Synthesis of poly(2-hydroxyethyl methacrylate) end-capped with asymmetric functional groups via atom transfer radical polymerization

Article abstract of DOI:10.1039/c3nj01398g

Poly(2-hydroxyethyl methacrylate) (PHEMA) end-capped with living chloride and alkyne groups was synthesized via ATRP of HEMA using CuCl/CuCl ₂/2,2′-bipyridine as a catalyst in a solvent mixture of methanol and 2-butanone. The effects of parameters including the initiator, solvent, temperature and initial monomer to initiator ratios on polymerization were studied in terms of polymerization kinetics, the degree of polymerization (DP) and molar mass dispersity (D) of the resulting PHEMA polymer. ATRP of HEMA using propargyl 2-bromoisobutyrate (PBiB) as an initiator was poorly controlled, but those using 3-(trimethylsilyl)propargyl 2-bromoisobutyrate (TMSPBiB) and 3-(triisopropysilyl)propargyl 2-bromoisobutyrate (TiPSPBiB) as initiators were well-controlled. Moreover, the apparent propagation rate constant for ATRP of HEMA using the TMSPBiB initiator was higher than that using the TiPSPBiB initiator. The solvent mixture of methanol-2-butanone at different compositions greatly affected the polymerization controllability. A high molecular weight PHEMA sample with a DP of 1000 and a D of 1.34 was obtained under appropriate conditions. The poly(2-hydroxyethyl methacrylate)-block-poly(butyl acrylate) (PHEMA-b-PBA) diblock copolymer was prepared through ATRP of BA using (CH ₃)₃Si-CC-PHEMA-Cl as a macroinitiator. The methoxyl polyethylene glycol-block-poly(2-hydroxyethyl methacrylate) (MPEG-b-PHEMA) diblock copolymer was prepared by click reaction between MPEG-N₃ and HCC-PHEMA-Cl. These two reactions demonstrated the reactivity of the asymmetric functional groups end-capping the PHEMA, and further provided modular examples for the synthesis of a novel well-defined (co)polymer with complex architectures. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01398g

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Synthesis of poly(2-hydroxyethyl methacrylate)
end-capped with asymmetric functional groups
via atom transfer radical polymerization†
Cite this: New J. Chem., 2014,
38, 2538
Chengmin Hou,ab Shudong Lin,ab Feng Liu,ab Jiwen Hu,*ab Ganwei Zhang,ab
Guojun Liu,ac Yuanyuan Tu,ab Hailiang Zouab and Hongsheng Luoab
Poly(2-hydroxyethyl methacrylate) (PHEMA) end-capped with living chloride and alkyne groups was
synthesized via ATRP of HEMA using CuCl/CuCl2/2,20-bipyridine as a catalyst in a solvent mixture of
methanol and 2-butanone. The effects of parameters including the initiator, solvent, temperature and
initial monomer to initiator ratios on polymerization were studied in terms of polymerization kinetics, the
degree of polymerization (DP) and molar mass dispersity (Ð) of the resulting PHEMA polymer. ATRP of
HEMA using propargyl 2-bromoisobutyrate (PBiB) as an initiator was poorly controlled, but those using
3-(trimethylsilyl)propargyl 2-bromoisobutyrate (TMSPBiB) and 3-(triisopropysilyl)propargyl 2-bromoisobutyrate
(TiPSPBiB) as initiators were well-controlled. Moreover, the apparent propagation rate constant for ATRP of
HEMA using the TMSPBiB initiator was higher than that using the TiPSPBiB initiator. The solvent mixture of
methanol–2-butanone at different compositions greatly affected the polymerization controllability. A high
molecular weight PHEMA sample with a DP of 1000 and a Ð of 1.34 was obtained under appropriate
conditions. The poly(2-hydroxyethyl methacrylate)-block-poly(butyl acrylate) (PHEMA-b-PBA) diblock
copolymer was prepared through ATRP of BA using (CH3)3Si–CRC–PHEMA–Cl as a macroinitiator. The
methoxyl polyethylene glycol-block-poly(2-hydroxyethyl methacrylate) (MPEG-b-PHEMA) diblock copolymer
was prepared by click reaction between MPEG-N3 and HCRC–PHEMA–Cl. These two reactions demon-
strated the reactivity of the asymmetric functional groups end-capping the PHEMA, and further provided
modular examples for the synthesis of a novel well-defined (co)polymer with complex architectures.
