Synthesis of well-defined semitelechelic poly[N-(2-hydroxypropyl) methacrylamide] polymers with functional group at the α-end of the polymer chain by RAFT polymerization

Article abstract of DOI:10.1021/ma400042u

N-(2-Hydroxypropyl)methacrylamide polymer precursors (pHPMA) with very narrow distribution of molecular weights and polymer chain terminating in a single reactive group have big potential in the synthesis of various polymer drug and gene delivery systems. This paper shows that pHPMA can be prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization; the pHPMAs prepared by RAFT polymerization conducted to high conversions contained at the α-end of the polymer chain a single functional group originated from both, the chain transfer agent (CTA) and initiator (INI) in ratio depending on polymerization conditions. The well-defined monofunctional pHPMAs with narrow molecular weight distribution and with single reactive group situated at the α-polymer chain end can be prepared in one step by RAFT polymerization initiated by the tailor-made CTA and INI, both containing the same functional group in their structure. Synthesis of pHPMAs terminating in various reactive groups (azide, propargyl, thiazolidin-2-thione, amino, hydrazide) was also described.

Full text of DOI:10.1021/ma400042u

Article
pubs.acs.org/Macromolecules
Synthesis of Well-Defined Semitelechelic Poly[N‑(2-
hydroxypropyl)methacrylamide] Polymers with Functional Group at
the α‑End of the Polymer Chain by RAFT Polymerization
̌
V. Subr,* L. Kostka, J. Strohalm, T. Etrych, and K. Ulbrich
Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, v.v.i., Heyrovsky Sq. 2, 162 06 Prague 6, Czech
Republic
S
* Supporting Information
ABSTRACT: N-(2-Hydroxypropyl)methacrylamide polymer precur-
sors (pHPMA) with very narrow distribution of molecular weights
and polymer chain terminating in a single reactive group have big
potential in the synthesis of various polymer drug and gene delivery
systems. This paper shows that pHPMA can be prepared by reversible
addition-fragmentation chain transfer (RAFT) polymerization; the
pHPMAs prepared by RAFT polymerization conducted to high
conversions contained at the α-end of the polymer chain a single
functional group originated from both, the chain transfer agent (CTA)
and initiator (INI) in ratio depending on polymerization conditions. The
well-defined monofunctional pHPMAs with narrow molecular weight distribution and with single reactive group situated at the
α-polymer chain end can be prepared in one step by RAFT polymerization initiated by the tailor-made CTA and INI, both
containing the same functional group in their structure. Synthesis of pHPMAs terminating in various reactive groups (azide,
propargyl, thiazolidin-2-thione, amino, hydrazide) was also described.
end is close to one, especially if reactive group has to be
introduced.
INTRODUCTION
■
Over the last 15 years, reversible addition−fragmentation chain
transfer (RAFT) radical polymerization has become an
important method for the synthesis of structurally well-defined
polymers with narrow molecular weight distributions.1−4 The
mechanism of RAFT polymerization based on chain transfer
agents (CTAs) containing thiocarbonylthio or trithiocarbonyl
groups has been described in many papers and reviews.5−8
Because RAFT polymerization is no more than a conventional
free radical polymerization conducted in the presence of a
suitable thiocarbonylthio species, traditional methods for the
generation of free radicals using azo compounds, such as 2,2′-
azobis(2-methylpropionitrile) (AIBN) or 4,4′-azobis(4-cyano-
pentanoic acid) (INI-(COOH)2), have been employed.7
Progress in the field of RAFT polymerization has enabled the
relatively easy synthesis of a broad range of statistical and block
copolymers and α- or ω-end-functionalized polymers with well-
defined architectures.9−14
These polymers were quickly found to have applications in
polymer drug delivery systems.15−19 For example, α-end-
functionalized polymers were used in the synthesis of polymer
protein/peptide conjugates.20−23 Generally, α- or ω-end-
functionalized polymers can be synthesized by RAFT polymer-
ization via two different methods. In the first method, polymers
prepared by RAFT polymerization are usually terminated at the
ω-end with a thiocarbonylthio group originating from the CTA.
