e acetylen e an d two azide fun ction alities or vice versa wh ich un -
dergo oligom erization by 1,3-dipolar cycloaddition . Th is approach , h ow-
ever, suffers from several drawbacks such as crosslin kin g of obtain ed
h yperbran ch ed m acrom olecules an d low m olecular weigh t of a soluble
polym er fraction . An in terestin g feature of th is approach is th e con com i-
tan t polym erization upon in troduction of th e polym erizable group. Re-
cen tly, a copper(I)-catalyzed cycloadditon of azides an d term in al acetylen es
was described6. At presen t, th e application of th is procedure to th e syn th e-
sis of h yperbran ch ed polytriazoles is un der in vestigation .
EXPERIMENTAL
But-2-yn e-1,4-diyl Bis(ch loroacetate) (2a)
Ch loroacetyl ch loride (25 g, 0.22 m ol) was added dropwise durin g 15 m in to a stirred solu-
tion of but-2-yn e-1,4-diol (1; 8.6 g, 0.10 m ol) an d Et3N (23 g, 0.22 m ol) in dry THF (300 m l)
at 0 °C un der argon . Th e resultin g suspen sion was stirred at room tem perature for 30 m in .
Th e m ixture was filtered an d th e precipitate was wash ed with dieth yl eth er (2 × 100 m l). Com -
bin ed filtrates were wash ed with brin e (2 × 100 m l) an d 1 M h ydroch loric acid (2 × 100 m l)
saturated with NaCl. Th e organ ic layer was dried over an h ydrous m agn esium sulfate an d
evaporated in vacuum . Bis(ch loroacetate) 2a was obtain ed as an oil in 82% yield. IR: 2951,
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1104
Smet, Metten, Dehaen:
2552, 2112, 1753, 1426, 1380, 1340, 1178, 977. 1H NMR (300 MHz, CDCl3): 4.81 s, 4 H
(CH2O); 4.10 s, 4 H (CH2Cl). 13C NMR (75 MHz, CDCl3): 167.1, 80.8, 53.6, 40.8.
But-2-yn e-1,4-diyl Bis(4-ch lorobutan oate) (2b)
4-Ch lorobutan oyl ch loride (31 g, 0.22 m ol) was added dropwise durin g 15 m in to a stirred
solution of but-2-yn e-1,4-diol (1; 8.6 g, 0.10 m ol) an d Et3N (23 g, 0.22 m ol) in dry THF
(300 m l) at 0 °C un der argon . Th e resultin g suspen sion was stirred at room tem perature for
30 m in . Th e m ixture was filtered an d th e precipitate was wash ed with dieth yl eth er (2 ×
100 m l). Com bin ed filtrates were wash ed with brin e (2 × 100 m l) an d 1 M h ydroch loric acid
(2 × 100 m l) saturated with NaCl. Th e organ ic layer was dried over an h ydrous m agn esium
sulfate an d evaporated in vacuum . Th e bis(ch lorobutan oate) 2b was obtain ed as an oil in
86% yield. 1H NMR (300 MHz, CDCl3): 4.74 s, 4 H (CH2O); 3.60 t, 4 H, J = 6.6 (CH2Cl);
2.55 t, 4 H, J = 6.9 (CH2COO); 2.11 m , 4 H (CH2CH2CH2). 13C NMR (75 MHz, CDCl3):
172.3, 81.1, 52.6, 44.3, 31.3, 27.9.
But-2-yn e-1,4-diyl Bis(4-iodobutan oate) (2c)
A solution of bis(ch lorobutan oate) 2b (10 g, 34 m m ol) an d sodium iodide (11.5 g, 77 m m ol)
in aceton e (120 m l) was h eated at reflux tem perature un der argon for 48 h . Th e reaction
m ixture was cooled to room tem perature an d evaporated in vacuum . Water (50 m l) was
added an d th e m ixture was extracted with dieth yl eth er (2 × 50 m l). Com bin ed organ ic
layers were dried over an h ydrous m agn esium sulfate an d evaporated. Th e diiodide 2c was
obtain ed as an oil in 90% yield. 1H NMR (300 MHz, CDCl3): 4.73 s, 4 H (CH2O); 3.25 t, 4 H,
J = 7.0 (CH2I); 2.51 t, 4 H, J = 7.0 (CH2COO); 2.15 m , 4 H (CH2CH2CH2). 13C NMR (75 MHz,
CDCl3): 171.9, 81.2, 52.6, 34.9, 28.7, 5.64.