Received (in Montpellier, France)
11th November 2013,
Accepted 8th March 2014
DOI: 10.1039/c3nj01398g
www.rsc.org/njc
Recently, the development of polymer synthesis techniques
enabled the synthesis of well-defined PHEMA-based (co)polymers
Introduction
2-Hydroxyethyl methacrylate (HEMA) is one of the most impor- with complex architectures and significantly broadened the
tant functional methacrylate commercial monomers because of application scope of PHEMA-based (co)polymers in novel
its hydrophilic nature and hydroxyl groups with the capability nanomaterials.3b
of undergoing a wide variety of reactions.1 Since Wichterle and
Usually, conventional free radical polymerization (CFRP)
Lim reported the use of the PHEMA hydrogel for biological techniques are employed to prepare HEMA-based (co)polymers
applications in 1960,2 several types of novel and important due to their applicability for a broad range of monomers and
PHEMA-based polymeric materials have been developed, such tolerance to a variety of impurities often encountered in most
as contact lenses, dental fillings, surgical implants, tissue industrial processes.4 However, the major drawbacks of this
engineering scaffolds, biosensors, catheters, hemodialysis technique include: (1) the difficulty in controlling the mole-
membranes, wound dressings, drug delivery agents and so on.3 cular weight (Mn) and molar mass dispersity (Ð) of the resulting
(co)polymers and (2) the difficulty in preparing well-defined
polymers with complex structures due to rapid transfer and
termination reactions.5
a Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou,
510650, P. R. China. E-mail: hjw@gic.ac.cn; Fax: +86-020-85232307;
Tel: +86-020-85232307
b Key Laboratory of Cellulose Lignocellulosics Chemistry,
University of Chinese Academy of Sciences, 510650, P. R. China
c Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario,
Canada K7L 3N6
It is well known that well-defined (co)polymers with con-
trolled Mn, low Ð, as well as predesigned architecture are very
important for fundamental research and commercial applica-
tions, including the disclosure of the relationship between
the properties and the polymer structure, and some specific
applications such as the overall design of the scaffold and
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3nj01398g
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nanomaterials with tailored structure.6 Therefore, the con-
Along with the progress of the ATRP technique, electron
trolled (co)polymerization of HEMA via living polymerization transfer ATRP (AGET-ATRP) and single-electron transfer living
techniques has drawn many researchers’ attention over the radical polymerization (SET-LRP) were also developed for the
years.3b Given the labile protons in hydroxyl groups,7 synthesis preparation of well-defined PHEMA. For example, in 2006,
of well-defined PHEMA by anionic polymerization required Matyjaszewski et al. reported well-defined PHEMA prepared
multiple processes in which the HEMA monomers were firstly by AGET-ATRP of HEMA in a solvent mixture of 2-butanone and
protected with a trimethylsilyl group and finally de-protected by MeOH (3/2, v/v) using ethyl 2-bromoisobutyrate as an initiator
removing the protecting groups.7
and CuX2/bpy (tris(2-pyridylmethyl)-amine) (TPMA) and tin(II)
However, the emergence and development of living radical 2-ethylhexanoate (Sn(EH)2) used as reducing agents as the
polymerization (LRP) techniques over the past two decades have catalytic system at 50–70 1C,20 the monomer conversion reached
allowed for the synthesis of well-defined polymers from a wide 53% after B21 h with a targeted maximum DP of 250. Recently,
variety of functional monomers including HEMA. Currently, the Baker et al. also reported well-defined PHEMA with a target DP of
three most effective LRP techniques including nitroxide mediated less than 100 prepared from AGET-ATRP in MeOH using Me-Br
polymerization (NMP), atom transfer radical polymerization as an initiator and CuX2/TPMA/ascorbic acid or hydrazine at
(ATRP) and reversible addition-fragmentation chain transfer 25 1C.21 In these cases, it was proposed that the active catalyst
(RAFT) polymerization have been widely employed.8 In com- CuX was generated from CuX2 reduced in situ with a reducing
parison with the few successful preparation methods available agent such as Sn(EH)2,20 ascorbic acid and hydrazine.21 Very
for PHEMA-based (co)polymers via NMP9 and RAFT,10 ATRP is recently, Percec et al. reported SET-LRP of HEMA using the
also one of the most promising methods for preparing well- methyl a-bromophenylacetate initiator and the Cu(0)/Me6TREN
defined (co)polymers based on PHEMA.3b
catalytic system in DMSO at 25 1C.22 PHEMA samples with Mn in
The ATRP of HEMA was first reported in 1999 by the the range of 333 500–1 017900 g molꢀ1 and Ð less than 1.50 were
Matyjaszewski group using the alkyl bromide initiator and the synthesized by this approach.
CuCl/2,20-bipyridine (bpy) catalytic system in a mixed solvent of
The most important factors for the preparation of PHEMA via
2-butanone and 1-propanol at a volume ratio of 7/3.1 In this controlled ATRP are the initiator system, temperature, and solvent.
mixed solvent, well-controlled polymerization of HEMA was The studies on the polymerization kinetics of HEMA reported so
demonstrated at 50 1C at [M]0/[I]0 = 100/1 in terms of the kinetic far mainly focused on a [M]0/[I]0 of B100, and the polymerizations
study, and PHEMA with Mn less than 40000 g molꢀ1 and Ð lower were controlled to a narrow targeting DP in some regions.
than 1.5 was achieved at 80% conversion after 20 h. In 2001,
Herein, we revisit the current report on ATRP of HEMA using a
Armes’s group reported ATRP of HEMA in methanol or binary new kind of initiator bearing the alkynyl group and a solvent mixture
mixtures of methanol and water (1/1, v/v) using the Br-capped of 2-butanone and MeOH with the aim of exploring appropriate
oligo(ethylene glycol) initiator and the CuBr/bpy catalytic system polymerization conditions to prepare PHEMA with a wide range of
at 20 1C.11 The same group further prepared PHEMA homo- targeting DPs (80 to 1000) in a controlled manner. Some factors
polymers and the PHEMA based block or random copolymers including initiators (unprotected or protected propargyl 2-bromoiso-
using the CuCl/bpy catalyst and the 2-(N-morpholino)ethyl butyrate), solvents (MeOH or MeOH–2-butanone mixture in different
2-bromo-2-methylpropionate (Me-Br) initiator in MeOH at compositions), temperature (30, 50 and 60 1C) and [M]0/[I]0 (80 : 1,
20 1C in 2004.12 In these two polymerization systems, both high 200 : 1, 500 : 1 and 1000 : 1) that affected the polymerization of HEMA
monomer conversions (more than 90% within a few hours) and were studied, and the ATRP’s controllability of HEMA was investi-
narrow Ð (1.2–1.3) were obtained with desired DP less than 100. gated in the terms of kinetic study, Mn and Ð of the resulting
By virtue of the controllable ATRP, they demonstrated that PHEMA sample. We also report the preparation of two diblock
PHEMA homopolymers with the DP less than 20 are water copolymers from PHEMA end-capped with asymmetric functional
soluble, and those with the DP in the range of 20–45 are partially groups to demonstrate the successful preparation of PHEMA in a
water soluble, whereas those with the DP more than 45 are water controlled/living fashion and provide model examples of the design
insoluble.12 Hocker et al. also reported the controlled ATRP of and preparation of copolymers (Scheme 1).