This thiocarbonylthio group can be subsequently removed or
transformed into the desired ω-end-functional group.11,24,25
Unfortunately, not always functionality of the polymer chain ω-
In the second method, chain transfer agents with the desired
functional groups were synthesized and then used in RAFT
polymerization, resulting in polymers with α-end-functional
groups.20,26 In this case, the ω-end thiocarbonylthio group was
removed by a method described by Perrier.11
Tao et al.20 described the synthesis of a novel thiazolidine-2-
thione-functionalized CTA and its use in the preparation of an
α-end-functionalized pHPMA by RAFT polymerization
initiated with AIBN. This α-end-functionalized pHPMA was
subsequently conjugated with lysozyme. The obtained semite-
1
lechelic pHPMA was characterized by H NMR and SEC.
Unfortunately, although the exact content of the α-end
thiazolidine-2-thione group can be easily measured spectro-
photometrically, it was not determined.27
Yang et al.28 described the synthesis of a biodegradable
multiblock pHPMA polymer. The pHPMA was prepared by
RAFT polymerization using a CTA containing a glycyl-
phenylalanyl-leucyl-glycyl alkyne functional group initiated
with AIBN. Postpolymerization modification of the ω-end
thiocarbonylthio group with 4,4′-azobis(azidopropyl-4-cyano-
pentanoate) resulted in the formation of a heterotelechelic
pHPMA containing α-terminal alkyne and ω-terminal azide
groups. The biodegradable multiblock pHPMA was prepared
Received: January 7, 2013
Revised: February 21, 2013
© XXXX American Chemical Society
A
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N-(2-Hydroxypropyl)methacrylamide (HPMA) was synthesized by
a modified reaction of methacryloyl chloride with 1-aminopropan-2-ol
in dichloromethane in the presence of sodium carbonate.32
from a heterotelechelic pHPMA using click chemistry. The
multiblock pHPMA was separated from the unreacted mono
and diblock pHPMA by size exclusion chromatography.
Presence of only 38 wt % of the multiblock pHPMA in the
final product indicated that the functionality of the starting
heterotelechelic pHPMA was not close to one and thus the
reaction was not completed. Unfortunately, the authors did not
provide detailed analysis of the end-chain groups.
The impact of azo initiators on the structure of α-end-
functionalized polymers in the papers mentioned above and in
many other published papers studying RAFT polymerization
has been neglected.7 We found only one paper that reported on
quantitative analysis of the effect of azo initiators on the
structure of the α-end of the polymer chain in RAFT
polymerization.29
Here, we focused on the synthesis of semitelechelic α-end-
functionalized polymers based on N-(2-hydroxypropyl)-
methacrylamide (pHPMAs) and the quantitative analysis of
the effect of azo initiators on the structure and functionality of
the α-end of the polymer chain prepared by RAFT polymer-
ization. The study was performed with HPMA polymerized by
a RAFT mechanism with 4,4′-azobis(4-cyanopentanoyl-valine
or leucine methyl ester) (INI-(Val-OMe)2 or INI-(Leu-OMe)2)
as initiators and 4-cyano-4-(thiobenzoylthio)pentanoyl-alanine
methyl ester (CTA-Ala-OMe) as the chain transfer agent.
Amino acids were used as labels, one amino acid in the CTA
molecule and two in each INI molecule. The ratio of amino
acids (Val/Ala or Leu/Ala) incorporated into the α-end of the
polymer chain was determined by amino acid analysis. On the
basis of these results, a series of azo initiators (INI) based on
4,4′-azobis(4-cyanopentanoic acid) and 4-cyano-4-(thioben-
zoylthio)-pentanoic acid as chain transfer agents (CTA)
containing different functional groups (azide, propargyl,
thiazolidin-2-thione, -NH-Boc and -NHNH-Boc) were synthe-
sized and used for the preparation of well-defined α-end-
functionalized pHPMAs. The importance of such well-defined
polymers is not only because of their use in the preparation of
HPMA−protein or antibody conjugates20−23 but such polymers
can be used for preparation of biodegradable multiblock
polymers, for polymer coating of nanoparticles and viral gene
delivery vectors and also in the synthesis of water-soluble
polymer drug carriers and conjugates of grafted block or star
dendrimer-derived architectures.30,31
Characterization Methods. The CTA and INI were characterized
on HPLC Shimadzu system with reverse-phase column Chromolith
HighResulution RP-18e, 100 × 4.6 mm (Merck, Germany) equipped
with UV/vis photodiode array detector. Gradient elution with 5−95%
of acetonitrile for 15 min at a flow rate of 1.0 mL/min was used. The
molecular weight of CTA and INI was determined using mass
spectrometer (MS LCQ Fleet, Thermo Fisher Scientific) and structure
1
was confirmed by H NMR (300 MHz) spectra recorded on (Bruker
DPX 300) spectrometer and elemental analysis.
