Free radical and nitroxide mediated polymerization of hydroxy-functional acrylates prepared via lipase-catalyzed transacylation reactions

Article abstract of DOI:10.1002/pola.24041

3-Hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-methyl-3-hydroxypropyl acrylate, 2-hydroxypropyl acrylate, neopentyl glycol acrylate, glyceryl acrylate, and dihydroxyhexyl acrylate were prepared via transacylation reaction of methyl acrylate with diols and triols catalyzed by Candida antarctica lipase B. After removal of the enzyme by filtration and the methyl acrylate by distillation, the monomers were polymerized via free radical polymerization (FRP) with azobisisobutyronitrile as initiator and nitroxide mediated polymerization (NMP) employing Blocbuilder alkoxyamine initiator and SG-1 free nitroxide resulting in hydroxy functional poly(acrylates). The NMP kinetics are discussed in detail. In addition, the polymers obtained by FRP and NMP are compared and the results are related to the amount of bisacrylates that are present in the initial monomer mixtures resulting from the transacylation reactions

Full text of DOI:10.1002/pola.24041

Free Radical and Nitroxide Mediated Polymerization of Hydroxy–
Functional Acrylates Prepared via Lipase–Catalyzed Transacylation
Reactions
¨
DRAGOS POPESCU, RICHARD HOOGENBOOM, HELMUT KEUL, MARTIN MOLLER
Institute of Technical and Macromolecular Chemistry, RWTH Aachen and DWI an der RWTH Aachen e.V., Pauwelsstr. 8,
Aachen D-52056, Germany
Received 15 January 2010; accepted 25 March 2010
DOI: 10.1002/pola.24041
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: 3-Hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-
methyl-3-hydroxypropyl acrylate, 2-hydroxypropyl acrylate, neo-
pentyl glycol acrylate, glyceryl acrylate, and dihydroxyhexyl acry-
late were prepared via transacylation reaction of methyl acrylate
with diols and triols catalyzed by Candida antarctica lipase B.
After removal of the enzyme by filtration and the methyl acrylate
by distillation, the monomers were polymerized via free radical
polymerization (FRP) with azobisisobutyronitrile as initiator and
hydroxy functional poly(acrylates). The NMP kinetics are dis-
cussed in detail. In addition, the polymers obtained by FRP and
NMP are compared and the results are related to the amount of
bisacrylates that are present in the initial monomer mixtures
resulting from the transacylation reactions. VC 2010 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 48: 2610–2621, 2010
KEYWORDS: controlled radical polymerization; enzyme-cata-
lyzed transacylation; hydrophilic polymers; hydroxy functional
acrylates; radical polymerization; water-soluble polymers
nitroxide mediated polymerization (NMP) employing Blocbuil-
TM
der
alkoxyamine initiator and SG-1 free nitroxide resulting in
INTRODUCTION Free radical polymerization (FRP) repre-
Acrylates are an important class of industrial monomers that
are found in a variety of consumer products. Acrylates with
various functional groups are used for the preparation of
polymers and resins as basis for specialty plastics, surface
coatings and latices. Therefore, the development of novel
functional acrylates is an area of major interest because of
their industrial importance. Hydroxy functional acrylate
monomers are versatile and ideal comonomers for crosslink-
ing via the pendant hydroxy group and as precursors for fur-
ther chemical modification leading to novel monomeric
building blocks. Hydroxy functional acrylates are also inter-
esting precursors for hydrophilic and water-soluble polymers
that are promising functional polymers for biotechnological
sents the most widespread industrial method to produce
1
polymeric materials, such as plastics, rubbers, and fibers.
The advantages of radical polymerizations over ionic or coor-
dination polymerization reactions are numerous: a large vari-
ety of vinyl monomers can be polymerized or copolymerized
covering a wide range of functional monomers requiring
only the absence of oxygen. The major drawback of conven-
tional radical polymerization is the lack of control over
2
the polymer microstructure, which is particularly important
for the preparation of more complex polymeric structures,
such as block copolymers or gradient copolymers. The rapid
development of controlled radical polymerization (CRP)
techniques, such as atom transfer radical polymerization
9
–14
applications.
Although 2-hydroxyethyl acrylate (HEA) is
3
,4
5,6
(
ATRP), nitroxide mediated polymerization (NMP), and
commercially available and widely used, other hydroxyl-func-
tionalized monomers, such as 3-hydroxypropyl acrylate
(3HPA), 4-hydroxybutyl acrylate (4HBA), neopentylglycol
acrylate (NPGA), and glyceryl acrylate (GA) are not commer-
cially available or only available at a very high price.
Therefore, this study aimed to develop a simple and straight-
forward method to prepare these monomers by enzymatic
transacylation followed by radical polymerization yielding
the poly(hydroxyl acrylate)s. Numerous reports have
appeared in the literature on the use of acrylic and metha-
crylic esters in transacylation reactions catalyzed by
reversible addition-fragmentation chain transfer polymeriza-
7
,8
tion (RAFT)
enabled the straightforward preparation of
well-defined polymers and copolymers with desired compo-
sitions, architectures, and functionalities. All these CRP meth-
ods are based on a fast dynamic equilibrium between the
dormant and the active species. The equilibrium constants,
which are very low in these systems, keep the concentration
of active species very low. As a result, termination and trans-
fer reactions are minimized, and a CRP can be achieved
under appropriate polymerization conditions.
Correspondence to: R. Hoogenboom (E-mail: r.hoogenboom@tue.nl) or M. M o¨ ller (E-mail: moeller@dwi.rwth-aachen.de)
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 2610–2621 (2010) V
2010 Wiley Periodicals, Inc.
C
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TM
SCHEME 1 Chemical structures of Blocbuilder , SG-1, and the investigated monomers. Boxes indicate structural isomers that are
obtained via acylation of non-symmetrical diols and triols.