HEMA by using MPEO-Br or Br-PEO-Br as a macroinitiator and
While the current MeOH and solvent mixture of MeOH/
the CuCl/bpy catalytic system in ethylene glycol at 80 1C.13 They 2-butanone was employed for ATRP and AGET-ATRP of HEMA
found that HEMA conversion reached a very high value within a with a targeting DP of less than 250 using different initiating
very short time, i.e., 98% monomer conversion over 7 min, and systems reported by Armes12 and Matyjaszewski,20 respectively,
the polydispersity of the resulting copolymers was in the range the initiator containing the alkynyl group for ATRP of HEMA
of 1.2–1.47. By a similar approach as mentioned above, ATRP was rarely reported. Only one paper was found reporting the
of HEMA using different initiators and CuX/ligand catalytic preparation of PHEMA with a DP of B112 and a narrow poly-
systems in protic and polar aprotic solvents or even toluene has dispersity of 1.24 using the 3-(trimethylsilyl)propargyl 2-bromo-
been utilized to synthesize PHEMA based copolymers with isobutyrate initiator in MeOH at 50 1C. However, relevant
complex architectures, such as random copolymers,14 block kinetic data were absent.19 As shown in this paper, well-defined
copolymers,15 grafting copolymers,16 polymer brushes,17 multi- PHEMA samples with targeting DPs from 80 to 1000 were
arm star copolymers,18 macrocycles,19 and other nano-structured prepared successfully. More importantly, due to the hydroxyl
polymeric materials/hybrids.3b
groups end-capped with asymmetric functional groups,
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Scheme 1 Schematic illustration for synthesis of (CH3)3Si–CRC–PHEMA–Cl, PBA-b-PHEMA, and MPEG-b-PHEMA.
the resulting PHEMA provided various reaction sites for the evaluated in terms of the kinetic study and the SEC profile of the
design and synthesis of other novel well-defined copolymers and resulting polymer. Three polymerization reactions of HEMA were
(co)polymers with complex architectures. Examples of these reac- carried out in methanol at 50 1C using CuCl/CuCl2/bpy as a
tions included click chemistry based on the alkynyl group after the catalyst system in the presence of different initiators by setting
removal of a protecting group, chain-extension reactions from the [HEMA]0 : [CuCl]0 : [CuCl2]0 : [initiator]0 : [bpy]0 = 80: 1 : 0.05: 1 : 2.
halogen-terminated end,23 ring-opening polymerization (ROP) of The kinetic data were collected with 1H NMR spectra of the
cyclic esters from the hydroxyl groups, and many others.3b
samples taken out from the reaction mixture at different reaction
time intervals (Fig. S2 in ESI†), and the SEC profiles of the
resulting polymers at the same monomer conversion of 50%
were obtained in THF using PS as standard.
Results and discussion
Fig. 1a shows that the semilogarithmic plot was apparently
curved when PBiB was employed as the initiator, indicating that the
ATRP of HEMA using PBiB as an initiator was not well controlled.
This was also evidenced by a high Ð of 1.9 for the resulting PHEMA
sample with a monomer conversion of 50% and a SEC profile with
a small shoulder as shown in Fig. 1b. The ill-controlled polymeri-
zation of HEMA by using PBiB as an initiator was believed to be
caused by irreversible termination reaction among radical species
or Glaser coupling reaction between terminal alkynes.25
1. Synthesis of different alkyne-containing initiators
Three different initiators bearing alkyne groups (Scheme 2) were
used for ATRP of HEMA to facilitate the introduction of a PHEMA
homopolymer with terminal alkyne groups (R–CRC–PHEMA–Cl).
The initiators were successfully prepared as observed from the
corresponding 1H NMR spectra (Fig. S1 in ESI†).24 The peak
centrally located at 4.67 ppm from the proton of methylene
adjacent to the alkyne group (–CRC–CH2–O–) could be used
for evaluating the DP of the obtained PHEMA polymer.
In contrast, both kinetic plots were almost linear when
TMSPBiB and TiPSPBiB were utilized as initiators. The linear
correlation coefficients of the guided line were 0.984 and 0.955
2. ATRP of HEMA using different alkyne-containing initiators
At the initial stage of the study, the controllability of ATRP of for TMSPBiB and TiPSPBiB, respectively (Fig. 1a). The Ð was
HEMA by using these three different initiators was preliminarily 1.26 and 1.29 for PHEMA at 50% monomer conversion by using
Scheme 2 The synthesis routes towards the initiators of (a) propargyl 2-bromoisobutyrate (PBiB), (b) 3-(trimethylsilyl)propargyl 2-bromoisobutyrate
(TMSPBiB), and (c) 3-(triisopropylsilyl)propargyl 2-bromoisobutyrate (TiPSPBiB).
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Fig. 1 ATRP of HEMA using different alkynyl-containing initiators in methanol at 50 1C by setting [HEMA]0 : [CuCl]0 : [CuCl2]0 : [initiator]0 : [bpy]0
80 : 1 : 0.05 : 1 : 2. (a) First-order kinetic data; (b) SEC curves of PHEMA with monomer conversion of 50%. The dashed lines are guides to the eye. The
molar mass (Mn) from SEC was B12 kg molꢀ1
=
.
TMSPBiB and TiPSPBiB, respectively. Moreover, both SEC well controlled. The apparent rate constants (Kappp) for different
curves were symmetric as shown in Fig. 1b. These results solvent systems were different as calculated from linear kinetic
indicated that the ATRP of HEMA using either TMSPBiB or plots (Fig. 2a). For example, Kappp was 1.3 ꢁ 10ꢀ4 sꢀ1 in
TiPSPBiB was better controlled compared to that using PBiB as methanol, 1.0 ꢁ 10ꢀ4 sꢀ1 and 8.33 ꢁ 10ꢀ5 sꢀ1 in methanol/
an initiator. This was also consistent with the previous reports 2-butanone at 3/2, and 2/3 (m/m), respectively. This indicated
that acetylene-protection constrained the side reactions of that the methanol–2-butanone mixture, rather than methanol,
Glaser coupling, leading to better control over the ATRP of resulted in a decrease of the apparent rate constant. Moreover,
styrene, methyl acrylate and tert-butyl acrylate.24b,26
the apparent rate constant decreased with the increase of
Fig. 1a further shows that the polymerization rate was 2-butanone content in the solvent mixture. A similar trend
higher using TMSPBiB as an initiator compared to that using was also observed in the previously reported AGET-ATRP of
TiPSPBiB as an initiator for ATRP of HEMA under identical HEMA in methanol/2-butanone.20
conditions. For example, the monomer conversion was up to
The samples in small volumes were taken out from the
93% at 300 min when TMSPBiB was used as an initiator, while reaction mixture at predesigned intervals. The monomer conver-
it was only 63% at identical reaction times when TiPSPBiB was sions and the molecular weights (Mn,NMR) of purified samples
used as an initiator. The apparent propagation constants (Kpapp
)
were calculated from 1H NMR spectra (Fig. S3 in ESI†). The
derived from the kinetic plots were 1.3 ꢁ 10ꢀ4 sꢀ1 and 5.0 ꢁ dependence of Mn,NMR on monomer conversion is shown in
10ꢀ5 sꢀ1 using TMSPBiB and TiPSPBiB as initiators, respec- Fig. 2b. The Mn,NMR of PHEMA linearly increased with the
tively. Therefore, TMSPBiB was chosen to initiate the polymeri- increase of monomer conversion in methanol when the mono-
zation of HEMA in the following studies.
