Well-controlled ATRP of 2-(2-(2-azidoethyoxy)ethoxy)ethyl methacrylate for high-density click functionalization of polymers and metallic substrates

Article abstract of DOI:10.1002/pola.27969

The combination of atom transfer radical polymerization (ATRP) and click chemistry has created unprecedented opportunities for controlled syntheses of functional polymers. ATRP of azido-bearing methacrylate monomers (e.g., 2-(2-(2-azidoethyoxy)ethoxy)ethyl methacrylate, AzTEGMA), however, proceeded with poor control at commonly adopted temperature of 50 °C, resulting in significant side reactions. By lowering reaction temperature and monomer concentrations, well-defined pAzTEGMA with significantly reduced polydispersity were prepared within a reasonable timeframe. Upon subsequent functionalization of the side chains of pAzTEGMA via Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry, functional polymers with number-average molecular weights (M_n) up to 22 kDa with narrow polydispersity (PDI < 1.30) were obtained. Applying the optimized polymerization condition, we also grafted pAzTEGMA brushes from Ti6Al4 substrates by surface-initiated ATRP (SI-ATRP), and effectively functionalized the azide-terminated side chains with hydrophobic and hydrophilic alkynes by CuAAC. The well-controlled ATRP of azido-bearing methacrylates and subsequent facile high-density functionalization of the side chains of the polymethacrylates via CuAAC offers a useful tool for engineering functional polymers or surfaces for diverse applications.

Full text of DOI:10.1002/pola.27969

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Well-Controlled ATRP of 2-(2-(2-azidoethyoxy)ethoxy)ethyl
Methacrylate for High-Density Click Functionalization
of Polymers and Metallic Substrates
Pingsheng Liu,* Jie Song
Department of Orthopedics and Physical Rehabilitation, Department of Cell and Developmental Biology,
University of Massachusetts Medical School, Worcester, Massachusetts, 01655
Correspondence to: J. Song (E-mail: Jie.song@umassmed.edu)
Received 29 September 2015; accepted 28 October 2015; published online 12 November 2015
DOI: 10.1002/pola.27969
ABSTRACT: The combination of atom transfer radical polymer-
ization (ATRP) and click chemistry has created unprecedented
opportunities for controlled syntheses of functional polymers.
ATRP of azido-bearing methacrylate monomers (e.g., 2-(2-
(2-azidoethyoxy)ethoxy)ethyl methacrylate, AzTEGMA), how-
ever, proceeded with poor control at commonly adopted tem-
perature of 50 8C, resulting in significant side reactions. By
lowering reaction temperature and monomer concentrations,
well-defined pAzTEGMA with significantly reduced polydisper-
sity were prepared within a reasonable timeframe. Upon sub-
sequent functionalization of the side chains of pAzTEGMA via
Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) click chem-
istry, functional polymers with number-average molecular
weights (Mn) up to 22 kDa with narrow polydispersity
(PDI < 1.30) were obtained. Applying the optimized polymeriza-
tion condition, we also grafted pAzTEGMA brushes from
Ti6Al4 substrates by surface-initiated ATRP (SI-ATRP), and
effectively functionalized the azide-terminated side chains with
hydrophobic and hydrophilic alkynes by CuAAC. The well-
controlled ATRP of azido-bearing methacrylates and subse-
quent facile high-density functionalization of the side chains of
the polymethacrylates via CuAAC offers a useful tool for engi-
neering functional polymers or surfaces for diverse applica-
C
V
tions.
2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2016, 54, 1268–1277
KEYWORDS: alloy; atom transfer radical polymerization (ATRP);
azido-bearing methacrylate; click chemistry; kinetics (polym.);
SI-ATRP
tures and functional materials.10,16–20 CuAAC and ATRP can
share the same catalyst systems (e.g., Cu(I)/ligand), making
it possible to carry out the polymerization and subsequent
click conjugation in one pot without to the need for isolation
of azide/alkyne-containing precursor polymers.21–23
INTRODUCTION Click chemistry, such as Cu(I)-catalyzed
azide-alkyne cycloaddition (CuAAC),1,2 Diels–Alder reaction,3
thiol-ene coupling,4–6 and strain-promoted azide-alkyne
cycloaddition (SPAAC),7 has gained significant attention
within the polymer chemistry/material sciences community
for tethering polymer segments (end group coupling), func-
tionalizing polymer side chains, conjugating drugs to poly-
mers, or encapsulating biological cargos within in situ
formed polymeric hydrogels.8–13 Among these click reactions,
CuAAC possesses the advantages of high orthogonality (e.g.,
compared to thiol-ene coupling) and relatively low reagent
cost (e.g., compared to SPAAC).2,8
Whereas end-group CuAAC of azide/alkyne-terminated poly-
mers prepared by ATRP,17,24–29 after substituting the termi-
nal halide originated from the ATRP initiator with azide,30,31
can be extended to covalently conjugate drugs, imaging
probes or biomolecules of interest, the functional density
introduced is limited by one copy per polymer. For high-
density functionalization of polymers, combining ATRP of
azido-bearing monomers with subsequent CuAAC functionali-
zation of pendant side chains, as first demonstrated by Maty-
jaszewski et al.,32 is more appealing. For instance, ATRP of
3-azidopropyl methacrylate (AzPMA) followed by CuAAC
CuAAC has been combined with atom transfer radical poly-
merization (ATRP), known for excellent control over the
molecular weight distributions of the polymer,14,15 for fabri-
cating a wide range of well-controlled polymeric architec-
Additional Supporting Information may be found in the online version of this article.