1
5–18
lipases.
Patents on this subject (as for example by BASF,
diols and triols was investigated for the preparation of a
library of hydroxy functional mono(meth)acrylate monomers
under mild conditions.
Ludwigshafen, Germany) describe the enzymatic synthesis of
polyol acrylates showing the industrial relevance of this
research.
3
4
1
9
In the current work, the direct polymerization of these
hydroxyfunctionalized acrylates, that is, without further puri-
fication, is investigated as straightforward cascade reaction
method for the synthesis of novel poly(hydroxyl acrylate)s.
To evaluate the scope of this cascade reaction concept for
the preparation of poly(hydroxyl acrylate)s, both FRP and
nitroxide mediated polymerization (NMP) were evaluated,
including a detailed kinetic study of the NMP. NMP was
chosen, as the use of acyclic phosphonylated nitroxides like
CRP of hydroxyl-functionalized acrylates was only reported
in a limited number of studies. The controlled radical poly-
2
0
merization of 2HPA via RAFT was recently reported while
NMP was previously described by our group, including a ki-
netic study on the copolymerization of both isomers
2
1,22
(1MeHEA and 2HPA).
However, NMP of the other mono-
mers have, to the best of our knowledge, not yet been
2
3–25
described. Nonetheless, ATRP
and more recently SET-
2
6
0
0
LRP (single electron transfer-living radical polymerization)
of protected GA (solketal acrylate) was reported as well as
the free radical polymerization of 2HPA, GA, and NPGA
and the microemulsion polymerization of 4HBA. In addi-
tion, Save et al. performed a laborious study on the ATRP of
unprotected glyceryl methacrylate and 2-hydroxypropyl
N-tert-butyl-N-(1 -diethylphosphono-2,2 -dimethylpropyl) free
2
2,35,36
nitroxide (SG-1)
and the corresponding highly reactive
2
7
28,29
0
alkoxyamine initiators, such as the N-tert-butyl-N-(1 -di-
ethylphosphono-2,2 -dimethylpropyl)-O-(2-carboxyl-prop-2-yl)
alkoxyamine (Blocbuilder initiator registered trademark of
3
0
0
TM
37
Arkema) greatly improves the versatility, rate and efficiency of
3
1
38–41
methacrylate and Giacomelli examined the ATRP of sily-
lated glycerol methacrylate in organic medium.
this polymerization method.
Scheme 1 provides an overview
3
2
of the monomers used in this study as well as the structure of the
TM
Blocbuilder alkoxyamine initiator and the SG-1 free nitroxide.
In our previous work, we reported a simple cascade reaction
of enzymatic transacylation of methyl acrylate (MA) and
methyl methacrylate (MMA) with various monoalcohols fol-
lowed by free radical polymerization of the resulting mono-
RESULTS AND DISCUSSION
In our previous work we developed a straightforward cas-
cade method for the preparation of functional poly[(meth)
acrylate]s by enzymatic transacylation followed by FRP.
3
3
mers yielding different poly[(meth)acrylate]s.
More
recently, the enzymatic transacylation of MA and MMA with
FRP AND NMP OF HYDROXY–FUNCTIONAL ACRYLATES, POPESCU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
SCHEME 2 General scheme for
the cascade reaction comprising:
(
1) an enzymatic transacylation
reaction catalyzed by Novozyme
35 and (2) a free radical poly-
merization reaction or con-
4
a
trolled radical polymerization, in
this work nitroxide mediated
polymerization.
Hydrophilic, hydrophobic as well as cationic poly[(meth)
acrylate]s were succesfully obtained. The present work
focuses on the synthesis of various hydroxy functional poly
hols as well as methyl acrylate. In addition, the enzyme
retains its catalytically active conformation in this solvent
and the tertiary hydroxy groups of 2Me2BuOH cannot be
converted by the enzyme. To suppress the formation of bisa-
crylates, the molar ratio of the acyl donor/polyol was set to
1/number OH groups in the polyol according to a theoretical
3
3
(
acrylate)s based on the enzymatic transacylation of MA
with different diols or triols followed by the radical polymer-
ization of the obtained monomers (Scheme 2). In the first
step (1), the hydroxyl-functionalized monomers are prepared
by lipase-catalyzed transacylation of MA with different sym-
metrical diols (1,3Pdiol, 1,4Bdiol, 2M1,3Pdiol and NPG), an
asymmetrical diol (1,2Pdiol), a symmetrical triol (glycerol),
2
8,29,45
model presented in the literature.
The equilibrium of
these enzymatic transacylations is reached after 24 h as pre-
3
4
viously demonstrated. The conversion of the transacylation
reactions and the product ratios in the reaction mixture
3
4
1
and an asymmetrical triol (Htriol). After removal of the
Novozyme 435 by filtration and removal of MA by rotary
evaporation, the resulting monomers are polymerized via
free radical polymerization or NMP (step 2 in Scheme 2)
resulting in multifunctional hydroxy poly(acrylate)s, which
are interesting new polymeric materials as well as promising
were determined by H NMR spectroscopy (Table 1). The ra-
tio of hydroxy acrylate, diol (triol) and 2Me2BuOH was
determined after cooling the reaction mixture to room tem-
perature, removal of Novozyme 435 by filtration and re-
moval of MA by distillation under reduced pressure. The re-
1
moval of MA was confirmed by the disappearance of the H
4
2
precursors for reactive polymers.
