Enzymatic ring-opening polymerization and atom transfer radical polymerization from a bifunctional initiator [5]

Article abstract of DOI:10.1021/ma011929m

The synthesis of the block copolymers combining enzymatic ring opening polymerization (ROP) and atom transfer radical polymerization (ATRP) was discussed. To synthesize block copolymers, the enzymatically polymerized PCL was precipitated and subsequently used to macroinitiate styrene. The PCL macroinitiator was then used for the ATRP of the styrene. A PCL end caped with the initiator was obtained in high selectivity by enzymatic ROP using a bifunctional initiator bearing initiator groups for both techniques. Sequential macroinitiation of styrene by ATRP results in block polymers in high yield without requiring an intermediate transformation step.

Full text of DOI:10.1021/ma011929m

Macromolecules 2002, 35, 2873-2875
2873
Sch em e 1. En zym a tic ROP a n d ATRP fr om a
Bifu n ction a l In itia tor
En zym a tic Rin g-Op en in g P olym er iza tion a n d
Atom Tr a n sfer Ra d ica l P olym er iza tion fr om
a Bifu n ction a l In itia tor
Ur su la Meyer , An ja R. A. P a lm a n s,
Ton Loon tjen s, a n d An d r ea s Heise*
DSM Research, P.O. Box 18, 6160 MD Geleen,
The Netherlands
Received November 5, 2001
This order was chosen because ATRP macroinitiation8
is more efficient and straightforward than enzymatic
macroinitiation due to its controlled character and the
sterical demands of the latter.9
In tr od u ction . This paper reports the first combina-
tion of an enzymatic polymerization with a “chemical”
polymerization technique, i.e., lipase-catalyzed ring-
opening polymerization (ROP) and atom transfer radical
polymerization (ATRP). We employ a bifunctional ini-
tiator that allows the synthesis of block copolymers from
two dissimilar monomers, i.e., vinyl and lactone, without
further transformation of the polymer end groups.
Enzymatic ROP is a promising alternative technique
to the “chemical” ROP employing organometallic com-
pounds as a catalyst. Intensive studies over recent years
indicate that various polyesters and polycarbonates can
be synthesized by the choice of monomer and lipase.1
While not strictly a controlled or living polymerization,
this new procedure has attributes comparable to tradi-
tional controlled polymerization techniques. It allows,
for example, reasonable control of molecular weight and
chain end functionality.2 Therefore, it may be a valuable
alternative synthetic tool to access copolymers and even
more complex polymer architectures. From the indus-
trial point of view, enzymatic reactions/polymerizations
are significant due to technological advantages such as
regio- and stereoselectivity, product purity (no residual
catalyst), and the fact that both batch and continuous
operation are possible.3,4 In addition, the environmental
tolerance of enzymatic procedures makes them attrac-
tive in industrial processes.4 However, to fully integrate
enzymatic polymerizations as an additional technique
into polymer synthesis and eventually the preparation
of novel materials, its versatility and compatibility with
other techniques must be proven.
All enzymatic polymerizations were conducted em-
ploying Novozym 435 from Novo Nordisk (lipase CALB
immobilized on an acrylic resin) and the initiator 1 in
dry toluene under nitrogen with a constant target
molecular weight around 5.0 × 103 g/mol.10 Since water
is an effective initiator in the enzymatic ROP, it can
compete with the hydroxy function of 1 in the initiation
step. This would result in PCL that is not end-function-
alized with the ATRP initiator but with a carboxylic acid
end group.11 Therefore, it is important to thoroughly dry
the reaction components in order to minimize the water
initiation. Equally important is the optimization of the
reaction time of the enzymatic ROP since lipases also
catalyze transesterification reactions.12 This side reac-
tion will become more prominent when most of the
monomer is consumed and will result in chain scission,
i.e., polymer degradation. To identify the optimum
reaction time, initiator 1 was employed in an ROP of
ꢀ-CL, and samples were withdrawn from the polymer-
ization reaction after 3 and 5 h. The samples were
analyzed by 1H NMR and GPC with triple detection
(UV, RI, viscosity) without prior purification. Figure 1
shows the GPC traces of the UV detection of the samples
taken before starting the reaction (t ) 0) and at t ) 3
and 5 h. The GPC trace of the pure initiator shows that
the peak at about 40 min retention time is the unreacted
initiator. During the course of the polymerization,
initiator is consumed as indicated by the decrease of
that signal. After 3 h the initiator concentration reaches
its minimum (95% consumed as calculated from signal
integration). Apparently, prolonged reaction time does
not result in further consumption of initiator. This
This study combines enzymatic ROP of ꢀ-caprolactone
(ꢀ-CL) and ATRP of styrene because the latter is a very
robust technique providing excellent control over mo-
lecular weight, polydispersity, and polymer end groups.5
Furthermore, the compatibility of ATRP with ROP
mediated by organometallic compounds was already
reported in the literature.6
1
corresponds to results obtained from the respective H
NMR measurements; it shows that all ꢀ-CL is consumed
after 3 h under the applied reaction conditions.