*Present address: Pingsheng Liu, Jiangsu Key Laboratory of Bio-Functional Materials, Nanjing Normal University, Nanjing, China
C
V
2015 Wiley Periodicals, Inc.
1268
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
coupling32 was shown to result in high conversion yield
(> 95%). The combination of facile surface-initiated ATRP
(SI-ATRP) of azido-bearing methacrylates and CuAAC (often
one-pot) has also been applied for surface functionalization
of substrates ranging from polymers, silicon wafers, gold
foils to magnetic nanoparticles.19,30,33–37 However, due to the
instability of azides (e.g., prone to thermal decomposition)
under the high temperature typically applied to ATRP (e.g.,
50 8C or higher), ATRP of AzPMA and subsequent CuAAC
tended to result in polymers with relative broad molecular
weight distributions (e.g., PDI > 1.5).38,39 Similarly, the qual-
ity of azide motifs grafted to 2D surfaces via SI-ATRP, which
affects ultimate surface functional group densities enabled
by CuAAC, was often overlooked. Overall, optimizing reaction
conditions to ensure well-controlled ATRP and SI-ATRP of
azido-bearing methacrylate monomers remain a significant
interest.
Synthesis of 2-(2-(2-Azidoethyoxy)ethoxy)ethanol
To prepare 2-(2-(2-Azidoethyoxy)ethoxy)ethanol (AzTEG),
sodium azide (0.69 mol) was added to mixture of
a
water (180 mL) and 2-(2-(2-chloroethyoxy)ethoxy)-ethanol
(0.36 mol). The mixture was stirred at 75 8C for 48 h and
then cooled to room temperature. Water was evaporated
under reduced pressure, and the residue was condensed over
NaOH pellets (0.2 g) to trap any HN3 potentially produced.
The residue was suspended in ether (300 mL) for filtration
and the filtrate was concentrated by rotary evaporation to
give the colorless liquid product (yield 90.5%).
1H NMR (400 MHz, CDCl3, d): 3.59 (m, 10H; -OCH2-), 3.32
(t, J 5 5.06 Hz, 2H; N3CH2-), 2.59 (b, 1H; -OH). 13C NMR (100
MHz, CDCl3, d): 72.72 (-O-CH2-CH2-OH), 70.75, 70.51, 70.15
(-O-CH2-), 61.76 (-CH2-OH), 50.77 (N3-CH2). NMR spectra are
shown in Supporting Information Figures S2 and S3.
Synthesis of 2-(2-(2-azidoethyoxy)ethoxy)ethyl
methacrylate
In this study, we first characterized the kinetics of solution
ATRP of 2-(2-(2-azidoethyoxy)ethoxy)ethyl methacrylate
(AzTEGMA) at 50 8C, a temperature that has been commonly
adopted for ATRP of azido-bearing methacrylates, and
identified thermal-induced complications contributing to
the poorly controlled outcome of the polymerization. We
then optimized the reaction conditions to achieve better-
controlled ATRP of AzTEGMA in reduced monomer concen-
trations at or slightly above room temperature, resulting in
polymers with higher molecular weight and narrower molec-
ular weight distributions. Implementing the optimized condi-
tion, we also grafted pAzTEGMA brushes with varying
degrees of polymerization from Ti6Al4V substrates using SI-
ATRP. Finally, facile orthogonal functionalization of the azide-
terminated side chains of the surface brushes with functional
alkynes by CuACC was demonstrated and characterized by
water contact angle measurements and X-ray photoelectron
spectroscopy (XPS).
To prepare 2-(2-(2-Azidoethyoxy)ethoxy)ethyl methacrylate
(AzTEGMA) monomer, AzTEG (40 mmol), TEA (45 mmol),
and 4-methylphenol (0.05 g) were added to 80 mL of ben-
zene and cooled to 0 8C in a two-neck round-bottom flask by
an ice-bath. Methacryloyl chloride (48 mmol) in 20 mL ben-
zene was added drop-wise into the mixture. The reaction
was slowly warmed to room temperature under stirring
overnight. The resulting mixture was filtered, concentrated,
and subjected to silica gel flash chromatography (hexane:
ethyl acetate/5:1 as eluent). The product fractions were con-
centrated in vacuum (yield 75.6%).
1H NMR (CDCl3, 400 MHz, d): 6.07 (m, 1H; 5CH2), 5.52 (m,
1H, 5CH2), 4.24 (m, 2H; -CH2-OC5O), 3.70 (m, 2H; -OCH2-
CH2-OC5O), 3.61(m, 6H; -OCH2-), 3.32 (t, J 5 5.02 Hz, 2H;
N3CH2-), 1.89 (m, 3H; -CH3). 13C NMR (CDCl3, 100 MHz, d):
167.39 (C5O), 136.29 (H2C5C-C5O), 125.80 (5CH2), 70.81,
70.22, 69.32 (-C-O-), 63.98 (-C-O-C5O), 50.78 (N3-C-), 18.41
(-CH3). NMR spectra are shown in Supporting Information
Figures S4 and S5.
EXPERIMENTAL
Materials
Methacryloyl chloride (97%), 4-methylphenol (99%), propar-
gyl alcohol (99%), phenylacetylene (98%), ethyl a-
bromoisobutyrate (EBiB, 98%), a-bromoisobutyryl bromide
(BiBB, 98%), 2,20-bipyridyl (BPY, 97%), copper(I) bromide
(CuBr, 99.999%), sodium azide (99.5%), trifluoroethanol
(TFE, 99%), benzene (99.8%), and dimethylformamide (DMF,
99.5%) were purchased from Sigma-Aldrich (St Louis, MO)
and used as received. Diethyl (hydroxymethyl)phosphonate
(P-OH, 98%) and 2-(2-(2-chloroethyoxy)ethoxy)-ethanol
(TEG-Cl, 96%) were purchased from TCI America (Portland,
OR) and used as received. Triethylamine (TEA, 99.5%,
Sigma-Aldrich) was dried by calcium hydride (CaH2, 99.99%,
Sigma-Aldrich) and distilled before use. Ti6Al4V plates
(1.3 mm thick, Titanium Metal Supply, Poway, CA) was cut
into 10 3 10 mm2 square pieces, which were sequentially
polished under water with 600, 1500, and 3000 grit silicon
carbide sandpapers and ultrasonically cleaned with hexane
(10 min), DCM (10 min), and acetone (10 min) before use.