NMR signal at d ¼ 3.68 ppm corresponding to the methyl
ester group of MA. The molar concentration of the acrylates
(monoacrylates and bisacrylates) in the remaining solution
was calculated using the density of the compounds (Table
1). If no densities were reported in literature, a density of 1
kg/L was assumed for the acrylates. It should be mentioned
that, especially for 3HPA and 4HBA, a relatively large amount
(ꢁ10 mol %) of bisacrylate was present in the monomer
mixture. In addition, GA, 2HPA and DHHA were obtained as
a mixture of isomers because of transacylation of nonequiva-
lent hydroxyl groups.
Lipase-Catalyzed Synthesis of Hydroxy Acrylates
3
4
Following our previous report, the transacylation of MA
ꢀ
with the diols and triols is performed at 50 C, which is
within the optimum temperature range for the enzyme sta-
ꢀ
43
bility (40–60 C), making this method suitable for the
preparation of heat-sensitive monomers. Because of the
hydrophilic nature of the diols and triols and their low solu-
bility in methyl acrylate, 2-methyl-2-butanol (2Me2BuOH)
4
4
was added as solvent, which is a good solvent for the alco-
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TABLE 1 Monomer Composition After Enzymatic
Transacylation Performed at T 5 50 8C for t 5 24 h in
Polymerizations
Free Radical Polymerization
2
Me2BuOH; Molar Concentration of the Monomer Mixture
At first, the free radical polymerization (FRP) of the hydroxy
functional acrylate was investigated, which is particularly
attractive as industrial process. For the FRP 1 mol % of AIBN
and MA Conversion
Monomer
Mixture
(
with respect to the monomer) was added to the monomer so-
lution that was obtained after removal of the enzyme and
excess MA from the enzymatic transacylation reaction mixture.
To reduce the viscosity of the polymerization mixture, 50 vol %
ethanol was added. The monomer conversion was determined
Concentration Concentration MA
a
b
c
No Name
(mol %)
(mol/L)
Conversion (%)
1
2
3
4
5
3HPA
BisPA
4HBA
BisBA
90
10
88
12
1.9
70.08
1
by H NMR spectroscopic analysis of the polymerization mix-
ture before and after the polymerization. The overall monomer
conversion was determined by comparing the total integral of
the acrylate signals (d ¼ 6.3 ꢂ 5.9 ppm) at time 0 and after 4 h
using the ethanol signal (d ¼ 3.44 ppm) as reference.
The final polymers resulting from free radical polymerization
as well as those prepared by nitroxide mediated polymeriza-
tion (cf. next section) were purified by dialysis in water using
a cellulose ester membrane with 100–500 Da molecular
weight cut off (MWCO) to remove low molecular weight ma-
terial. After dialysis, the polymers were isolated by freeze dry-
2.4
1.7
0.7
0.8
71.14
68
2Me3HPA 93
Bis2MePA
NPGA
BisNPGA
GA
7
95
5
44.2
54.2
94
3
DHIPA
BisGA
2HPA
3
ing. The molecular weight (M ) and the polydispersity indices
n
6
7
88
11
1
2.4
1.4
48.9
75.4
n w
(M /M ) of the purified polymers are summarized in Table 2.
1
MeHEA
Free radical polymerization of 3HPA and 4HBA did not lead
to soluble poly(hydroxyl acrylate)s, but a polymer gel was
formed after 15–20 min. This gel formation can be explained
by the high concentration of bisacrylates, namely 10 mol %
in 3HPA and 12 mol % in 4HBA, leading to crosslinking reac-
tions. However, the free radical polymerization of 2Me3HPA
that contains 7 mol % bisacrylates yielded soluble poly(2-
Me3HPA). Nonetheless, size exclusion chromatography of the
isolated polymer revealed a trimodal distribution with a
BisIPA
5,6DHHA 72
2
1
,6DHHA 14
,6DHHA
8
6
BisHA
a
1
Product distribution was calculated from H NMR spectroscopy.
b
The molar concentration was determined knowing the product distri-
1
bution from H NMR spectroscopy and the densities of the compounds
in the system. For the density values of the acrylates which are not
found in the literature we used 1 kg/L.
number average molecular weight M of 21,500 g/mol and a
n
c
1
polydispersity index of M /M ¼ 9 suggesting the formation
w
n
Calculated from H NMR spectroscopy according to the methyl ester
signal of MA (d ¼ 3.68 ppm) at time 0 and after 24 h, using 2Me2BuOH
d ¼ 0.82 ppm) as reference.
of coupled chains induced by the bisacrylates [Fig. 1(A)].
(
In contrast to these three monomers, all other monomers
showed the expected free radical polymerization behavior
TABLE 2 Conversions, Molecular Weights, and Polydispersity Indices of the Isolated Polymers
Free Radical Polymerization
Nitroxide Mediated Polymerization
a
b
c
c
b
d
c
c
No.
Monomer
X
p
(%)
M
n
PDI
X
p
(%)
M
n,th
M
n
PDI
e
e
e
e
e
1
2
3
4
5
6
7
a
3HPA
4HBA
2Me3HPA
NPGA
GA
n.d.
n.d.
97
n.d.
n.d.
n.d.
n.d.
9
65
54
66
43
94
74
63
9,000
8,200
11,100
26,300
10,500
9,100
5,200
9,000
6,100
2.1
3.5
1.7
1.3
1.6
1.4
1.2
e
21,500
17,600
5,100
9,900
86
2.3
2
7,300
96
14,100
10,000
12,300
2HPA
DHHA
98
14,000
6,100
2.3
1.3
d
98
Major monomer in the feed.
Theoretical number average molecular weight (Mn,th) calculated from
monomer to initiator ratio and monomer conversion.
b
c
1
Total monomer conversion determined by H NMR spectroscopy.