At the same time evolving polymer traces with very
weak UV activitysattributed to the attached initiators
can be detected in the GPC. (A “conventionally” obtained
PCL showed no UV activity.) Since the UV traces are
not discernible in Figure 1, the corresponding traces of
the RI detection are shown as an inset. Comparison of
the GPC traces from RI and viscosity detection of the
polymers obtained after 3 and 5 h indicates a shift of
the peak to lower molecular weight as well as a
broadening of the peak with longer reaction time. This
can be rationalized by enzyme-catalyzed transesterifi-
cation reactions becoming dominant at low monomer
concentration as described above.
Resu lts a n d Discu ssion . The bifunctional initiator
1 was prepared from the benzyl ester of bis(hydroxy)-
propionic acid (bis-MPA).7 One hydroxy group was
esterified with 2-bromo-2-methylpropionyl bromide. The
initiator contains a single primary alcohol group to
initiate enzymatic ROP and an activated bromide, which
is an effective initiator for ATRP (Scheme 1). The design
of the bifunctional initiator was chosen carefully since
the benzyl ester moiety provides a tag for 1H NMR
analysis as well as a chromophore for UV detection in
the gel permeation chromatography (GPC) analysis of
the block copolymer. To prove the efficiency of the
initiator, a block copolymer was synthesized in two
consecutive steps starting with the enzymatic ROP of
ꢀ-CL, followed by the ATRP macroinitiation of styrene.
To synthesize block copolymers, the enzymatically
polymerized PCL was precipitated and subsequently
10.1021/ma011929m CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/06/2002

2874 Communications to the Editor
Macromolecules, Vol. 35, No. 8, 2002
F igu r e 1. GPC traces of enzymatic ring-opening polymeri-
zation employing initiator 1 and ꢀ-caprolactone. UV detection
of crude samples taken at various reaction times shows the
consumption of the initiator. The inset shows the development
of the polymer traces by RI detection of the respective polymer
region (t ) 3 h: Mn ) 7.5 × 103 g/mol; polydispersity ) 1.6; t
) 5 h: Mn ) 4.0 × 103 g/mol; polydispersity ) 2.3; values
determined by GPC calibrated with PS).