Preparation of Poly[2-(2-(2-azidoethyoxy)ethoxy)ethyl
methacrylate] (pAzTEGMA) via ATRP
BPY (0.2 mmol) and TFE (1 mL) were charged into a dry
Schlenk flask. After three “freeze-pump-thaw” cycles to
remove oxygen, the flask was back filled with argon followed
by the addition of CuBr (0.1 mmol) under argon protection.
The mixture was stirred until a uniform dark brown catalyst
complex was formed. AzTEGMA (10 mmol), EBiB (0.1
mmol), and TFE (1 mL) were charged into another dry
Schlenk flask. The flask was then degassed by three “freeze-
pump-thaw” cycles, after which the uniform catalyst complex
was injected by syringe to start the polymerizations at 50 8C,
23 8C, or 34 8C. Small aliquots of the reaction mixture were
1
retrieved at predetermined time points for H NMR and GPC
monitoring of the polymerization. To terminate the polymer-
ization, the reactor was exposed to air and the reaction solu-
tion was diluted by acetone and passed through a pad of
silica gel (Alfa Aesar, silica gel 60, mesh 230-400) to remove
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277
1269

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
the deactivated green catalyst complex. Colorless pAzTEGMA
polymers were obtained after removing the solvent under
reduced pressure.
formed polymerization mixture was transferred into a flat-
bottom reactor containing Ti-Br substrates under argon at
room temperature. Upon terminating the nearly simultane-
ously carried out SI-ATRP and solution ATRP, the
pAzTEGMA-grafted Ti6Al4V substrates (e.g., Ti-AzTEGMA-10,
where 10 refers to the targeting degree of polymerization/
DP of 10) were washed with a copious amount of acetone
(30-min stirring each time, three times) to remove the free
polymers physically absorbed on the surface, and dried
under vacuum. pAzTEGMA brushes with different targeting
DPs (10 and 100) were grafted from Ti-Br by varying the
ratio of monomers relative to initiators accordingly.
Monomer Conversion Calculation
The AzTEGMA monomer conversion (C) was calculated
based on real-time 1H NMR data according to the following
equation:
Ip
3
C5
3 100%
Ip
3 1 I
m
2
where Ip is the integration of the broad proton peak (d 0.75–
1.25 ppm) that belongs to the methyl (–CH3) group on the
backbone of the polymer, whereas Im is the integration of
the two proton peaks (d 5.52–6.07 ppm) that belongs to the
methylene (5CH2) group of the unconsumed monomer.
Functionalization of the pAzTEGMA Brushes on Ti6Al4V
Substrates by CuAAC
To demonstrate the versatile functionalization of Ti-
pAzTEGMA substrates by CuAAC, both hydrophilic proparyl
alcohol and hydrophobic phenylacetylene were chosen to be
“clicked” to the azide-terminated pedant chains. Specifically,
propargyl alcohol or phenylacetylene (3.5 mmol) was added
into a solution of BPY (1.0 mmol) in dry DMF (5 mL). After
three “freeze-pump-thaw” cycles to remove oxygen, the flask
was back filled with argon before the addition of CuBr (0.5
mmol). The resulting mixture was stirred until forming a
uniform dark brown catalyst complex and was then injected
into another argon-protected flask containing pAzTEGMA-
grafted Ti6Al4V substrates (Ti-pAzTEGMA-100). After stir-
ring the mixture for 17 h at room temperature, the sub-
strates were removed and washed with a copious amount of
DMF (30-min stirring each time, three times).
Functionalization of pAzTEGMA via CuACC
pAzPEGMA polymer (0.608 g) and proparyl alcohol (3.5
mmol) were added into a solution of BPY (1.0 mmol) in dry
DMF (5 mL). After three “freeze-pump-thaw” cycles to
remove oxygen and back filled with argon, CuBr (0.5 mmol)
was added into the flask under argon protection. The result-
ing mixture was stirred overnight at room temperature
before being exposed to air. The reaction mixture was
diluted by methanol and passed through silica gel. Colorless
pAzTEGMA polymers were obtained upon removing solvent
under reduced pressure.
Gel Permeation Chromatography
Gel permeation chromatography (GPC) measurements were
performed on a Varian Prostar HPLC system equipped with
two 5-mm PLGel MiniMIX-D columns (Polymer Laboratory,
Amherst, MA), an UV–vis detector and a PL-ELS2100 evapo-
rative light scattering detector (Polymer Laboratory,
Amherst, MA). THF was used as an eluent at a flow rate of
0.3 mL/min at room temperature. The number-average
molecular weight (Mn) and the polydispersity index (PDI) of
pAzTEGMA were calculated by the Cirrus AIA GPC Software
using the narrowly dispersed polystyrenes (ReadyCal kits,
PSS Polymer Standards Service, Germany) as the calibration
standards.
XPS Analysis
Surface compositional analyses of substrates before and after
modification were carried out on
a Thermo Scientific
K-Alpha XPS equipped with an Al Ka radiation source under
the spot size of 400 lm. Survey scan spectra were obtained
from two consecutive scans of a randomly chosen area of
interest while high-resolution scan spectra were obtained
from five consecutive scans under the snapshot mode. All
binding energies were referenced to the C1s hydrocarbon
peak at 285.0 eV.