Number average molecular weight (M ) and polydispersity index (PDI)
e
n
Not determined due to gelation of the polymerization mixtures.
of purified polymers determined by size exclusion chromatography (SEC)
using N,N-dimethylformamide as eluent. Entry 5, was measured in water.
FRP AND NMP OF HYDROXY–FUNCTIONAL ACRYLATES, POPESCU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 1 Size exclusion chromatography (SEC) traces using (A) N,N-dimethylformamide and (B) water as eluent for the isolated
polymers obtained by free radical polymerization of the monomers prepared in situ via enzymatic transacylation. [Color figure can
be viewed in the online issue, which is available at www.interscience.wiley.com.]
resulting in monomodal molecular weight distributions with
polydispersity indices around 2 [Fig. 1(A,B)]. The low Mn
and PDI values of P(DHHA) are most likely due to the lim-
ited solubility of this polymer in DMF, which is used as SEC
eluent, resulting in a smaller hydrodynamic volume. The low
In the following, the NMP of the hydroxy acrylates obtained
by enzymatic transacylation will be discussed based on the
amount of bisacrylate present in the monomers based on
diols: (i) ꢁ10 mol % bisacrylate (12 mol % for 3HPA and 10
mol % for 4HBA); (ii) 6–7 mol % bisacrylate (7% for
2Me3HPA and 6% for DHHA); (iii) ꢃ6 mol % (1% for HPA).
Finally, the NMP of NPGA (5 mol % bisacrylate) and GA (3
mol % bisacrylate) will be discussed separately (iv) since
the high viscosity of these monomer mixtures required addi-
tional investigations with regard to dilution.
M of P(GA) can be ascribed to the use of a PEO calibration,
n
as PEO is likely to have a very different hydrodynamic vol-
ume in solution compared with P(GA). These SEC results of
the poly(hydroxy acrylate)s indicate that up to 6 mol % bis-
acrylate may be present in the monomer for the preparation
of soluble poly(hydroxyl acrylate)s with a monomodal molar
mass distribution by FRP. The resulting soluble poly(hydroxy
acrylate)s will still contain unreacted acrylate units in the
side chains and, thus, might be used as reactive intermedi-
ates for crosslinking or further functionalization.
(
i) In the case of 3HPA and 4HBA, which contain a high
concentration of bisacrylates (10 mol % and 12 mol %,
respectively), the ln([M] /[M] ) versus time plots are lin-
0
t
ear up to 40% (ln ꢄ 0.5) indicating a constant radical
concentration up to this conversion [Fig. 2(A)]. The lin-
ear M versus conversion plots also suggest a controlled
n
Nitroxide Mediated Polymerization
polymerization up to 40% conversion with polydisper-
The nitroxide-mediated polymerization of the new hydroxy
functional acrylates was investigated using the previously
optimized conditions: 110 C polymerization temperature
sities (M /M ) ranging from 1.1 to 1.4. At higher con-
w
n
versions a positive deviation from linearity is observed
[Fig. 2(B)] suggesting the loss of control due to chain
coupling reactions caused by the presence of bisacry-
lates. The SEC traces for 4HBA [Fig. 3(A)] reveal a shift
of the elution curves to higher molar masses with time.
However, at a conversion >40%, a significant shoulder
is observed at lower elution volumes, which becomes
the dominant peak after 8 h indicating the occurrence
of bisacrylate coupling reactions. Nonetheless, the NMP
allows the formation of soluble and relatively well-
defined P(3HPA) and P(4HBA), which could not be
achieved by FRP. This difference can be ascribed to the
low concentration of active centres in NMP leading to
the formation of uniform chains, which suppresses the
formation of cross-linked structures at low monomer
conversions.
ꢀ
and 10% excess SG-1 free nitroxide relative to the Blocbuil-
TM
21,22
der
alkoxyamine initiator.
As the acrylate monomers
have a high propagation rate constant, excess of free nitro-
xide is required to reduce the polymerization rate and retain
control over the polymerization. Since the monomer solu-
tions are viscous because of the presence of diol/triol that
was used in excess during the monomer synthesis, a certain
amount of DMF, which is known to increase the polymeriza-
4
6
tion rate in NMP, was added to decrease the viscosity and
to prevent the loss of control over the polymerization due to
limited diffusion. The homopolymerization kinetics of the dif-
1
ferent monomers were determined by H NMR spectroscopy
and SEC. The monomer conversions were calculated using
the integral of the acrylate signals (d ¼ 6.3ꢂ5.9 ppm) and
the signals of DMF (d ¼ 7.95 ppm) as a reference. The evolu-
tion of molecular weight and PDI during the polymerizations
was determined by SEC.
(ii) The polymerizations of 2Me3HPA and DHHA that contain
similar amounts of bisacrylates (2Me3HPA: 7 mol %;
DHHA: 6 mol %) revealed comparable polymerization
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ARTICLE
FIGURE 2 Kinetic investigation of the nitroxide mediated polymerizations of 3HPA, 4HBA, and 2Me3HPA (AþB) as well as 2HPA
ꢀ
and DHHA (CþD) with 10 mol % SG-1 at 110 C in N,N-dimethylformamide at different monomer concentrations (see insert of the
0 t
graphs) and a monomer-to-initiator ratio of 100. Left: ln([M] /[M] ) versus time plots for the overall monomer conversion. Right:
n
Number-average molecular weight (M ) and polydispersity index (PDI) versus conversion plots.
results. Linear first-order kinetic plots were observed up
to 50% conversion [Fig. 2(A,C)] suggesting a constant
concentration of propagating radicals under these condi-
tions. For 2Me3HPA the linearity of the first order kinetic
plot is lost after 50% conversion, which can be ascribed
to the increasing viscosity of the polymerization mixture
at the longest polymerization time (6 h) comprise a high
molar mass shoulder because of coupling reactions of the
bisacrylates. For 2Me3HPA the control is lost at an earlier
stage of the polymerization compared to DHHA because
of the higher viscosity of the polymerization mixture
resulting from the higher initial monomer concentration.