But-2-yn e-1,4-diyl Bis(azidoacetate) (3a)
A solution of bis(ch loroacetate) 2a (10 g, 42 m m ol) an d sodium azide (6.0 g, 92 m m ol) in
DMF (80 m l) was stirred at room tem perature for 24 h un der argon atm osph ere. Th e m ixture
was poured in to water (250 m l) an d extracted with dieth yl eth er (3 × 100 m l). Com bin ed
eth er layers were wash ed with water (3 × 100 m l), dried over an h ydrous m agn esium sulfate
an d evaporated in vacuum . After colum n ch rom atograph y over silica gel (CH2Cl2), diazide
3a was obtain ed as an oil in 72% yield. IR: 2951, 2552, 2112, 1753, 1426, 1380, 1340, 1178,
977. 1H NMR (300 MHz, CDCl3): 4.85 s, 4 H (CH2O); 3.94 s, 4 H (CH2N3). 13C NMR (75 MHz,
CDCl3): 168.0, 81.1, 53.5, 50.5.
Meth yl 3,5-Bis(but-3-yn -1-yloxy)ben zoate (8)
To an ice-cool solution of m eth yl 3,5-dih ydroxyben zoate (3.0 g, 18 m m ol) an d but-3-yn -1-ol
(3.1 g, 45 m m ol) in dry THF (100 m l) un der argon atm osph ere was added dieth yl azodicarb-
oxylate (7.8 g, 45 m m ol) an d triph en ylph osph in e (11.7 g, 45 m m ol). After 15 m in stirrin g at
room tem perature, water was added (100 m l) an d th e m ixture was extracted with dich loro-
m eth an e (3 × 50 m l). Com bin ed organ ic layers were dried over an h ydrous m agn esium sul-
fate an d evaporated in vacuum . Th e diacetylen e 8 was obtain ed as a sticky oil in 73% yield.
1H NMR (300 MHz, CDCl3): 7.20 d, 2 H, J(2,4) = 2.2 (H-2,6); 6.68 t, 1 H, J(2,4) = 2.2 (H-4);
4.11 t, 4 H, J = 6.9 (CH2O); 3.90 s, 3 H (OCH3); 2.69 dt, 4 H, J1 = 6.9, J2 = 2.7 (CH2CH2O);
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New AB2 Monomers for Polymerization to Hyperbranched Polymers
1105
2.05 s, 2 H, J = 2.7 (HCC). 13C NMR (75 MHz, CDCl3): 166.6, 159.4, 132.1, 108.1, 106.9,
80.2, 70.0, 66.2, 52.2, 19.4.
3,5-Bis(but-3-yn -1-yloxy)ben zyl Alcoh ol (9)
A solution of ester 8 (2.0 g, 8.2 m m ol) in dry THF (20 m l) was added to an ice-cool suspen -
sion of LiAlH4 (0.31 g, 8.2 m m ol) in dry THF (20 m l) un der argon . After com plete addition
th e reaction m ixture was stirred at room tem perature for 1 h . A solution of 1 M NaOH was
added dropwise un der ice coolin g un til a readily filterable precipitate was form ed. Th e pre-
cipitate was filtered off an d wash ed with THF (2 × 10 m l). Com bin ed organ ic layers were
dried over m agn esium sulfate an d evaporated in vacuum . Th e product 9 was obtain ed as a
sticky oil in 85% yield. 1H NMR (300 MHz, CDCl3): 6.54 d, 2 H, J(2,4) = 2.6 (H-2,6); 6.41 t,
1 H, J(2,4) = 2.6 (H-4); 4.63 s, 2 H (CH2OH); 4.08 t, 4 H, J = 6.9 (CH2O); 2.67 dt, 4 H, J1
=
6.9, J2 = 2.6 (CH2CH2O); 2.05 s, 2 H, J = 2.6 (HCC); 1.72 s(br), 1 H (OH). 13C NMR (75 MHz,
CDCl3): 159.8, 143.5, 105.6, 100.9, 80.3, 69.9, 66.2, 65.2, 19.5.