Water Contact Angle Measurement
The static water contact angles of the substrates before and
after surface modifications were recorded on a CAM200
goniometer (KSV Instruments). A droplet (2 mL) of Milli-Q
water was placed on the substrate and the contact angles
(left and right) of the droplet were recorded after 30 s. The
left and right contact angles of each droplet and three sub-
strates of each sample group were averaged, and reported as
averages 6 standard deviation (SD).
Surface Immobilization of Initiator PA-O-Br on Ti6Al4V
Annealed Ti6Al4V substrates were cleaned in an air plasma
cleaner (Harrick, PDC-001) for 2 min before being placed in
a plastic dish, and submerged under 40 mL of 3-mM anhy-
drous methanol solution of PA-O-Br (prepared and purified
as we previously reported40) at room temperature in dark
for 24 h. All retrieved substrates (Ti-Br) were then annealed
at 110 8C for 15 min in a vacuum oven to stabilize the bond-
ing of the phosphonic acid group to the thin oxidized metal
surface before being subjected to extensive sonication in
methanol (10 min each time, twice) and vacuum drying.
RESULTS AND DISCUSSION
Weak Controllability of ATRP of AzTEGMA at 50 8C
Polymers with azido pendants prepared through radical poly-
merizations are promising candidates for preparing functional
materials through orthogonal CuAAC conjugation with alkynes.
However, most reported ATRP of azido-bearing monomers,
SI-ATRP Of AzTEGMA from Ti-Br Substrates
SI-ATRP of AzTEGMA was conducted in a similar fashion as
those described for the solution ATRP except that the freshly
1270
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
TABLE 1 ATRP of AzTEGMA in TFE with Varied Monomer Concentrations, Targeting DPs and Polymerization Temperatures
Temperature
Concentration
(mol/L)
Conversiona
(%)
Mn
Polymer
b
Entry
DP
(8C)
Time (h)
(Da)
PDI
Solubility
1
2
3
4
5
6
7
8
9
100
100
100
50
50
50
50
23
23
23
23
34
34
3
3
3
3
3
2
2
2
2
1
25.10
69.10
/
3,900
4,300
/
1.15
1.17
/
Soluble
Soluble
Gelled
6
18
5
58.80
79.73
45.48
76.15
76.64
96.36
6,300
8,200
7,300
10,100
11,800
13,800
1.17
1.27
1.10
1.13
1.13
1.17
Soluble
Soluble
Soluble
Soluble
Soluble
Soluble
50
21
6
50
100
100
100
31
3
15
a
b
Monomer conversion as determined by 1H NMR.
Mn of the polymers as determined by GPC.
carried out at 50 8C or higher temperatures, were not well
controlled, often resulting in polymers with relatively broad
molecular weight distribution (PDI > 1.4).20,32
molecular weight (Mn) increases as a function of monomer
conversion [Fig. 1(b)] did not support a well-controlled
ATRP as the linear increase of the molecular weight was
only observed up to 40% monomer conversion (polymeriza-
tion time up to 2 h). Prolonged polymerization time did not
result in further increase in the molecular weight [Mn in fact
decreased after 50% conversion; Fig. 1(b), Table 1]. It should
be noted that the reaction mixture maintained a brown color
throughout the ATRP process, eliminating deactivation of the
Cu(I)/BPY catalyst system (would have resulted in a green
color) as a potential cause for decreased polymerization rate
or termination of the ATRP. Accordingly, we hypothesize that
To identify the underlying factors contributing to the weak
controllability of the ATRP of azido-bearing monomers, we
first investigated the kinetics of ATRP of AzTEGMA at 50 8C
in TFE (Table 1, Entries 1–3). As shown by the plots of
AzTEGMA monomer conversion (black square) and conver-
sion index (blue star) over polymerization time [Fig. 1(a)],
first-order monomer conversion presenting a typical living
polymerization was observed. However, the number-average
FIGURE 1 Kinetics of the ATRP of AzTEGMA at 50 8C. (a) Kinetics plots of monomer conversion (%) and conversion index In([M]0/
[M]) over polymerization time; (b) number-average molecular weight (Mn) of the polymers as a function of monomer conversion;
(c) GPC curves of the polymerization mixture as a function of reaction time. Polymerization condition: [AzTEGMA]5 3 M, [AzTGMA]/
[EBiB]/[CuBr]/[BPY] 5 100:1:1:2. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277
1271

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
chain propagation of the ATRP gradually slowed down
(nearly halted by 2 h) at 50 8C, and further conversion of
the monomers likely contributed to side reactions giving rise
to lower molecular weight oligomers/polymers. It has been
reported that the azido and the methacrylate groups from
azido-bearing methacrylate monomers can form intermolecu-
lar triazoline rings during reversible addition2fragmentation
chain transfer (RAFT) polymerization at 50 8C.41 Alkyl azides
can also undergo a 1,3-dipolar cycloaddition with electron-
deficient olefins (e.g., N-isopropylacrylamide, dimethylacryla-
mide, methyl acrylate, and methyl methacrylate) to form tria-
zoles and aziridines during RAFT at 60 8C even in the
absence of any catalysts.42 The side reaction(s) involving the
consumption of vinyl group of the AzTEGMA monomer may
also occur in a first-order kinetics similar to that observed
with the ATRP. This would not have affected the conven-
tional monomer conversion calculation based on the reduc-
tion of the vinyl proton signals by 1H NMR monitoring.