used to macroinitiate styrene.13 To determine whether
1
the initiator is attached to the PCL, H NMR spectra
were carefully examined (Figure 2). Beside the domi-
nant polymer signal, the resonances of the initiator
1
group can be assigned in the H NMR spectrum of the
PCL. All initiator signals experience a shift upon
polymerization, which is highest for the methylene
group c shifting from 3.75 ppm in the initiator to about
4.2 ppm in the PCL due to the ester formation of the
adjacent hydroxy function. Moreover, the signal as-
signed to the methyl groups f close to the activated
bromide (1.9 ppm) can also be detected in the PCL,
suggesting an intact ATRP initiator group. This indi-
cates that the presence of the ATRP initiator group does
not interfere with the activity of the enzyme. To quantify
the amount of water initiated PCL, oxalyl chloride was
added to the polymer solution directly in the NMR tube,
according to a literature procedure.14 Because of the
reaction of the carboxylic acid end group of the water-
initiated PCL with the oxalyl chloride, the proton NMR
signal of the methylene group adjacent to the carboxylic
acid group shifts into an uncrowded region of the
spectrum at 2.9 ppm and can be quantified by integra-
tion. Although all components, i.e., solvent and mono-
mers, were carefully dried prior to polymerization, the
proportion of water initiation can be as high as 40% in
some cases. This is close to the theoretically expected
initiation by water attached to the enzyme beads as
determined by Karl Fischer titration. When Novozym
435 beads were dried over P2O5 prior to polymerization,
the amount of water-initiated PCL could be reduced to
about 5% as determined by 1H NMR.15 Unlike PCL
obtained without drying the enzyme, PCL obtained
under dry conditions showed a symmetrical trace in the
GPC with a polydispersity of 1.6 (Figure 3).16
F igu r e 2. 1H NMR (CDCl3) spectra of the initiator 1 (A); poly-
(caprolactone) (5.8 × 103 g/mol; polydispersity: 1.7) (B) and
poly(styrene-b-caprolactone) (PS-block 5.1 × 103 g/mol; poly-
dispersity block copolymer: 1.4) (C). Molecular weights were
calculated from the 1H NMR integrated peak areas of the
initiator peak a and polymer peaks C and A. The polydisper-
sity was determined by GPC calibrated with PS.
F igu r e 3. (A) GPC (RI detection) of enzymatically polymer-
ized poly(caprolactone) (5.8 × 103 g/mol, polydispersity 1.7)
employing initiator 1 and (B) poly(styrene-b-caprolactone) (PS
block, 15 × 103 g/mol; polydispersity block copolymer, 1.2).
Molecular weight and polydispersity were determined as
described in Figure 2.
To obtain the block copolymer, the PCL macroinitiator
was then used for the ATRP of styrene. The attempted
block length of PS block was about 3 times that of the
PCL block as calculated from the macroinitiator/styrene
feed ratio. The SEC traces of both the macroinitiator
and the block copolymer are shown in Figure 3 with the
expected increase in molecular weight and decrease in
polydispersity of the block copolymer due to the con-
trolled character of the ATRP. To further confirm the
block copolymer structure, it was dissolved in 1,4-
dioxane (85 °C) and hydrolyzed with aqueous hydro-
chloric acid solution to give the cleaved polystyrene

Macromolecules, Vol. 35, No. 8, 2002
Communications to the Editor 2875
1
mmol) was added dropwise at 0 °C. After the reaction was
stirred for 2 h, the reaction mixture was filtered and the
filtrate extracted with 2 N HCl (2×), Na2CO3 (2×), and
NaHCO3 (2×) solution. The organic phase was dried over
Na2SO4 and the crude product purified by column chroma-
tography (silica gel, hexane/ethyl acetate 9:1) to give a
viscous liquid (3.7 g, 55%). 1H NMR (CDCl3): δ 1.3 (s, 3H,
-CH3); 1.9 (s, 6H, -C(Br)-CH3); 3.7 (dd, 2H, -CH2-OH);
4.3-4.5 (dd, 2H, -CH2-); 5.2 (s, 2H, Ar-CH2-); 7.3 (s, 5H,
ArH). 13C NMR (CDCl3): δ 17.1 (-CH3); 29.9 (-C(Br)-
CH3); 48.1 (-C-CH2-OH); 54.8 (-C-Br); 64.5 (Ar-CH2-
); 66.1 (-CH2-O-); 66.4 (-CH2-OH); 127.4-127.6 (ArCH);
135.8 (ArC-C); 170.4 ((CdO)-C-Br); 171.7 ((CdO)-C-
CH3). IR (cm-1): 3580-3650 (OH val), 3000-3100 (aro-
matic), 2850-2960 (CH val), 1750 (CdO abs). Anal. Calcd
for C16H21BrO5: C, 51.4; H, 5.6; O, 21.5; Br, 21.4. Found:
C, 51.2; H, 5.7; O, 22.1; Br, 21.0.
block. Analysis of the product by H NMR shows the
disappearance of the all PCL resonances and compari-
son of the GPC traces show a shift to lower molecular
weight with low polydispersity (1.2).