(iii) Transacylation of MA with 1,2Pdiol led to a mixture of
monoacrylate isomers containing only 1% bisacrylate af-
ter 24 h. The first order kinetic plot for the NMP of this
monomer mixture [Fig. 2(C)] as well as the Mn versus
conversion plot [Fig. 2(D)] were linear demonstrating
the absence of significant termination, chain transfer
reactions and/or coupling reactions. The monomodal
SEC traces [Fig. 3(B)] further illustrate the controlled
character of the polymerization. Recently, we reported
the NMP kinetics for the commercially available isomeric
mixture of hydroxypropyl acrylate (78% 2HPA and 22%
(
initial monomer concentration 1.5 M). This did not
occur for the polymerization of DHHA because of the
lower monomer concentration (initial monomer concen-
tration 1.1 M). Furthermore, the linear increase of molec-
ular weight up to 70% conversion for 2Me3HPA and up
to 50 % conversion for DHHA together with the relatively
low molecular weight distributions confirm the control
over the polymerizations [Fig. 2(B,D)]. Note that the SEC
of P(DHHA) prepared by NMP was performed in DMAc,
while the polymer prepared by FRP was analyzed by SEC
in DMF resulting in distinctly different calculated molar
masses. The SEC traces of P(2Me3HPA) and P(DHHA)
revealed a defined monomodal distribution up to 50%
conversion [Fig. 3(C,D)]. However, the polymers obtained
2
1
1MeHEA). In that study it was found that both 2HPA
and 1MeHEA revealed linear kinetic plots following the
2/3 47,48
persistent radical effect model (ln([M] /[M] ) vs. t ).
0
t
FRP AND NMP OF HYDROXY–FUNCTIONAL ACRYLATES, POPESCU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 Size exclusion chromatography (SEC) traces using N,N-dimethylacetamide as eluent for the polymerization of 4HBA (A)
and DHHA (D) and N,N-dimethylformamide as eluent for the polymerization of 2HPA (B) and 2Me3HPA (C) in N,N-dimethylforma-
ꢀ
mide with 10 mol % SG-1 at 110 C using a monomer-to-initiator ratio of 100. [Color figure can be viewed in the online issue,
which is available at www.interscience.wiley.com.]
At this point it is worth to mention that the hydroxy
acrylates used in the current work revealed linear first
order kinetics without a strong persistent radical effect
commonly observed for NMP. Nonetheless, a slight posi-
tive deviation from linearity is observed in many kinetic
plots (Figs. 2 and 4, ln([M] /[M] ) vs. t). The absence of
a strong persistent radical effect is most likely because
of the lower monomer concentration used in the current
study, which suppresses radical termination reactions
and, thus, suppresses the accumulation of SG-1 free
nitroxide.
concentration for GA. Figures 4 and 5 depict the kinetic
results for NPGA and GA polymerizations at these two
different concentrations. The first order kinetic plots for
the polymerizations of NPGA and of GA were linear at
both concentrations implying that the radical concentra-
tion remained constant even at high conversions (94%
for GA 0.8 M) [Fig. 4(A,C)]. The molecular weight
increased linearly with conversion and the PDI values
are below 1.4 in the case of NPGA [Fig. 4(B)] indicating
the controlled character of the polymerization. In the
case of GA, SEC analysis was performed in water as elu-
ent showing only a small increase of molecular weight
with conversion. This big discrepancy between the aque-
ous SEC molecular weights and the targeted molecular
weights as well as the broad molecular weight distribu-
tions of the P(GA) (PDI ¼ 1.7) [Fig. 4(D)] are presum-
ably caused by the unsuitable calibration protocol that
is based on poly(ethylene oxide) standard. Nonetheless,
0
t
(
iv) The enzymatic transacylation of MA with glycerol or
NPG resulted in highly viscous monomer mixtures con-
taining 3 mol % and 5 mol % bisacrylates, respectively.
To study the influence of monomer concentration (and
viscosity) on the polymerization rate, two different con-
centrations were chosen: 0.6 M and 0.3 M monomer
concentration for NPGA and 0.8 M and 0.4 M monomer
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ARTICLE
FIGURE 4 Kinetic investigation of the nitroxide mediated polymerizations of NPGA (AþB) and GA (CþD) with 10 mol % SG-1 at
ꢀ
1
10 C in N,N-dimethylformamide at different monomer concentrations (see insert of the graphs) and a monomer-to-initiator ratio
0 t n
of 100. Left: ln([M] /[M] ) versus time plots for the overall monomer conversion. Right: Number-average molecular weight (M )
and polydispersity index (PDI) versus conversion plots. [Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
FIGURE 5 Size exclusion chromatography (SEC) traces using N,N-dimethylacetamide as eluent for the polymerization of NPGA (A)
and water as eluent for the polymerization of GA (B) in N,N-dimethylformamide at 0.8 M monomer concentration with 10 mol %
ꢀ
SG-1 at 110 C using a monomer-to-initiator ratio of 100. [Color figure can be viewed in the online issue, which is available at
www.interscience.wiley.com.]
FRP AND NMP OF HYDROXY–FUNCTIONAL ACRYLATES, POPESCU ET AL.