Meanwhile, real-time GPC monitoring of the ATRP of
AzTEGMA at 50 8C indeed detected a lower molecular weight
component (elution time around 18.5–19 min; note that the
monomer was eluted at 19–19.5 min) at as early as 30 min
after the initiation of the ATRP [Fig. 1(c), black line]. The sig-
nal for this component became stronger and broader (note
the increasing shoulder peak eluted around 18.5 min) over
time, with its intensity exceeding that of the pAzTEGMA
(eluted around 15–18 min) after 2 h. These data combined
support that the side reaction occurred early on during the
ATRP process at 50 8C and it became a dominant event after
2 h.
of polymerization time agreed with first-order monomer con-
version [Fig. 2(a)]. Unlike the polymerization conducted at
50 8C, the molecular weight of the polymer increased linearly
as a function of the monomer conversion at 23 8C, with the
narrow polydispersity (PDI < 1.20) maintained throughout
the polymerization process [Fig. 2(b)]. In addition, no low
molecular weight components resulting from side reactions
(elution time around 18-18.5 min as observed in ATRP at
50 8C) was detected by GPC monitoring during the 21 h
polymerization at 23 8C with monomer conversions over
80% [Fig. 2(c)]. The shift of pAzTEGMA peak to earlier elu-
tion, consistent with the increase of molecular weight of the
polymer, also correlated well with the reduction of monomer
peak intensity over time [Fig. 2(c)]. These observations sup-
port that the room temperature ATRP of AzTEGMA was well-
controlled and that lowering polymerization temperature
effectively suppressed the side reaction of azido-bearing
monomers and polymers that were observed at 50 8C.
To further optimize the controllability of ATRP of AzTEGMA
at room temperature, we lowered the monomer concentra-
tion from 3 to 2 M while keeping the initiator concentration
the same (Table 1, Entries 6 and 7). Consistent with first-
order controlled polymerization kinetics,29 the room temper-
ature ATRP proceeded at a slower rate when the monomer
concentration decreased from 3 to 2 mol/L, with the appa-
rent propagation rate constant (kapp) decreasing from 4.89
3 1025 to 2.84 3 1025 s21 [Fig. 3(a)]. The slower ATRP car-
ried out in the more diluted AzTEGMA solution, however,
resulted in pAzTEGMA with narrower polydispersity (PDI
ꢀ1.10) at comparable monomer conversions (Table 1, Entry
4 vs. Entry 6, Entry 5 vs. Entry 7).
With further prolongation of the polymerization time (18 h) at
50 8C, the brown reaction mixture eventually gelled (Table 1,
Entry 3) with many bubbles encapsulated. Thermal decom-
position of the azide groups43,44 and the insertion reaction43
are likely responsible for releasing nitrogen and crosslinking
the polymer chains, respectively, resulting in the gelation of
the mixture and the entrapment of the in situ released nitro-
gen gas.
To take advantage of better controlled ATRP of AzTEGMA in
lower monomer concentration while aiming to accelerate the
polymerization without high temperature-induced side
effects, we carried out ATRP of AzTEGMA at 2-M monomer
concentration at 34 8C (Table 1, Entries 8 and 9). The 11 8C
increase from room temperature significantly accelerated the
polymerization (DP 5 100) to reach 76% monomer conver-
sion in 3 h and 96.36% conversion by 15 h, resulting in pAz-
TEGMA with much higher molecular weights while still
maintaining a low polydispersity [Fig. 3(b)]. Overall, the
combination of an intermediate temperature of 34 8C and a
lower monomer concentration of 2 M resulted in an acceler-
ated yet well-controlled ATRP of AzTEGMA.
Optimization of ATRP of AzTEGMA
To mitigate the thermal decomposition of azides and the
accompanying side reactions often observed during the prep-
aration of polymers with azide-terminated side chains via
radical polymerization processes, shortening polymerization
time and lowering polymerization temperature are both logi-
cal strategies to pursue. Pierce et al. implemented both strat-
egies to optimize the polymerization of azido-bearing vinyl
monomers by RAFT.42 For optimizing the ATRP of azido-
bearing monomers, however, shortening polymerization time
has been more broadly implemented than lowering polymer-
ization temperature. Here we first investigated the efficacy of
lowering the temperature of ATRP of AzTEGMA from 50 8C
to room temperature (23 8C) on improving the controllability
of the polymerization (Table 1, Entries 4 and 5).
Postfunctionalization of pAzTEGMA by CuAAC
To examine the efficacy of CuAAC for high-density functional-
ization of the azide-terminated pendant chains of pAzTEGMA
(Mn 5 17,300, PDI 5 1.29), prepared by the optimized ATRP
at 34 8C with an initial monomer concentration of 2 M,
proparyl alcohol was allowed to conjugate with the polymer
at room temperature in the presence of Cu(I) catalyst
[Fig. 4(a)]. After the click conjugation, the molecular weight
of the polymer increased from 17,300 to 22,000 while the
PDI remained largely unchanged [Fig. 4(b)]. 1H NMR con-
firmed the complete disappearance and up-filed shifts of the
The kinetics plots of AzTEGMA monomer conversion (black
square) and the conversion index (blue cycle) as a function
1272
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 2 Kinetics of the ATRP of AzTEGMA at room temperature. (a) Kinetic plots of monomer conversion (%) and conversion
index In([M]0/[M]), (b) molecular weight and molecular weight distribution, and (c) GPC curve as a function of reaction time. Poly-
merization condition: [AzTEGMA] 5 3 M, [AzTGMA]/[EBiB]/[CuBr]/[BPY] 5 50:1:1:2. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
methylene signals alpha and beta to the azide [Protons 8
and 7, bottom spectrum, Fig. 4(c)] consistent with the meth-
ylenes alpha and beta to the triazole nitrogen [Protons 80
and 70, top spectrum, Fig. 4(c)], supporting quantitative con-
version of azides by CuAAC. Signal ascribable to the triazole
ring proton (Proton 90) was also observed and its integration
relative to that of the methyl group on the polymer back-
bone (Proton 10) closely matched with the theoretic ratio
FIGURE 3 Tailoring kinetics of ATRP of AzTEGMA by monomer concentration and polymerization temperature. (a) Monomer con-
version index In([M]0/[M]) of ATRP of AzTEGMA as a function of time with monomer concentration of 2 M (red) versus 3 M (black)
at room temperature. [AzTGMA]/[EBiB]/[CuBr]/[BPY] 5 50:1:1:2. (b) GPC traces of pAzTEGMA prepared by ATRP at 34 8C with a
monomer concentration of 2 M. [AzTGMA]/[EBiB]/[CuBr]/[BPY] 5 100:1:1:2. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277
1273

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
FIGURE 4 Functionalization of pAzTEGMA by CuAAC. (a) Schematic illustration of the functionalization of pAzTEGMA with propar-
gyl alcohol at room temperature. (b) GPC traces of pAzTEGMA before (red line) and after the click reaction with propargyl alcohol.