1-(Brom om eth yl)-3,5-bis(but-3-yn -1-yloxy)ben zen e (10)
Alcoh ol 9 (1.37 g, 5.6 m m ol) an d triph en ylph osph in e (2.2 g, 8.4 m m ol) were added succes-
sively to an ice-cool solution of CBr4 (2.8 g, 8.4 m m ol) in dry THF (50 m l) un der argon at-
m osph ere. Th e m ixture was stirred at 0 °C for 10 m in , poured in water an d extracted with
CH2Cl2 (2 × 50 m l). Com bin ed organ ic layers were dried over m agn esium sulfate an d evapo-
rated in vacuum . Th e com poun d 10 was obtain ed as an oil after colum n ch rom atograph y
over silica gel (CH2Cl2) in 92% yield. 1H NMR (300 MHz, CDCl3): 6.55 d, 2 H, J(2,4) = 2.2
(H-2,6); 6.41 t, 1 H, J(2,4) = 2.2 (H-4); 4.40 s, 2 H (CH2Br); 4.07 t, 4 H, J = 7.0 (CH2O);
2.67 dt, 4 H, J1 = 7.0, J2 = 2.9 (CH2CH2O); 2.05 s, 2 H, J = 2.9 (HCC). 13C NMR (75 MHz,
CDCl3): 159.7, 139.8, 108.0, 101.7, 80.2, 70.0, 66.0, 33.4, 19.5.
1-(Azidom eth yl)-3,5-bis(but-3-yn -1-yloxy)ben zen e (11)
A m ixture of brom ide 10 (1.68 g, 5.4 m m ol) an d NaN3 (0.55 g, 8.4 m m ol) in DMF (50 m l)
was stirred at room tem perature for 24 h un der argon atm osph ere. Th e m ixture was poured
in to water (100 m l) an d extracted with dieth yl eth er (3 × 50 m l). Com bin ed organ ic layers
were dried over m agn esium sulfate an d evaporated in vacuum . Th e azide 11 was obtain ed as
a sticky oil in 86% yield. 1H NMR (300 MHz, CDCl3): 6.48 d, 2 H, J(2,4) = 2.2 (H-2,6); 6.44 t,
1 H, J(2,4) = 2.2 (H-4); 4.26 s, 2 H (CH2N3); 4.00 t, 4 H, J = 7.0 (CH2O); 2.67 dt, 4 H, J1
=
7.0, J2 = 2.6 (CH2CH2O); 2.05 s, 2 H, J = 2.6 (HCC). 13C NMR (75 MHz, CDCl3): 159.9,
137.7, 107.0, 101.4, 80.2, 70.0, 66.1, 54.7, 19.5.
Oligom erization of Mon om ers 3a an d 11
A solution of an appropriate m on om er 3a (2 m m ol) in aceton itrile or 11 (5 m l) in dich loro-
eth an e was refluxed for 4 days. Th e reaction m ixture was evaporated in vacuum . Th e sticky
residue readily un derwen t crosslin kin g upon stan din g.
11-Azidoun decan -1-ol (13)
A m ixture of 11-brom oun decan -1-ol (12; 2.0 g, 8.0 m m ol), NaN3 (0.80 g, 12 m m ol) an d KI
(0.50 g, 3.0 m m ol) in aceton itrile was h eated at reflux tem perature for 48 h un der argon . Af-
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1106
Smet, Metten, Dehaen:
ter coolin g to room tem perature, th e solven t was evaporated in vacuum , water was added
(50 m l) an d th e m ixture was extracted with dich lorom eth an e (3 × 50 m l). Com bin ed or-
gan ic layers were dried over m agn esium sulfate an d th e solven t was evaporated in vacuum .
Th e title com poun d was obtain ed as an oil in 95% yield. 1H NMR (300 MHz, CDCl3): 3.63 t,
2 H, J = 6.6 (CH2OH); 3.26 t, 2 H, J = 6.9 (CH2N3); 1.57–1.62 m , 5 H (OH, CH2CH2OH,
CH2CH2N3); 1.27–1.31 m , 14 H (in tern al CH2 groups). 13C NMR (75 MHz, CDCl3): 63.5,
52.0, 33.3, 30.0, 29.9, 29.6, 29.3, 27.2, 26.2.