2617

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
where kp is the propagating rate constant, K is the equilib-
ꢆ
rium constant, [P ] the concentration of active radicals,
[
SG1] and [P-SG1] are the initial concentrations of SG1 and
0 0
TM
Blocbuilder , respectively, and r is the initial molar ratio of
SG1 to Blocbuilder . The value of k
TM
ꢅ
p
[P ] is given by the
slope of the plot of ln([M] /[M] ) versus time. All the calcu-
0
t
lated k K values, at different molar concentrations, are sum-
p
marized in Table 3. It should be noted that these
polymerizations are random copolymerizations rather than
homopolymerizations, because of the presence of monoacry-
lates and bisacrylates and in the case of GA, 2HPA, and
DHHA also the presence of isomeric structures. Conse-
SCHEME 3 Activation–deactivation equilibrium of SG-1 nitroxide
mediated polymerization of hydroxy acrylates.
the monomodal SEC traces shifted to higher molecular
weights with conversion indicating the control over the
quently, k K is a global rate constant for the isomeric mono-
p
mer mixtures.
polymerizations [Fig. 5(A,B)]. However,
a shoulder
appeared at higher elution volumes at higher conver-
sions of GA, which might indicate interactions between
the polymer and the SEC column material. Alternatively
this shoulder could be due to chain transfer reactions,
which is unlikely as it has not been observed for the
other monomers.
From these k K values it can be concluded that a higher po-
p
larity of the monomer results in a higher polymerization
ꢂ6 ꢂ1
rate: The highest k K value of 9.6 ꢈ 10
s
is obtained for
p
GA, which is the most polar monomer, at 0.8 M while NPGA,
the least polar monomer, revealed the lowest k K value of
p
ꢂ7 ꢂ1
6.6 ꢈ 10
s
at 0.3 M. This is accordance with literature
where it is reported that an increase of the polymerization
Further analysis of the first order kinetic plots that were
obtained for the NMP of the hydroxy-functional acrylates
was performed to determine the effect of the hydroxyl
groups on the polymerization rate constants. The key steps
of SG1-mediated polymerization (Scheme 3) consist of the
5
2
rate is observed for more polar monomers. It is worth to
notice that the values obtained in this work are in good
agreement with those from literature for higher monomer
ꢂ6 ꢂ1
ꢂ6
concentration: k K ¼ 4.3 ꢈ 10
s
in bulk and 5.1 ꢈ 10
p
ꢂ1
s
in 50 wt % DMF solution was reported for 2-hydrox-
activation-deactivation equilibrium (K
¼
k /k ), which
d c
ꢀ
51
ꢂ6
yethyl acrylate polymerized at 110 C ; k K ¼ 4.6 ꢈ 10
p
defines the concentration of active chains (active centres)
and the addition of the active species to the monomer. This
ꢂ1
ꢂ6 ꢂ1
s
in bulk and 5.7 ꢈ 10
s
in 50 wt % DMF for 2-
ꢀ
46
ꢂ6
4
6,49–51
(dimethylamino)ethyl acrylate at 112 C ; k K ¼ 5.3 ꢈ 10
p
equilibrium can be described by eq 1:
ꢂ1 ꢂ6 ꢂ1
s
for N-acryloyl morpholine; 2.5 ꢈ 10
s
for N,N-di-
for 2HPA using a 2 M
ꢂ6 ꢂ1
ꢆ
methyl acrylamide and 1.4 ꢈ 10
s
kp ꢅ ½P ꢇ ꢅ ½SG1ꢇ
0
ꢆ
kp ꢅ K ¼
¼ kp ꢅ ½P ꢇ ꢅ r
(1)
monomer concentration in DMF and 20% free SG-1 at 110
½
P ꢂ SG1ꢇ0
p
TABLE 3 The k K Values for the Hydroxy-Functional Acrylates at Different Molar Concentrations
a
a
b
ꢂ1
)
Monomer
Chemical Structure of the Monomer
[M] (mol/L)
k
p
K (s
ꢂ6
ꢂ6
ꢂ6
3
4
2
HPA
1.65
2.1
3.5 10
*
HBA
2.5 10
*
Me3HPA
1.5
5.8 10
*
ꢂ6
4.65 10
*
2
HPA
2.1
1.1
0.6
ꢂ6
3.2 10
*
DHHA
NPGA
ꢂ6
1,9 10
*
ꢂ7
0
.3
6.6 10
*
ꢂ6
ꢂ6
GA
0.8
.4
9.6 10
*
0
2.8 10
*
a
Major monomer.
b
1
The molar concentration was determined knowing the product distribution from H NMR spectroscopy and the densities of the compounds in the sys-
tem. For the density values of the acrylates which are not found in the literature we used 1 kg/L. All the [M] values are determined after addition of DMF.
2618
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
ꢀ
22
ꢀ
C. All these results suggest that the hydroxyl groups have
a catalytic effect via hydrogen bonding to the nitroxide,
series at 30 C with a flow rate of 1.0 mL/min. For DMAc as
eluting solvent a Shimadzu system with a SCL-10A system
controller, a LC-10AD pump, a RID-10A refractive index de-
tector and PSS gram 30 (pore size 30 Å; bead size 10 lm)
and PSS gram 1000 (pore size 1000 Å; bead size 10 lm) col-
5
2
which was found to accelerate the NMP. In the current arti-
cle, only a small volume of DMF was added to the reactions
mixture to decrease the viscosity of the solution. In contrast,
a higher amount of DMF was reported to suppress the for-
ꢀ
umns were used in series at 60 C with a flow rate of 1.0
2
2
mation of hydrogen bonds between the HPA and SG-1.
mL/min. The molecular weights and polydispersities were
calculated using poly(methyl methacrylate) (PMMA) calibra-
tion. For water as eluting solvent an isocratic pump (Agilent
100 series), a refractive index detector (Wyatt Optilab DSP)
and four PSS suprema columns filled with modified acrylate-
copolymer-network particles were applied at 30 C with a
flow rate of 1.0 mL/min. The length of the precolumn was
0 mm and 300 mm for the other three separation columns.