(c) 1H NMR spectra of the pAzTEGMA before (bottom) and after (top) the click reaction with propargyl alcohol. [Color figure can
be viewed in the online issue, which is available at wileyonlinelibrary.com.]
(1:3 theoretical; 1:2.97 detected), further supporting quanti-
tative coupling by CuAAC.
trolled surface modifications, we first grafted pAzTEGMA by SI-
ATRP (Fig. 5) from Ti6Al4V substrates under the optimized
conditions identified above. Phosphonic acid-terminated initia-
tor PA-O-Br (Fig. 5) was synthesized, covalently attached to the
metal alloy surfaces and annealed as we previously described.40
The initiator-bound substrate Ti-Br was then subjected to graft-
ing of pAzTEGMA by SI-ATRP at room temperature.
Controlled Grafting of pAzTEGMA Brushes from Ti6Al4V
Substrates
To investigate the possibility of applying the optimized ATRP
conditions and subsequent CuAAC functionalization to con-
1274
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
FIGURE 5 Schematic illustration of the grafting of well-controlled pAzTEGMA brushes from Ti6Al4V and the subsequent function-
alization via CuAAC. R 5 -OH or -Bn. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
XPS survey scans of the pAzTEGMA-grafted substrates (Ti-
pAzTEGMA, DP 5 100) detected a N1s peak (binding energy
ꢀ400 eV) along with residue P2p signal ascribable to the
azide groups on the pendant side chains and the phosphonic
acid anchor that were absent from that of the pristine
Ti6Al4V [Fig. 6(a), Table 2]. Meanwhile, the intensity of the
Ti2p signal significantly decreased on Ti-pAzTEGMA com-
pared to Ti6Al4V [Fig. 6(a), Table 2] as the metal alloy sub-
strate became masked by the grafted polymer brushes. High-
resolution scan of N1s using the snapshot mode that reduces
the X-ray irradiation- induced in situ decomposition of azide
(see conventional scans in Supporting Information Fig. S6)
revealed 2 peaks at 405.5 and 401.5 eV [Fig. 6(b)] attribut-
able to the terminal N’s and the center N1 species, respec-
tively.45,46 The detected abundance of the terminal N’s
relative to that of the center N1 species of the azide, aver-
aged from three randomly selected Ti-pAzTEGMA substrates,
closely approximates that of the theoretical ratio [2.3 6 0.1
detected vs. 2.0 theoretical; Fig. 6(b)], further supporting
successful grafting of the azide-rich pAzTEGMA brushes by
SI-ATRP.
By introducing free initiator into the reaction system, a
method for controlling the chain length of surface grafted
polymer brushes,40,47,48 we grafted pAzTEGMA with targeted
degrees of polymerization of 10 and 100 from Ti-Br. As
revealed by XPS analyses, upon grafting the shorter
pAzTEGMA-10, the surface carbon content and nitrogen con-
tent of the Ti-Br substrates increased from ꢀ26 to ꢀ35%
and 0 to ꢀ5%, respectively, while the surface Ti content
decreased by ꢀ1% [Fig. 6(c), Table 2]. When longer pAz-
TEGMA brushes (DP 5 100) was grafted, the surface C and N
content sharply increased to 55 and >10%, respectively,
while the surface Ti content substantially decreased to <3%
FIGURE 6 Surface modification of Ti6Al4V by grafting of pAzTEGMA brushes via SI-ATRP. (a) XPS survey scans and (b) N1s high-
resolution scans of the pristine Ti6Al4V and Ti-pAzTEGMA surfaces. The Ti6Al4V substrates were grafted with pAzTEGMA at a tar-
geted degree of polymerization of 100 (DP 5 100). (c) Relative contents of N, Ti, C, and O elements detected by XPS on
Ti-pAzTEGMA surfaces with varying degrees of polymerization (DP 510 vs. DP 5100). *P < 0.05 (ANOVA). [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277
1275

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 2 Surface Elemental Composition of Surface-Modified Ti6Al4V Substrates (n 5 3) as Determined by XPS
Elemental Composition (%)
Substrates
N
Ti
C
O
Ti-Br
–
9.93 6 1.81
9.25 6 0.92
2.45 6 0.43
26.19 6 2.87
35.18 6 1.82
55.00 6 1.02
53.63 6 1.96
50.59 6 1.24
32.10 6 0.91
Ti-pAzTEGMA-10
Ti-pAzTEGMA-100
4.99 6 0.48
10.45 6 0.32
[Fig. 6(c), Table 2], supporting much dense surface coverage
by the longer pAzTEGMA brushes.