Figure 2C shows for better clarity the 1H NMR
spectrum of a block copolymer with about equal block
length with resonances for both blocks assigned. Note-
worthy is the chemical shift from 1.9 to 1.1 ppm of the
methyl protons f of the ATRP initiator unit upon styrene
polymerization due to the replacement of the activated
bromide by the PS.17 This proves the feasibility of the
bifunctional initiator in the block copolymer synthesis
employing enzymatic ROP and ATRP in two consecutive
polymerizations.
(8) Matyjaszewski, K.; Teodorescu, M.; Acar, M. H.; Beers, K.
L.; Coca, S.; Gaynor, S. G.; Miller, P. J .; Paik, H. Macromol.
Symp. 2000, 157, 183. Huan, K.; Bes, L.; Haddleton, D. M.;
Khoshdel, E. J . Polym. Sci., Part A: Polym. Chem. 2001,
39, 1833. Matyjaszewski, K. Macromolecules 1998, 31, 3489.
Yijin, X.; Caiyuan, P. Macromolecules 2000, 33, 4750. Heise,
A.; Trollsås, M.; Magbitang, T.; Hedrick, J . L.; Frank, C.
W.; Miller, R. D. Macromolecules 2001, 34, 2798.
(9) Co´rdova, A.; Hult, A.; Hult, K.; Ihre, H.; Iversen, T.;
Malmstro¨m, E. J . Am. Chem. Soc. 1998, 120, 13521.
(10) Novozym 435 (0.5 g) and 1 (0.5 g, 1.3 mmol) were weighed
into a flame-dried round-bottom flask. Then ꢀ-CL (5.0 g, 43
mmol) and toluene (5 mL) were added under nitrogen, and
the mixture was heated to 60 °C. After 3 h the polymer was
precipitated in cold methanol to yield 3.1 g of polymer.
(11) Macdonald, R.; Pulapura, S.; Svirkin, Y. Y.; Gross, R. A.;
Kaplan, D. L.; Akkara, J .; Swift, G. Macromolecules 1995,
28, 73. Uyama, H.; Takeya, K.; Hoshi, N. Kobayashi, S.
Macromolecules 1995, 28, 7046.
Con clu sion . In conclusion, we described a versatile
route for the synthesis of block copolymers combining
enzymatic ROP and ATRP. A PCL end-capped with the
initiator was obtained in high selectivity by enzymatic
ROP using a bifunctional initiator bearing initiator
groups for both techniques. Sequential macroinitiation
of styrene (or other vinyl monomers) by ATRP results
in block copolymers in high yield (90-95%) without
requiring an intermediate transformation step.
We are currently investigating whether both poly-
merizations can be conducted in one pot. Therefore, we
have to examine the reaction kinetics in more detail and
evaluate how well the enzyme works under ATRP
conditions. The results of this study will be reported in
a forthcoming paper.
(12) Wallace, J . S.; Morrow, J . C. J . Polym. Sci., Part A: Polym.
Chem. 1989, 27, 3271. Athawale, V. D.; Gaonkar, S. R.
Biotechnol. Lett. 1994, 16, 149. Mezoul, G.; Lalot, T.;
Brigodiot, M.; Mare´chal, E. J . Polym. Sci., Part A: Polym.
Chem. 1995, 33, 2691. Co´rdova, A.; Iversen, T.; Hult, K.
Macromolecules 1998, 31, 1040. Kumar, A.; Kalra, B.;
Dekhterman, A.; Gross, R. A. Macromolecules 2000, 33,
6303.
(13) The PCL macroinitiator (0.5 g, ca. 0.1 mmol) and CuBr (15
mg, 0.1 mmol) were weighed into a flame-dried round-
bottom flask equipped with a three-way stopcock. Several
cycles of evacuation and subsequent purging with nitrogen
were conducted to remove most of the dissolved oxygen.