[4-(Brom om eth yl)ph en yl]m eth an ol (15)
To a solution of ben zen e-1,4-dim eth an ol (14; 3.0 g, 22 m m ol) in dry THF (100 m l) were
added successively CBr4 (7.4 g, 22 m m ol) an d PPh 3 (5.9 g, 22 m m ol) at 0 °C un der argon .
After stirrin g for 10 m in , water was added (100 m l) an d th e resultin g m ixture was extracted
with dich lorom eth an e (3 × 50 m l). Com bin ed organ ic layers were dried over m agn esium sul-
fate an d evaporated in vacuum . Th e desired com poun d 15 was obtain ed as an oil after col-
um n ch rom atograph y on silica gel (CH2Cl2) in 39% yield. 1H NMR (300 MHz, CDCl3):
7.37 m , 4 H (ph en yl); 4.70 d, 2 H (CH2OH); 4.50 s, 2 H (CH2Br); 1.65 s(br), 1 H (OH).
13C NMR (75 MHz, CDCl3): 141.2, 137.2, 129.3, 127.3, 64.9, 33.3.
[4-(Azidom eth yl)ph en yl]m eth an ol (16)
A m ixture of brom ide 15 (0.7 g, 3.5 m m ol), NaN3 (0.3 g, 4.5 m m ol) an d KI (0.10 g,
0.60 m m ol) in DMF (50 m l) was stirred un der argon for 2 days. Th e resultin g suspen sion
was poured in to water (100 m l) an d extracted with dieth yl eth er (3 × 50 m l). Com bin ed or-
gan ic layers were dried over m agn esium sulfate an d evaporated in vacuum . Th e azidoalcoh ol
16 was obtain ed as an oil in 95% yield. 1H NMR (300 MHz, CDCl3): 7.30 m , 4 H (ph en yl);
4.63 d, 2 H (CH2OH); 4.30 s, 2 H (CH2N3); 2.51 s(br), 1 H (OH). 13C NMR (75 MHz, CDCl3):
141.5, 135.1, 128.9, 127.9, 65.4, 55.0.
4-{[4-(Brom om eth yl)ben zyl]oxy}ben zyl Alcoh ol (19)
A suspen sion of 4-h ydroxyben zyl alcoh ol 17 (5.0 g, 40 m m ol), 1,4-bis(brom om eth yl)ben zen e
(18; 21 g, 81 m m ol) an d K2CO3 (6.8 g, 48 m m ol) in aceton e (100 m l) was refluxed un der
argon overn igh t. After coolin g to room tem perature, th e solven t was evaporated in vacuum ,
water (100 m l) was added an d th e m ixture was extracted with dich lorom eth an e (3 × 50 m l).
Com bin ed organ ic layers were dried over m agn esium sulfate an d th e solven t was evaporated
in vacuum . Th e alcoh ol 19 was obtain ed as an am orph ous solid after colum n ch rom atogra-
ph y over silica gel (CH2Cl2/EtOAc, 95:5) in 67% yield. 1H NMR (300 MHz, CDCl3): 7.40 m ,
4 H (brom om eth ylph en yl); 7.28 d, 2 H, J = 8.4 (CHCCH2OH); 6.95 d, 2 H, J = 8.4 (CHCO);
5.06 s, 2 H (CH2OPh ); 4.61 d, 2 H (CH2OH); 4.50 s, 2 H (CH2Br); 1.61 s(br), 1 H (OH).
13C NMR (75 MHz, CDCl3): 158.2, 137.6, 137.5, 133.5, 129.3, 128.6, 127.7, 114.9, 69.5,
65.0, 33.1.
4-{[4-(Azidom eth yl)ben zyl]oxy}ben zyl Alcoh ol (20)
A m ixture of brom ide 19 (3.0 g, 10 m m ol), NaN3 (0.78 g, 12 m m ol) an d KI (0.50 g, 3.0 m m ol)
in DMF (80 m l) was stirred un der argon atm osph ere for 2 days. Th e resultin g suspen sion
was poured in to water (100 m l) an d extracted with dieth yl eth er (3 × 50 m l). Com bin ed or-
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New AB2 Monomers for Polymerization to Hyperbranched Polymers
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gan ic layers were dried over m agn esium sulfate an d evaporated in vacuum . Th e azidoalcoh ol
20 was obtain ed as an am orph ous solid in 92% yield. 1H NMR (300 MHz, CDCl3): 7.47 d,
2 H, J = 8.0 (CHCCH2N3); 7.30–7.35 m , 4 H (CHCCH2OH an d CHCCH2OC); 6.97 d, 2 H, J =
8.4 (CHCO); 5.09 s, 2 H (CH2OPh ); 4.63 d, 2 H (CH2OH); 4.36 s, 2 H (CH2N3); 1.57 s(br),
1 H (OH). 13C NMR (75 MHz, CDCl3): 158.8, 137.6, 134.0, 129.1, 128.9, 128.3, 115.4, 70.1,
65.5, 55.0.