“clicked” to Ti-pAzTEGMA-100 and the extent of azide to tri-
azole conversion was examined by high-resolution N1s XPS
scans under the snapshot mode. As shown in Figure 7(a),
the N1 signal of the azide (405.5 eV) completely disappeared
upon CuAAC while a new N signal of comparable intensity
ascribable to the uncharged center N in the triazole was
detected (peaking ꢀ402 eV upon curve fitting). This observa-
tion was made upon CuAAC by both hydrophilic (benzyl-ter-
minated) and hydrophobic (hydroxyl-terminated) alkynes,
supporting quantitative conversion of azide-terminated pend-
ant chains in both cases. Finally, water contact angle meas-
urements confirmed significant increase in surface
hydrophilicity upon “clicking” the proparyl alcohol (Ti-pAz-
TEGMA-OH), with the water contact angle decreased from
708 for the Ti-pAzTEGMA to 43 8 for Ti-pAzTEGMA-OH [Fig.
7(b)]. Likewise, expected decrease in surface hydrophilicity
upon “clicking” the hydrophobic phenylacetylene was also
observed. Overall, these data show that well-controlled graft-
ing of polymer brushes with stable azide-terminated side
chains from metallic alloy surface could be accomplished by
SI-ATRP of AzTEGMA under the optimal conditions we iden-
tified. Subsequent quantitative high-density functionalization
of the azide-terminated side chains by CuAAC with small
molecule alkynes enabled further surface modifications.
Facile High-Density Side Chain Functionalization of the
Ti-pAzTEGMA Surface Brushes by CuAAC
To demonstrate the general applicability of CuAAC for high-
density side chain functionalization of the pAzTEGMA surface
brushes, both hydrophobic and hydrophilic alkynes are
CONCLUSIONS
In summary, we demonstrated that the poor controllability
of ATRP of azido-bearing methacrylate monomer AzTEGMA
at commonly adopted temperature of 50 8C could be
addressed by lowering polymerization temperature and
reducing the monomer concentrations. Lowing the polymer-
ization temperature to room temperature effectively sup-
pressed the side reaction induced by the thermal
decomposition of azido groups, thereby significantly
increased the molecular weight (up to 22 kD) and reduced
the polydispersity (PDI < 1.3) of pAzTEGMA. By carrying out
the ATRP of AzTEGMA at a reduced monomer concentration
while, at an intermediate temperature 34 8C, the ATRP was
accelerated without compromising the controllability of the
polymerization. The optimized ATRP of AzTEGMA was read-
ily extended to graft pAzTEGMA surface brushes with con-
trolled lengths from Ti6Al4V substrates. The azide-
terminated side chains of these polymers and surface poly-
mer brushes could be readily functionalized by functional
alkynes in a quantitative manner by facile CuAAC. The opti-
mized ATRP and SI-ATRP of AzTEGMA combined with
FIGURE 7 Facile high-density functionalization of Ti6Al4V sub-
strates via the combination of SI-ATRP and CuAAC. (a) XPS
N1s high-resolution scans of Ti-pAzTEGMA surface versus
those “clicked” with proparyl alcohol (Ti-pAzTEGMA-OH) or
phenylacetylene (Ti-pAzTEGMA-Bn). (b) Water contact angle of
the Ti-pAzTEGMA substrates before and after conjugation with
hydrophilic and hydrophobic alkynes by CuAAC. *P < 0.05 (Stu-
dent’s t-test). [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
orthogonal CuAAC provides
a
useful tool for facile
1276
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277

JOURNAL OF
POLYMER SCIENCE
WWW.POLYMERCHEMISTRY.ORG
ARTICLE
21 D. H. Han, X. Tong, Y. Zhao, Macromolecules 2011, 44,
5531–5536.
preparations of functional polymers and controlled modifica-
tion of surfaces for a wide range of applications.
22 Y. F. Zhang, C. H. Li, S. Y. Liu, J. Polym. Sci. Polym. Chem.
2009, 47, 3066–3077.
23 L. Mespouille, M. Vachaudez, F. Suriano, P. Gerbaux,
O. Coulembier, P. Degee, R. Flammang, P. Dubois, Macromol.
Rapid Commun. 2007, 28, 2151–2158.
ACKNOWLEDGMENTS
The authors acknowledge grant support from the National
Institutes of Health (R01AR055615, R01AR068418). XPS was
performed at the Center for Nanoscale Systems (CNS) at Har-
vard University supported by the National Science Foundation
award ECS-0335765.
24 A. P. Vogt, B. S. Sumerlin, Macromolecules 2006, 39, 5286–
5292.
25 P. L. Golas, N. V. Tsarevsky, B. S. Sumerlin, K.
Matyjaszewski, Macromolecules 2006, 39, 6451–6457.
26 M. M. Abdellatif, K. Nomura, ACS Macro Lett. 2012, 1, 423–
427.
27 C. Zhou, S. S. Qian, X. J. Li, F. Yao, J. S. Forsythe, G. D. Fu,
RSC Adv. 2014, 4, 54631–54640.
REFERENCES AND NOTES
28 E. Soto-Cantu, B. S. Lokitz, J. P. Hinestrosa, C. Deodhar, J.
M. Messman, J. F. Ankner, S. M. Kilbey, Langmuir 2011, 27,
5986–5996.
1 H. C. Kolb, M. G. Finn, K. B. Sharpless, Angew. Chem. Int.
Ed. 2001, 40, 2004–2021.
2 P. Wu, A. K. Feldman, A. K. Nugent, C. J. Hawker, A. Scheel,
B. Voit, J. Pyun, J. M. J. Frechet, K. B. Sharpless, V. V. Fokin,
Angew. Chem. Int. Ed. 2004, 43, 3928–3932.
29 W. K. Storms-Miller, C. Pugh, Macromolecules 2015, 48,
3803–3810.
30 Y. R. Zhang, L. Ren, Q. Tu, X. Q. Wang, R. Liu, L. Li, J. C.
Wang, W. M. Liu, J. Xu, J. Y. Wang, Anal. Chem. 2011, 83,
9651–9659.