mer conversion was below 83%. However, when the monomer
conversion was above 83%, the molecular weights (Mn,NMR) of
PHEMA were non-linearly increased with the increase of mono-
3. Effect of solvent on ATRP of HEMA
The increase in the reaction rate as well as the reactivity of mer conversion in methanol. This deviation may attribute to
free radical accelerated either termination or chain transfer irreversible termination which increased with the increase of
reaction for ATRP in polar solvents, constituting the main monomer conversion. These results were commonly observed in
disadvantage of poor controllability.21 ATRP in polar solvent the ATRP of HEMA performed by Beers,1 Armes11,12 and other
generally resulted in polymers with higher Ð than that in researchers.21 For ATRPs of HEMA in methanol/2-butanone at
nonpolar media or in bulk.5a,27 The polar solvents applied 2/3 and 3/2 (m/m), the Mn,NMR of PHEMA was linearly increased
for synthesis of PHEMA by ATRP included water,28 DMSO,22 with the increase of monomer conversion. It should be noted
ethylene glycol,13 methanol,12,21 2-butanone/1-propanol,1 that the Mn,NMR was higher than the theoretical value at corre-
2-butanone/methanol20 and methanol/water.11 It has been dis- sponding monomer conversion due to an initiator efficiency of
closed that the solvent polarity had a significant effect on the B75% as calculated from Fig. 2b.
controllability of the ATRP of HEMA. Speaking in detail, the
The variation of Ð of PHEMA with monomer conversion in
controllability of ATRP of HEMA increased with the decrease methanol, methanol/2-butanone at 2/3 and 2/3 (m/m) is shown
of solvent polarity.1 Considering the solubility and polarity of in Fig. 2c. The Ð of the PHEMA sample prepared in methanol/
the solvent, the polymerizations were performed in methanol 2-butanone at m/m = 2/3 was less than 1.3 when the monomer
and in the methanol–2-butanone mixture in the presented conversion was more than 90%, meanwhile the Ð of the PHEMA
studies.
sample as prepared in methanol or methanol/2-butanone 3/2
Fig. 2a shows kinetic plots for the ATRP carried out in (m/m) was below 1.3 when the monomer conversion was less
different solvents. The kinetic plots for ATRPs of HEMA were almost than 80%. The Ð increased to more than 1.7 in methanol and
linear for the solvents of methanol and methanol/2-butanone at 1.4 in methanol/2-butanone 3/2 (m/m) when the monomer
3/2 and 2/3 (m/m), suggesting that the polymerizations were conversion further increased to more than 90%. Moreover, the
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Fig. 2 ATRP of HEMA at 50 1C in different solvents by setting [HEMA]0 : [CuCl]0 : [CuCl2]0 : [TMSPBiB]0 : [bpy]0 = 80 : 1 : 0.05 : 1 : 2. (a) First-order kinetic
plots; (b) Mn,NMR variation of PHEMA with monomer conversion (the solid line represents the theoretical Mn), and (c) dependence of Ð on conversion. The
dashed lines are guides to the eye.
Ð of PHEMA prepared in methanol/2-butanone at m/m = at 2/3 (m/m) was the most homogenous, the controllability of
2/3 was lower than that prepared in methanol, or methanol/ ATRP of HEMA in MeOH/butanone at 2/3 (m/m) was better than
2-butanone at m/m = 3/2 at identical monomer conversion. those in methanol and MeOH/butanone (m/m = 3/2).
These results indicated that the controllability of ATRP (Ca) of
4. Effect of temperature on the ATRP of HEMA
HEMA exhibited a trend of solvent dependence which was
expressed as follows: Ca (methanol/2-butanone 2/3) 4 Ca In order to reveal the effect of temperature on the ATRP of
(methanol/2-butanone 3/2) 4 Ca (methanol).
HEMA, the polymerizations were carried out in the methanol/
The controllability variation of ATRP of HEMA with solvents 2-butanone mixture at 2/3 (m/m) at 30 1C, 50 1C and 60 1C,
was hypothesized to derive from homogeneity of the reaction respectively, by setting the same reaction recipe of [HEMA]0 :
mixture.29 To confirm this hypothesis, controlled experiments [CuCl]0 : [CuCl2]0 : [initiator]0 : [bpy]0 = 80 : 1 : 0.05 : 1 : 2 (Fig. 3).
were designed and carried out to evaluate the solubility of the
As shown in Fig. 3a, the kinetic plots for ATRP of HEMA
catalyst complex and PHEMA in reaction media. The solubility of carried out at 30 1C, 50 1C and 60 1C were almost linear,
CuCl2/bpy catalyst complexes in methanol, methanol/2-butanone suggesting that the polymerizations were well controlled. The
(3 : 2, 2 : 3, or 1 : 4) and 2-butanone was measured using UV/vis apparent rate constants of the polymerizations at different
spectroscopy by comparing ultraviolet absorption intensity of temperatures were calculated from linear kinetic plots, i.e.,
the dissolved sample (see experimental details and Fig. S4 in 1.5 ꢁ 10ꢀ4 sꢀ1 at 60 1C, 8.33 ꢁ 10ꢀ5 sꢀ1 at 50 1C and 7.33 ꢁ
ESI†). The results showed that the solubility (S) of CuCl2/bpy 10ꢀ5 sꢀ1 at 30 1C. It is found that the lower reaction tempera-
catalyst–ligand complexes exhibits the following trend: S(CuCl2/ ture resulted in a decrease of the apparent rate constant.