Esterification of Dibrom ofum aric Acid with Azidoalcoh ols 13, 16 an d 20
To an ice-cool solution of dibrom ofum aric acid (1.0 g, 3.7 m m ol) an d an appropriate
azidoalcoh ol (8.8 m m ol) in dry THF (20 m l) un der argon atm osph ere were added suc-
cessively dieth yl azodicarboxylate (1.54 g, 8.8 m m ol) an d PPh 3 (2.31 g, 8.8 m m ol). After
stirrin g at 0 °C for 15 m in , water (20 m l) was added an d th e m ixture was extracted with
dich lorom eth an e (3 × 25 m l). Com bin ed organ ic layers were dried over m agn esium sulfate
an d th e solven t was evaporated in vacuum . Th e desired dibrom ofum arates were purified by
colum n ch rom atograph y over silica gel (CH2Cl2/petroleum eth er, 50:50–80:20).
Bis(11-azidoundecyl) dibromofumarate (22). Yield: 66%. 1H NMR (300 MHz, CDCl3): 4.30 t,
4 H, J = 7.0 (CH2O); 3.26 t, 4 H, J = 7.0 (CH2N3); 1.73 m , 4 H (CH2CH2O); 1.60 m , 4 H
(CH2CH2N3); 1.27–1.31 m , 28 H (oth er CH2 groups). 13C NMR (75 MHz, CDCl3): 162.1,
112.5, 67.1, 51.9, 29.8, 29.6, 29.5, 29.3, 28.7, 27.1, 26.2.
Bis[4-(azidomethyl)benzyl] dibromofumarate (23). Yield: 82%. 1H NMR (300 MHz, CDCl3):
7.38–7.40 m , 8 H (ph en yl); 5.31 s, 4 H (CH2O); 4.35 s, 4 H (CH2N3). 13C NMR (75 MHz,
CDCl3): 162.4, 136.5, 134.9, 133.3, 129.5, 129.0, 68.7, 54.9.
Bis(4-{[4(-azidomethyl)benzyl]oxy}benzyl) dibromofumarate (24). Yield: 66%. 1H NMR
(300 MHz, CDCl3): 7.44 d, 4 H, J = 8.1 (CHCCH2O); 7.31–7.35 m , 8 H (C6H5CH2N3); 6.96 d,
4 H, J = 8.1 (CHCO); 5.23 s, 4 H (CH2OCO); 5.07 s, 4 H (CH2OPh ); 4.34 s, 4 H (CH2N3).
13C NMR (75 MHz, CDCl3): 162.5, 159.5, 137.4, 135.7, 131.0, 128.9, 128.3, 127.3, 115.4,
113.2, 70.1, 69.0, 54.9.
Deprotection an d Oligom erization of Dibrom ofum arates 22–24
A suspen sion of zin c powder (0.81 g, 13 m m ol) in dry THF (30 m l) was refluxed un der argon
atm osph ere for 15 m in . After coolin g to room tem perature, a solution of an appropriate
fum arate (1.3 m m ol) in dry THF (20 m l) was added an d th e reflux was con tin ued un der
argon for 24 h . Th e suspen sion was allowed to cool to room tem perature, th e precipitate
was filtered off with celite an d wash ed with THF (2 × 10 m l) an d aceton e (2 × 10 m l). Th e
filtrate was dried over m agn esium sulfate an d th e solven t was evaporated in vacuum yield-
in g a sticky m ixture of oligom ers, wh ich readily crosslin ked to give an in soluble m aterial.
We thank the University of Leuven, the FWO-Vlaanderen and the Ministerie voor Wetenschaps-
beleid for financial support. M. Smet thanks the FWO-Vlaanderen for a postdoctoral fellowship.
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Smet, Metten, Dehaen:
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