3 K. C. Nicolaou, S. A. Snyder, T. Montagnon, G.
Vassilikogiannakis, Angew. Chem. Int. Ed. 2002, 41, 1668–1698.
4 C. E. Hoyle, C. N. Bowman, Angew. Chem. Int. Ed. 2010, 49,
1540–1573.
31 X. H. He, L. Y. Liang, K. Wang, S. L. Lin, D. Y. Yan, Y. Q.
Zhang, J. Appl. Polym. Sci. 2009, 111, 560–565.
5 M. J. Kade, D. J. Burke, C. J. Hawker, J. Polym. Sci. Polym.
Chem. 2010, 48, 743–750.
32 B. S. Sumerlin, N. V. Tsarevsky, G. Louche, R. Y. Lee, K.
Matyjaszewski, Macromolecules 2005, 38, 7540–7545.
6 K. L. Killops, L. M. Campos, C. J. Hawker, J. Am. Chem. Soc.
2008, 130, 5062–5064.
33 W. Li, Y. H. Xu, Y. Zhou, W. H. Ma, S. X. Wang, Y. N. Dai,
Nanoscale Res. Lett. 2012, 7, 485
7 N. J. Agard, J. A. Prescher, C. R. Bertozzi, J. Am. Chem. Soc.
2004, 126, 15046–15047.
34 W. T. Song, C. S. Xiao, L. G. Cui, Z. H. Tang, X. L. Zhuang,
X. S. Chen, Colloid Surface B. 2012, 93, 188–194.
8 E. Lallana, A. Sousa-Herves, F. Fernandez-Trillo, R. Riguera,
E. Fernandez-Megia, Pharm. Res. Dordr. 2012, 29, 1–34.
35 S. Saha, M. L. Bruening, G. L. Baker, Macromolecules 2012,
45, 9063–9069.
9 P. Thirumurugan, D. Matosiuk, K. Jozwiak, Chem. Rev. 2013,
113, 4905–4979.
36 L. Q. Xu, D. Wan, H. F. Gong, K. G. Neoh, E. T. Kang, G. D.
Fu, Langmuir 2010, 26, 15376–15382.
10 P. L. Golas, K. Matyjaszewski, QSAR Comb. Sci. 2007, 26,
37 H. He, Y. Zhang, C. Gao, J. Wu, Chem. Commun. 2009,
1116–1134.
1655–1657.
11 R. K. Iha, K. L. Wooley, A. M. Nystrom, D. J. Burke, M. J.
Kade, C. J. Hawker, Chem. Rev. 2009, 109, 5620–5686.
38 N. K. Devaraj, J. P. Collman, Qsar Comb. Sci. 2007, 26,
1253–1260.
12 J. Xu, E. Feng, J. Song, J. Am. Chem. Soc. 2014, 136, 4105–
4108.
39 Y. Li, J. W. Yang, B. C. Benicewicz, J. Polym. Sci. Polym.
Chem. 2007, 45, 4300–4308.
13 J. W. Xu, T. M. Filion, F. Prifti, J. Song, Chem. Asian J.
2011, 6, 2730–2737.
40 P. S. Liu, E. Domingue, D. C. Ayers, J. Song, ACS Appl.
Mater. Interfaces 2014, 6, 7141–7152.
14 K. Matyjaszewski, J. H. Xia, Chem. Rev. 2001, 101, 2921–
2990.
41 M. J. Yanjarappa, K. V. Gujraty, A. Joshi, A. Saraph, R. S.
Kane, Biomacromolecules 2006, 7, 1665–1670.
15 J. S. Wang, K. Matyjaszewski, J. Am. Chem. Soc. 1995, 117,
5614–5615.
42 V. Ladmiral, T. M. Legge, Y. L. Zhao, S. Perrier, Macromole-
cules 2008, 41, 6728–6732.
16 H. F. Gao, K. Matyjaszewski, Macromolecules 2006, 39,
43 C. J. Ruud, J. P. Jia, G. L. Baker, Macromolecules 2000, 33,
8184–8191.
4960–4965.
44 K. A. Murray, A. B. Holmes, S. C. Moratto, R. H. Friend,
Synth. Met. 1996, 76, 161–163.
17 J. F. Lutz, H. G. Borner, K. Weichenhan, Macromolecules
2006, 39, 6376–6383.
45 A. C. Gouget-Laemmel, J. Yang, M. A. Lodhi, A.
Siriwardena, D. Aureau, R. Boukherroub, J. N. Chazalviel, F.
Ozanam, S. Szunerits, J. Phys. Chem. C 2013, 117, 368–375.
18 P. D. Topham, N. Sandon, E. S. Read, J. Madsen, A. J.
Ryan, S. P. Armes, Macromolecules 2008, 41, 9542–9547.
19 F. He, B. W. Luo, S. J. Yuan, B. Liang, C. Choong, S. O.
Pehkonen, RSC Adv. 2014, 4, 105–117.
46 J. P. Collman, N. K. Devaraj, T. P. A. Eberspacher, C. E. D.
Chidsey, Langmuir 2006, 22, 2457–2464.
20 M. A. Olson, A. B. Braunschweig, L. Fang, T. Ikeda, R. Klajn,
A. Trabolsi, P. J. Wesson, D. Benitez, C. A. Mirkin, B. A.
Grzybowski, J. F. Stoddart, Angew. Chem. Int. Ed. 2009, 48,
1792–1797.
47 D. Koylu, K. R. Carter, Macromolecules 2009, 42, 8655–8660.
48 E. Marutani, S. Yamamoto, T. Ninjbadgar, Y. Tsujii, T.
Fukuda, M. Takano, Polymer 2004, 45, 2231–2235.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE, PART A: POLYMER CHEMISTRY 2016, 54, 1268–1277
1277