This suppression of the hydrogen bonding effect at lower
monomer concentration is also evident from the observation
that the polymerizations of NPGA and GA at higher monomer
concentrations (0.6 M for NPGA; 0.8 M for GA) were about
three times faster than the polymerizations at half the mono-
mer concentrations (0.3 M for NPGA and 0.4 M for GA).
1
ꢀ
5
The diameter of each column was 8 mm, the diameter of the
copolymer particles 10 lm, and the nominal pore widths of
the separation columns were 30, 1000, and 3000 Å, respec-
tively. The molecular weights and polydispersities were cal-
culated using poly(ethylene oxide) (PEO) calibration.
EXPERIMENTAL
Materials
Methyl acrylate (MA, 99%, Acros Organics), 1,3-propanediol
(
9
9
1,3Pdiol, 99.6þ%, Aldrich), 1,4-butanediol (1,4Bdiol,
9þ%, Acros Organics), 2-methyl-1,3-propanediol (2M1,3Pdiol,
9%, Aldrich), neopentyl glycol (NPG, 99%, Aldrich), glycerol
Syntheses
(
9
99.5þ%, Sigma Aldrich), 1,2-propanediol (1,2Pdiol,
9.5þ%, Sigma-Aldrich), 1,2,6-hexanetriol (Htriol, 97%,
Acros Organics), azobisisobutyronitrile (AIBN, 98þ%, Fluka),
Representative Procedure for the Lipase-Catalyzed
Synthesis of Hydroxy Acrylates
1,3Pdiol (8.8 g, 116 mmol) was dissolved in 2Me2BuOH
3
4
2
-methyl-2-butanol (2Me2BuOH, 96þ%, Fluka), N,N-di-
(13.6 mL, 50 vol % of the reaction mixture). To this mix-
ture, MA (5 g, 58 mmol) and Novozyme 435, (500 mg, 10 wt
% with respect to MA), were added and stirred for 24 h at
methylformamide (DMF, 98%, Fluka) and all solvents were
used as received. MA was passed through a neutral alumin-
ium oxide column before use to remove the inhibitor. A
commercial lipase, Novozyme 435 (Lipase B from Candida
antarctica immobilized on a macroporous acrylic resin,
ꢀ
1
50 C. The reaction progress was monitored by means of H
NMR spectroscopy. After 24 h the following molar ratio of
products was calculated: MA: 3HPA: BisPA ¼ 35%: 65%:
5%. The enzyme was removed by filtration, MA by distilla-
1
0,000 U/g Novo Nordisk) was dried under reduced pres-
ꢀ
sure at room temperature for 24 h and stored under nitro-
gen before it was used as a biocatalyst for the transacylation
reactions. Blocbuilder initiator and SG-1 (85%) free nitro-
tion under reduced pressure at 40 C, and the reaction mix-
ꢀ
ture was stored at 5 C before polymerization. All other reac-
TM
tions were performed according to this procedure, (Table 4,
entry 1).
xide were kindly provided by Arkema and used as received.
All reactions were carried out in nitrogen atmosphere. Nitro-
gen (Linde, 5.0) was passed over molecular sieves (4 Å) and
finely distributed potassium on aluminium oxide before use.
Representative Procedure for Free Radical Polymerization
In a Schlenk tube, 3HPA stock solution in 2Me2BuOH (7 mL,
1
0
3.223 mmol acrylate), ethanol (7 mL) and AIBN (21.7 mg,
.13 mmol), was bubbled with nitrogen for at least 30 min
Measurements
1
to deoxygenate the solution and then immersed into an oil
H NMR spectra were recorded on a Bruker DPX-400 FT-
ꢀ
bath, preheated at 70 C (Table 1, entry 1). After 240 min, a
NMR spectrometer at 400 MHz, using deuterated dimethyl
sample (200 lL) was taken for analysis of the crude product.
For SEC analysis, 100 lL of this sample were diluted with
sulfoxide (DMSO-d ) as the solvent. Chemical shifts are
6
reported relative to the residual solvent signals. The concen-
tration of the hydroxy acrylates resulting from the enzymatic
transacylation was calculated using the signal of the methyl
protons of 2Me2BuOH (d ¼ 1.05 ppm) as reference signal.
The overall monomer conversions for homopolymerizations
were calculated according to the total integral of the acrylate
signals (d ¼ 6.3 ꢂ 5.9 ppm) using DMF (d ¼ 7.95 ppm) or
ethanol (d ¼ 3.44 ppm) as reference signal.
2
.0 mL DMF, DMAc or water and 100 lL of the sample were
1
diluted with 600 lL DMSO for H NMR spectroscopy. Subse-
quently, the reaction mixture was cooled to room tempera-
ture and the crude polymerization mixture was diluted with
3
0 mL distilled water. The polymer was purified by dialysis
against water, using a cellulose ester membrane with 100–
00 Da molecular weight cut off (MWCO), and isolated by ly-
5
ophilization. In this particular example, the mixture became
a gel after 20 minutes and no further work up was
performed.
Size exclusion chromatography (SEC) was carried out using
DMF (containing 1.0 g LiBr/L), DMAc (containing 2.1 g LiCl/
L) or 0.1 M aqueous NaNO solution with methanol as inter-
3
nal standard as eluting solvents. For DMF as eluting solvent
a high pressure liquid chromatography pump (Bischoff
HPLC), a Jasco 2035-plus RI detector and four MZ-DVB gel
columns (30 Å, 100 Å, 1000 Å, and 3,000 Å) were used in
Representative Procedure for Nitroxide Mediated
Polymerization
In a Schlenk tube, 3HPA stock solution in 2Me2BuOH (7 mL,
TM
13.223 mmol acrylate), DMF (1 mL), Blocbuilder (50 mg,
FRP AND NMP OF HYDROXY–FUNCTIONAL ACRYLATES, POPESCU ET AL.