bpy/methanol/2-butanone at 2 : 3) 4 S(CuCl2/bpy/methanol/ Additionally, a linear relationship of Ln(Kappp) and 1/T was
2-butanone at 3 : 2) 4 S(CuCl2/bpy/methanol) 4 S(CuCl2/bpy/ obtained (see Fig. S5 in ESI†), and the reaction activation
methanol/2-butanone at 1 : 4) 4 S(CuCl2/bpy/2-butanone). The energy (Ea) extrapolated from the linear plot was 16.05 kJ molꢀ1
solubility of PHEMA in solvent was directly evaluated by dis- based on the Arrhenius equation.31 This value was comparable
solving the polymer sample in solvent (see experimental details to that of 13.6 kJ molꢀ1 reported in the literature.21
and Fig. S4 in ESI†). The results clearly showed that the solvent
In Fig. 3b, the Mn,NMR of PHEMA increased linearly with
mixture of MeOH/butanone at 2/3 was the best solvent for monomer conversion at three different temperatures. The
PHEMA with a DP of B800. This could be further verified by Mn,NMR of PHEMA was higher than its theoretical values, and
the closest solubility parameter of MeOH/butanone at 2/3 to the initiator efficiency approached to 75% regardless of tem-
that of PHEMA as calculated from the Hansen model30 (see perature. All the Ð of PHEMAs were below 1.30. Furthermore,
Table S1, ESI†). Since the reaction mixture in MeOH/butanone the Ð values obtained at 30 1C were higher than those obtained
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Fig. 3 ATRP of HEMA carried out in a methanol/2-butanone mixture (m/m = 2/3) at various temperatures by setting [HEMA]0 : [CuCl]0 : [CuCl2]0 :
[initiator]0 : [bpy]0 = 80 : 1 : 0.05 : 1 : 2. (a) First-order kinetic plots; (b) Mn,NMR variation of PHEMA with monomer conversion (the solid line represents the
theoretical Mn) and (c) dependence of Ð from SEC on conversion. The dashed lines are guides to the eye.
at 50 1C at identical monomer conversion (Fig. 3c). The Ð of monomer conversion increased to 90%. The results indicated
PHEMAs obtained at 60 1C was below 1.30 when the monomer that the ATRP of HEMA using the current system could be well
conversion was below 75%, but increased to 1.53 when the controlled at moderate temperatures, i.e, 50 1C.
Fig. 4 ATRP of HEMA in methanol/2-butanone (m/m = 3/2) at 50 1C by varying [M]0/[I]0. (a) First-order kinetic plots; (b) Mn,NMR variation of PHEMA with
monomer conversion (the solid lines represent the theoretical Mn) and (c) dependence of Ð on conversion. The dashed lines are guides to the eye.
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5. Preparation of PHEMA with controlled molecular weight
In order to investigate the feasibility of the PHEMA with well-
controlled molecular weight, polymerizations of PHEMA were
carried out under identical conditions except the variation of
[M]0/[I]0. Fig. 4a shows that the kinetic plots were all linear for
[M]0/[I]0 at 80/1, 200/1, 500/1, and 1000/1. However, the apparent
polymerization rate decreased with the increase of [M]0/[I]0.
This was not surprising since a higher [M]0/[I]0 resulted in a
lower concentration of active species in the reaction system.
The Mn,NMR increased linearly with the increase of monomer
conversion regardless of [M]0/[I]0, indicative of the feasibility
for preparation of PHEMA with controlling molecular weight
using the current reaction system. The initiator efficiency, as
calculated from the ratio between the Mn,NMR and the corre-
sponding theoretical values, decreased with the increase of
[M]0/[I]0. The initiator efficiency was 75%, 73%, 68%, and
59% for [[M]0/[I]0 = 80, 200, 500, and 1000, respectively]. This
could be explained by the fact that a portion of the initiator could
be easily oxidized and inactivated in a deoxidization process
at higher [M]0/[I]0.32 Although the Ð of PHEMA increased with
[M]0/[I]0 at identical monomer conversion, all the prepared
PHEMA samples had a Ð of less than 1.34, indicating that all
the polymerizations occurred under control (Fig. 4c).
Fig. 5 SEC curves of PHEMA, MPEG, and the obtained PHEMA-b-PBA and
MPEG-b-PHEMA diblock copolymers, respectively.
Conclusions
The controllable ATRP of HEMA was achieved by tuning the
reaction parameters, including temperature, solvents, initiators
and recipes. Well-defined PHEMA polymers end-capped with
living chloride and alkyne terminal groups were achieved by
using TMSPBiB as an initiator and CuCl/CuCl2/bpy as a catalyst
in methanol/2-butanone. The solvent mixture of methanol/
2-butanone at a mass ratio of 3/2 led to better polymerization
controllability. PHEMA samples with a wide range of DPs
(the degree of polymerization) from 80 to 1000 and Ð below
1.34 were obtained under appropriate conditions. The activity
of chloride and alkyne terminal groups was confirmed by the
successful preparation of diblock copolymers PHEMA-b-PBA
and MPEG-b-PHEMA, which were obtained by ATRP of BA
using (CH3)3Si–CRC–PHEMA–Cl as a macroinitiator and click
reaction between azide-terminated MPEG and HCRC–PHEMA–Cl,
respectively. The studies provided a facile synthetic modular route
for the design and synthesis of novel copolymers.
6. Synthesis of a diblock copolymer
To evaluate the reactivity characteristics of the asymmetric end-
groups of the (CH3)3Si–CRCPHEMA–Cl homopolymers, two
chain growth experiments were carried out, namely ATRP of
n-butyl acrylate (nBA) using (CH3)3Si–CRCPHEMA–Cl as a
macroinitiator and click reaction between HCRC–PHEMA–Cl
and MPEG-N3. Firstly, block copolymer PHEMA-b-PBA was
obtained in high yield of B68.4% within 10 h. (CH3)3Si–
CRC–PHEMA–Cl as a macroinitiator initiated the ATRP of
butyl acrylate (BA) in 50% m/m DMF solution at 90 1C by
setting [BA]0 : [PHEMA-Br]0 : [CuCl]0 : [PMDETA]0 = 100 : 1 : 1 : 1.