Finally, styrene (1.5 g, 14 mmol) and PMDETA (17 mg, 0.1
mmol) were added under nitrogen by a syringe, and the
mixture was heated to 95 °C for 4 h. The solidified polymer
was dissolved in THF and precipitated in methanol, yielding
1.2 g of polymer.
Refer en ces a n d Notes
(1) Enzymes in Polymer Synthesis; Gross, R. A., Kaplan, D. L.,
Swift, G., Eds.; ACS Symp. Ser. 684; American Chemical
Society: Washington, DC, 1998. Gross, R. A.; Kumar, A.;
Kalra, B. Chem. Rev. 2001, 7, 2097. Kobayashi, S.; Uyama,
H.; Kimura, S. Chem. Rev. 2001, 7, 3793.
(2) Cordova, A.; Iversen, T.; Hult, K. Polymer 1999, 40, 6709.
Bisht, K. S.; Deng, F.; Gross, R. A.; Kaplan, D. L.; Swift, G.
J . Am. Chem. Soc. 1998, 120, 1363. Uyama, H.; Kikuchi,
H.; Kobayashi, S. Bull. Chem. Soc. J pn. 1997, 70, 1691.
(3) Kemp, A. R.; Macrae, A. R. Eur. Pat. EP 0506159, 1992.
van Gemert, R.; Cuperus, F. P. WO 9836058, 1998. Binns,
F.; Taylor, A. WO 9412652, 1994. Binns, F.; Harffey, P.;
Roberts, S.; Taylor, A. J . Chem. Soc., Perkin Trans. 1 1999,
2671.
(4) Schmid, A.; Dordick, J . S.; Hauer, B.; Kiener, A.; Wubbolts,
M.; Witholt, B. Nature (London) 2001, 409, 258.
(14) Bankova, M.; Kumar, A.; Gross, R. A.; Impallomeni, G.;
Ballisteri, A. Polym. Prepr. 2000, 41 (2), 1822.
(5) Wang, J . S.; Matyjaszewski, K. J . Am. Chem. Soc. 1995, 117,
5614. Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura,
T. Macromolecules 1995, 28, 1721. Controlled/ Living Radi-
cal Polymerization; Matyjaszewski, K., Ed.; ACS Symp. Ser.
768; American Chemical Society: Washington, DC, 2000.
(6) Hawker, C. J .; Hedrick, J . L.; Malmstro¨m, E. E.; Trollsa˚s,
M.; Mecerreyes, D.; Moineau, G.; Dubois, Ph.; J erome, R.
Macromolecules 1998, 31, 213. Mecerreyes, D.; Moineau, G.;
Dubois, P.; J erome, R.; Hawker, C. J .; Hedrick, J . L.;
Malmstro¨m, E.; Trollsås, M. Angew. Chem., Int. Ed. 1998,
37, 1274.
(7) Synthesis of benzyl ester of bis(hydroxy)methylpropionic
acid (bis-MPA) precursor: Ihre, H.; Hult, A.; Soderlind, E.
J . Am. Chem. Soc. 1996, 118, 6388. Synthesis of initiator
1: The precursor (5.0 g, 18 mmol) was dissolved in 25 mL
of dichloromethane and triethylamine (1.8 g, 18 mmol)
added. Then 2-bromo-2-methylpropionyl bromide (4.1 g, 18
(15) The water content of the immobilized enzyme determined
by Karl Fischer titration is 2.9 mass % before drying. For
the applied reaction conditions the molar amount of water
is almost equal to the amount of initiator; i.e., roughly half
of the polymer could be initiated by water. On the other
hand, the dried enzyme contains 0.15 mass % water, which
can theoretically result in 3-4% of water initiated polymer
provided all other components are dry.
(16) Comparison of GPC traces before and after precipitation in
methanol shows that low molecular weight fractions were
cut out due to the better solubility compared to the high
molecular weight fraction.
(17) Heise, A.; Diamanti, S.; Hedrick, J . L.; Frank, C. W.; Miller,
R. D. Macromolecules 2001, 34, 3798.
MA011929M