2619

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 4 Cascade Reaction: Starting Materials and Reaction Conditions
a
b
c
Enzymatic Transacylation
Free Radical Polymerization
Nitroxide Mediated Polymerization
d
e
d
e
No.
Alcohol
g (mmol)
V
transacylation (mL)
C
M
(M)
V
transacylation (mL)
M
C (M)
f
1
1,3Pdiol
1,4Bdiol
2M1,3Pdiol
NPG
8.8 (116)
10.5 (116)
20.9 (232)
20 (200)
7
6
6
0.9
1.2
7
7
7
1.65
2.1
f
2
g
3
0.85
0.4
0.4
1.2
0.7
1.5
h
i
i
4
10
10
6
8/4
8/4
7
0.6/0.3
g
i
i
5
Glycerol
1,2Pdiol
Htriol
32 (348)
0.7/0.4
2.1
g
6
17.7 (232)
46.7 (348)
g
7
10
7
1.1
a
T ¼ 50 ꢀC; t ¼ 24 h; 10 wt % Novozyme 435 with respect to MA and 50
e
C
M
is the monomer molar concentration in the final monomer solution
1
vol % 2Me2BuOH.
b
and was determined, knowing the product distribution from H NMR spec-
troscopy and the densities of the compounds in the system. For the density
values of the acrylates which are not available in the literature we used 1 kg/
ꢀ
T ¼ 70 C; t ¼ 4 h; 1 mol % AIBN and 50 vol % EtOH.
c
ꢀ
T ¼ 110 C; t ¼ 8 h; [M]:[BB]:[SG-1] ¼ 100:1:0.1 molar. Reactions 1 to
6
were performed with addition of 1 mL DMF, reaction 7 with 3 mL
M
L. All C values of the table are determined after addition of EtOH or DMF
f
DMF.
d
5 g (58 mmol) MA.
10 g (116 mmol) MA.
8.6 g (100 mmol) MA.
50 vol % DMF.
g
h
i
Vtransacylation is the monomer stock solution volume taken from the en-
zymatic transacylation reaction mixture after Novozyme 435 and MA
removal.
0
.13 mmol) and SG-1 free nitroxide (3.9 mg, 0.013 mmol),
tions caused by the presence of bisacrylates. In the case of
5,6DHHA (6 mol % bisacrylate) and 2Me3HPA (7 mol %
bisacrylate), a controlled NMP could be achieved up to 50%
monomer conversion, whereas in the case of NPGA (contain-
ing 5 mol % bisacrylate) and GA (containing 3 mol % bisa-
crylate) the first order kinetic plots remain linear up to high
conversions. Moreover, for these two latter monomers, the
polymerization rate increased by a factor of three when the
monomer concentration was doubled, which was ascribed to
accelerating hydrogen bonding effect, which was suppressed
at lower concentrations in DMF.
were mixed and bubbled with nitrogen for at least 30 min to
deoxygenate the solution and then immersed into an oil
bath, preheated to 110 C (Table 1, entry 1). For the kinetic
investigations, samples (600 lL) were withdrawn from the
polymerization mixture after 7, 15, 30, 60, 120, 240, and
ꢀ
480 min. For SEC analysis, 100–500 lL of each sample was
diluted with 2.0 mL DMF, DMAc or water and 100 lL of the
sample was diluted with 600 lL DMSO for H NMR spectros-
1
copy. After 8 h polymerization time, the reaction mixture
was cooled to room temperature and the crude polymeriza-
tion mixture was diluted with 20 mL distilled water. The
polymer was purified by means of dialysis against water,
using a cellulose ester membrane with 100–500 Da molecu-
lar weight cut off (MWCO), and isolated by means of lyophi-
lization. SEC analysis of the isolated polymer using DMF as
Finally, the cascade reaction–enzymatic transacylation, nitro-
xide mediated polymerization or free radical polymerization-
described herein represents a mild, straightforward and
rapid process for the preparation of hydroxy functional poly-
mers. As such, well defined hydroxy functional poly(acry-
late)s were prepared bearing all the prerequisites for a vari-
ety of applications, including thermoresponsive polymers as
eluent revealed a M
n
¼ 11,100 and M
w
/M
n
¼ 2.1.
5
3
will be shown in an upcoming publication.
This work was carried out in the framework of the IP-project
‘Sustainable Microbial and Biocatalytic Production of
Advanced Functional Materials’’ (BIOPRODUCTION/NMP-2-T-
007-026515) funded by the European Commission. R. Hoo-
CONCLUSIONS
Various hydroxy functional polyacrylates were prepared via
FRP as well as NMP of hydroxy functional monomers that
were prepared by in situ lipase-catalyzed transacylation reac-
tions of MA with different diols and triols yielding a mixture
of monoacrylates and bisacrylates. In the current work, we
have shown that up to 6 mol % of bisacrylate (2HPA, NPGA,
GA, DHHA) may be present in the monomer mixture result-
ing from enzymatic transacylation to obtain soluble (non
cross-linked) polymers by FRP. With 7 mol % bisacrylate
‘
2
genboom is grateful to the Alexander von Humboldt foundation
and the Netherlands Scientific Organization (NWO; Veni-grant)
for financial support. Arkema is thanked for providing the Bloc-
TM
builder alkoxyamine initiator and SG-1 free nitroxide.
(
2Me3HPA) significant polymer coupling was observed, while
bisacrylate concentrations of 10 and 12 mol % (3HPA,
HBA) led to gel formation.
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