Secondly, diblock copolymer MPEG-b-PHEMA was successfully
synthesized by click reaction between alkynyl-terminated
PHEMA and azide-terminated MPEG. The alkyne terminated
Experimental section
Materials and reagents
PHEMA was obtained by the removal of the trimethylsilyl group 2-Hydroxyethyl methacrylate (HEMA, 98%, Sigma Aldrich) was
via hydrolysis catalyzed by tetrabutyl ammonium fluoride using purified according to procedure described in our previous
a similar procedure.33 Azide-terminated MPEG was obtained papers.1,34 HEMA solution (25% v/v) in water was washed with
through the reaction of MPEG with 2-bromoisobutyryl bromide hexane for 5 times to remove ethylene glycol diacrylate. Then,
followed by the reaction of MPEG-Br with sodium azide. Azide HEMA was salted out from the aqueous phase by addition of
terminated MPEG (1.2 equivalent) was coupled to the alkynyl NaCl and separated by extraction with diethyl ether for 4 times.
end of PHEMA by ‘‘click reaction’’ using CuBr/PMDETA as a The organic layer was collected and dried over anhydrous
catalyst in DMF at 30 1C for 24 h (see Scheme 2) in a yield of MgSO4 before filtration and evaporation in a rotary evaporator
82.6%. The successful preparation of these diblock copolymers at 35 1C. HEMA was obtained via distillation at 80 1C under
was verified by 1H NMR spectroscopy. The peaks were well reduced pressure. n-Butyl acylate (BA, 98%, Sigma Aldrich) was
analyzed and assigned (see Fig. S6 in ESI†). In the SEC curves washed thrice with an equal volume of 5 wt% NaOH aqueous
(Fig. 5), the obtained PHEMA-b-PBA and MPEG-b-PHEMA solution, and then washed with water until the acidity–alkalinity
diblock copolymers showed obvious shift to left owing to the of the water was tested to be neutral. It was dried over anhydrous
increase of molecular weight compared to the precursor MgSO4 before being distilled under reduced pressure. The
PHEMA and MPEG. The Ð of the obtained PHEMA-b-PBA and purified HEMA and BA were kept at 4 1C prior to use. Cuprous
MPEG-b-PHEMA diblock copolymers were 1.34 and 1.37, chloride (CuICl, 95%, Sigma Aldrich) was purified by stirring
respectively.
with acetic acid overnight, filtration, repeatedly washing in
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sequence with ethanol and diethyl ether, and drying under extracted with diethyl ether (100 ꢁ 4 mL) four times. The
vacuum conditions.35 2-Bromoisobutyryl bromide (98%), copper organic layers were washed with 15 mL of 10 wt% NaCl solution,
chloride (CuCl2), 2,20-bipyridine (bpy, 99%), 3-(trimethylsilyl)- dried with anhydrous MgSO4 and rotary evaporated to generate
propargyl alcohol (TMS-OH, 98%), triisopropylchlorosilane 1.70 g of 3-(triisopropylsilyl)propargyl alcohol at 79% yield.
(TiPS-Cl, 97%), ethyl magnesium bromide (3.0 M solution in 1H NMR (400 MHz, CDCl3, d, ppm): 4.60 (s, 2H, –CRC–CH2–
diethyl ether), ethylene diamine tetraacetic acid disodium salt OH), 1.06–1.05 (m, 18H, ((CH3)2CH)3Si–), 0.14–0.19 (m, 3H,
(EDTA), tetrabutyl ammonium fluoride (TBAF, 1 M solution in ((CH3)2CH)3Si–).
THF), sodium azide (NaN3) (99%), and propargyl alcohol (PA, 98%)
2-Bromoisobutyryl bromide (1.88 mL, 15.2 mmol) was added
were all from Aldrich and used as received. Triethylamine dropwise to a solution of obtained 3-(triisopropylsilyl)propargyl
(Et3N, 98%), 2-butanone (98%) and diethyl ether (99%) were alcohol (1.7 g, 8.0 mmol) and triethylamine (2.12 mL,
purified by refluxing with sodium for 12 h before distillation. 15.2 mmol) in THF (50 mL) at 0 1C, and stirred for 2 h at room
Methanol (99%) was purified by magnesium and iodine to temperature. The mixture was filtered, rotary evaporated and
remove water. Water employed in all experiments was double- purified using column chromatography (hexane/ethyl acetate =
distilled. All other reagents and solvents were of analytical 95 : 5). The final colorless oil was 1.88 g and the yield was 65%.
grade and used without further purification, if not specified.
1H NMR (400 MHz, CDCl3, d, ppm): 4.67 (s, 2H, –CRC–CH2–
O2C), 1.95 (s, 6H, –O2C–C(CH3)2Br), 1.07 (m, 18H, ((CH3)2CH)3Si–),
0.14–0.19 (m, 3H, ((CH3)2CH)3Si–).
Synthesis of initiators
The initiators, propargyl 2-bromoisobutyrate (PBiB), 3-(trimethyl-
silyl)propargyl 2-bromoisobutyrate (TMSPBiB), and 3-(triisopropyl-
ATRP of HEMA
silyl)propargyl 2-bromoisobutyrate (TiPSPBiB), were synthesized by ATRP is strongly solvent-dependent. Protic solvents lead to
the esterification reaction between the hydroxyl groups of quick and poorly controllable reaction possibly due to compe-
unmodified or modified propargyl alcohol and 2-bromoisobutyryl titive coordination of solvent to the Cu(II) center, therefore,
bromide, as shown in Scheme 2.
the controlled polymerization of HEMA in protic solvents by
PBiB was synthesized following the literature procedure.24a employing a catalyst system initially containing a sufficient
In brief, propargyl alcohol (3.0 g, 0.054 mol), triethylamine Cu(II)–halide complex.20 Thus, in our system, CuCl2 ([CuCl]/
(6.0 g, 0.059 mol) and anhydrous diethyl ether (150 mL) were [CuCl2] = 20/1) was employed in all the reaction recipes.
added into a 250 mL flask which was pre-cooled with ice bath.
The ATRP of HEMA was performed in a home-made dual-
2-Bromoisobutyryl bromide (13.8 g, 0.059 mol) was added flask apparatus in which two 25 mL flasks were connected via a
dropwise via a syringe over 30 min. After stirring at room glass pipe with a diameter of 0.8 cm and a length of 5 cm. In a
temperature for 12 h, the reaction mixture was filtered, washed typical experiment, 2,20-bipyridine (91 mg, 0.58 mmol),
in sequence with 5 mL of 1 mol Lꢀ1 HCl, 50 mL of 3% NaHCO3 TMSPBiB (80 mg, 0.29 mmol), HEMA (3.00 g, 23.1 mmol) and
solution and water for several times until the pH value of the methanol (1.2 mL) were added into one flask, while CuCl
aqueous phase was around 7. The organic layer was collected, (29 mg, 0.29 mmol), CuCl2 (2 mg, 0.015 mmol), 2-butanone
dried over anhydrous MgSO4, rotary evaporated and distilled (1.8 mL) and a magnetic stir bar were loaded into the other
under reduced pressure to obtain a colorless liquid (6.17 g) at flask. The liquid mixture in the two dual-flasks were bubbled
a yield of 61%. 1H NMR (400 MHz, CDCl3, d, ppm): 1.96 with argon for half an hour and then subjected to three freeze–
(6H, –OCO–C(CH3)2Br), 2.38–2.42 (1H, CHRC–CH2–OCO–), pump–thaw cycles under an argon atmosphere, and then
4.76–4.78 (2H, HCC–CH2–OCO–).
thoroughly transferred into one of the flasks via a vacuum line
TMSPBiB was synthesized with the same synthesis and system. The dual-flask apparatus was immediately placed in
purification procedure as PBiB except that 3-(trimethylsilyl)- thermostatic oil baths at 50 1C (defining t = 0). The reaction
propargyl alcohol (7.0 g, 0.054 mol) were added. The finally mixture was initially dark brown and transparent and the
colorless liquid was weighted to be 10.93 g and the yield was polymerization occurred immediately, leading to an increase
73%. 1H NMR (400 MHz, CDCl3, d, ppm): 0.14–0.19 (9H, in viscosity over 30 min. Samples for the kinetics study were
protons of trimethylsilyl groups), 1.91 (6H, –OCO–C(CH3)2Br), taken out via argon purged syringe at predesigned time inter-
4.76 (2H, –CRC–CH2–OCO–).
vals and quickly transformed to vials which were pre-cooled
TiPSPBiB was synthesized in two steps according to the with liquid nitrogen for DMF, SEC and 1H NMR analysis,
literature.36 Firstly, the alkynyl group of propargyl alcohol was respectively. The reaction was stopped after 5 h by freezing
protected with the triisopropylsilyl group. Secondly, the hydroxyl the flask with liquid nitrogen before exposure to air. Once
group of 3-(triisopropylsilyl)propargyl alcohol was reacted with switched with air, the dark brown reaction solution turned
2-bromoisobutyryl bromide.
blue, indicative of aerial oxidation of Cu(I) to Cu(II). Sub-
Propargyl alcohol solution (0.57 g, 10.2 mmol) in THF sequently, the dark bluish reaction mixture was diluted with
(10 mL) was added dropwise into 10 mL of ethylmagnesium 15 mL of methanol and passed through a silica column to
bromide (30 mmol) solution in THF and refluxed for 18 h. And remove copper–ligand complexes and the resulting colorless
then, triisopropylchlorosilane (2.79 g, 14.5 mmol) in THF aqueous solution was concentrated to about 5 mL via rotary
(10 mL) was added dropwise and refluxed for 5 h. After pouring evaporation and precipitated out over diethyl ether (200 mL) to
into 15 mL of 10% (m/m) HCl solution, the solution was remove the HEMA monomer and other impurities. The crude
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product was further purified by re-dissolving in 5 mL of water was added, and then extracted by CH2Cl2 three times. The
methanol and precipitated out over 100 mL of diethyl ether. This combined CH2Cl2 solution was washed with cool water thrice
procedure was repeated thrice. The precipitates were collected and condensed and dried with anhydrous MgSO4 overnight, and
dried under vacuum for 72 h to generate white PHEMA polymers. then precipitated in cold diethyl ether. 0.92 g of MPEG-N3 at a
1H NMR (400 MHz, CDCl3, d, ppm): 0.14–0.19 (9H, protons of yield of 93% was obtained as white powder by drying at 30 1C
1
trimethylsilyl groups), 0.86–1.22 (–CH2–C(CH3)–), 1.97 (–CH2– for 24 h. H NMR (400 MHz, DMF-d6, d, ppm): 3.38 (CH3–O–),
C(CH3)–), 3.86–4.23 (–OCH2CH2O–), 4.67 (–CRC–CH2–O2C). 3.75 (–CH2–CH2–), 3.92 (–OCH2CH2O–CO–), 1.96 (–CH2–N3).
Mn and Ð from SEC were 14000 g molꢀ1 and 1.26, respectively.
Preparation of PHEMA-b-PBA by chain extension via ATRP
The click reaction was performed as follows (Scheme 2):
6.4 mg of CuBr (0.063 mmol), 11.7 mg of PMDETA (0.063 mmol),
0.1 g of HCRC–PHEMA–Cl (0.063 mmol of terminal alkynyl
PHEMA-b-PBA (Scheme 2) was prepared via ATRP of BA by groups), MPEG-N3 (0.333 g, 0.065 mmol) and 8 mL of DMF were
using chloro-terminated PHEMA (Ð = 1.16 from SEC, DP = 121 added into the flask and degassed for 50 min by blowing with
1
evaluated from H NMR) as a macroinitiator. The polymeriza- argon. The solution was stirred for 48 h at 80 1C. After reaction,
tion was carried out at 90 1C for 10 h by using a recipe described the mixture was diluted with 15 mL of dichloromethane and
as follows: macroinitiator (0.51 g, or 0.032 mmol of terminal passed through the silica column to remove the catalyst. Then,
chloride groups), DMF (9 g, 50% m/m), CuCl (3.17 g, the mixture was rotary-evaporated to remove CH2Cl2 before dialyzed
0.032 mmol), PMDETA (5.55 g, 0.032 mmol), and BA (0.82 g, against water (molecular weight cut-off is 14 000 g molꢀ1) by
6.40 mmol). The isolation and purification procedure of this changing water every 4 h over 7 days. Finally, the solution in the
block copolymer (CH3)3Si–CRC–PHEMA-b-PBA was the same as dialysis bag was collected and freeze dried for 12 h to generate
the procedure used for the (CH3)3Si–CRC–PHEMA–Cl homo- 0.35 g product as a white powder at a yield of 82.6%. SEC: Ð =
polymer. The yield was 68.4%. SEC: Ð = 1.26, Mn = 37200 g molꢀ1
.
1.19, Mn = 22 100 g molꢀ1. 1H NMR (400 MHz, CDCl3, d, ppm):
1H NMR (400 MHz, CDCl3, d, ppm): 0.14–0.19 (9H, protons of 0.86–1.22 (–CH2–C(CH3)–), 1.97 (–CH2–C(CH3)–), 3.32 (–OCH3),
trimethylsilyl groups), 0.86–1.22 (–CH2–C(CH3)– of PHEMA), 1.97 3.86–4.31 (–OCH2CH2O–).
(–CH2–C(CH3)– of PHEMA or –CH2–CH– of PBA), 3.86–4.23 (–OCH2-
Characterization
CH2O– of PHEMA), 4.21 (–OCH2– of PBA), 2.27 (–OCH2–CH2– of
PBA), 1.27–1.67 (–OCH2CH2–CH2CH3 of PBA).
1H NMR spectra were recorded in DMSO-d6 or CDCl3 on a
Bruker 400 spectrometer at 30 1C. The peak integral ratios
of the two protons of the propargyl group (–CRC–CH2–O)
introduced from the initiator at 4.67 ppm and the two oxyethyl
or ethylene protons of the PHEMA at 4.34 ppm or 5.58 ppm
were used to calculate the monomer conversion (conv. =
2(d4.34/2 ꢀ d5.58)/d4.34 ꢁ 100%), the degree of polymerization
(DP = 2(d4.34/2 ꢀ d5.58)/d4.67) and the theoretical molecular weight
of PHEMA (Mn,theor = 130.15 ꢁ ([M]0/[I]0) ꢁ conv. + Minitiator).
The molecular weight and molar mass dispersity of samples
were measured at 25 1C using a size exclusion chromatography
(SEC) system consisting of a Waters 1515 pump, a Styragel
Packed Column, and a Waters Model 410 refractive index
detector. Narrow disperse polystyrene with molecular weights
in the range of 1.31 ꢁ 103 to 3.64 ꢁ 106 g molꢀ1 was used as a
standard for calibration. Tetrahydrofuran (THF) was used as
the eluent at a flow rate of 1 mL minꢀ1. The concentration of
the sample was around 3 mg mLꢀ1 and the sample solution was
filtered through the 0.45 mm membrane before injection.
To have the PHEMA well dissolved in THF which was used as
an eluent in the SEC analysis, the PHEMA homopolymer and
the diblock copolymer containing the PHEMA block were
acetylated with acetic anhydride in dry pyridine before SEC
measurements.
Preparation of MPEG-b-PHEMA by click reaction
1.5 g of (CH3)3Si–CRC–PHEMA–Cl (DP = 121, Ð = 1.16,
0.095 mmol of terminal trimethylsilyl groups) and 1 mL of
TBAF (1.15 mmol, 10.0 eq.) were dissolved in 8.5 mL of THF and
stirred at room temperature for overnight (Scheme 1). Then, the
reaction mixture was rotary evaporated to eliminate THF, and
the polymer was obtained by precipitation into diethyl ether,
1
and dried under vacuum. H NMR (400 MHz, CDCl3, d, ppm):
0.86–1.22 (–CH2–C(CH3)–), 1.97 (–CH2–C(CH3)–), 3.86–4.23
(–OCH2CH2O–), 4.67 (–CRC–CH2–O2C–). The disappearance
of peak at d = 0.14–0.19 ppm in 1H NMR spectra confirmed the
complete removal of the trimethylsilyl terminal group of the
purified polymer HCRC–PHEMA–Cl.
Monomethoxy polyethylene glycol (Mn = 5000 g molꢀ1) with
azido terminal functional group (MPEG-N3) was obtained
according to the literature method.37 First, monomethoxy
polyethylene glycol (5.0 g, or 1.0 mmol of hydroxyl group),
and dry triethylamine (0.15 g, 1.5 mmol) were dissolved in
THF (40 mL) and 2-bromoisobutyryl bromide (0.35 g, 1.5 mmol)
was added dropwise. The mixture was stirred at room tempera-
ture for 24 h. After filtration, the solvent was rotary evaporated
and the product was redissolved in CH2Cl2 and then precipi-
tated into cold diethyl ether (100 mL) and dried under vacuum.
The polymers were re-dissolved in CH2Cl2 (4 mL) and precipi-
tated into diethyl ether (20 mL). MPEG-Br (4.69 g, the yield was
91%) was obtained by drying under vacuum for 24 h. Second,
Acknowledgements
MPEG-Br (1.0 g, 0.2 mmol) and sodium azide (26.0 mg, We thank the National Natural Science Foundation of China
0.4 mmol) were dissolved in DMF (10 mL) and stirred at (No. 20474068, 51173204, 51203191), the Leading Talents Pro-
120 1C for 6 h. After cooling to room temperature, 10 mL of gram of Guangdong Province, Guangzhou Invited-Talents Special
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16 H. L. Kang, W. Y. Liu, R. G. Liu and Y. Huang, Macromol.
Chem. Phys., 2008, 209, 424–430.
Project for Solving Difficult Problems (No. 11D34060038),
Guangdong Natural Science Foundation (No. S2012010009063)
and the Guangzhou Science and Technology Tackling Fund
(No. 11A24060596) for financial support.
17 Y. H. Chen, C. Y. Wang, J. X. Chen, X. X. Liu and Z. Tong,
J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 1354–1367.
18 Z. Q. Cao, W. G. Liu, G. X. Ye, X. L. Zhao, X. Z. Lin, P. Gao
and K. D. Yao, Macromol. Chem. Phys., 2006, 207, 2329–2335.
19 X. S. Fan, G. W. Wang and J. L. Huang, J. Polym. Sci., Part A:
Polym. Chem., 2011, 49, 1361–1367.
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