Number-average molecular weight (Mn), weight-average molecular
weight (Mw), and polydispersity (Đ) were measured using size-
exclusion chromatography (SEC) on a HPLC Shimadzu system
equipped with UV, an Optilab rEX differential refractometer and
multiangle light scattering DAWN 8 (Wyatt Technology, USA)
detectors. For these experiments, a 20% 0.3 M acetate/80% methanol
(v/v) buffer and TSKgel G3000SW or TSKgel G4000SW column
were used.
The content of alanine and valine or leucine in the polymer was
determined by amino acid analysis of hydrolyzed pHPMA samples (6
M HCl, 115 °C, 18 h in a sealed ampule) on a reverse-phase column
Chromolith HighResulution RP-18e, 100 × 4.6 mm (Merck,
Germany) using precolumn derivatization with phthalaldehyde
(OPA) and 3-sulfanylpropanoic acid (excitation at 229 nm, emission
at 450 nm). Gradient elution with 10−100% of solvent B for 35 min at
a flow rate of 1.0 mL/min was used (solvent A, 0.05 M sodium acetate
buffer, pH 6.5; solvent B, 300 mL of 0.17 M sodium acetate and 700
mL of methanol).
The content of thiazolidine-2-thione (TT) groups was determined
spectrophotometrically on a Specord 205 (Jena Analytics) spectro-
photometer (ε305 = 10 700 L.mol−1.cm−1; methanol)27 and content of
amino groups was determined by TNBSA method (ε420 = 11 550
L.mol−1.cm−1; borate buffer pH 9.3).33
Synthesis of 2-[1-Cyano-1-methyl-4-oxo-4-(2-thioxo-thiazolidin-
3-yl)-butylazo]-2-methyl-5-oxo-5-(2-thioxothiazolidin-3-yl)-
pentanenitrile (INI-(TT)2). The INI-(TT)2 was prepared by reaction of
INI-(COOH)2 with 4,5-dihydro-thiazole-2-thiol in the presence of
DCC in tetrahydrofuran catalyzed by DMAP.34 HPLC analysis gave
two symmetrical peaks at 305 nm with a retention times of 10.24 and
10.35 min. Anal. Calcd/found: C, 44.79/44.35; H, 4.59/5.71; N,
17.41/9.45; S, 26.57/28.92. 1H NMR 300 MHz (DMSO, 296 K),
ppm: 1.7 (s, 6H), 2.39−2.45 (m, 4H), 3.12−3.21 (m, 4H), 3.29−3.36
(m, 4H), 4.42−4.54 (m, 4H). ESI−MS: m/z = 504.83 (M−Na)+.
Synthesis of 4,4′-Azobis(4-cyanopentanoylvaline methyl ester)
(INI-(Val-OMe)2). The INI-(COOH)2 (2.0 g; 7.14 mmol), Val-
OMe.HCl (2.63 g, 15.7 mmol) and DIPEA (2.82 mL, 16.4 mmol)
were dissolved in dichloromethane (15 mL) and then EDC (3.61 g;
18.8 mmol) was added in several portions. Reaction mixture was
stirred for 3 h and then extracted with distilled water (3 × 20 mL) and
with 2 wt % NaHCO3 aqueous solution. Organic layer was separated
and dried with Na2SO4. The DCM was evaporated and the product
was crystallized from mixture ethyl acetate/diethyl ether. Yield was 1.8
g (49%). HPLC gave a single peak at 220 nm with a retention time of
9.67 min. Anal. Calcd/found: C, 56.90/56.56; H, 7.56/7.64; N, 16.59/
EXPERIMENTAL SECTION
■
Materials. Methacryloyl chloride (≥97%), 1-amino-propan-2-ol,
2,2′-azobis(2-methylbutyronitrile) (AIBN, ≥ 98%), 4,4′-azobis(4-
cyanopentanoic acid) (INI-(COOH)2, ≥ 98%), 4,5-dihydro-thiazole-
2-thiol (TT), L-alanine methyl ester hydrochloride (≥99%), L-leucine
methyl ester hydrochloride (≥99%), L-valine methylester hydro-
chloride (≥99%), 3-amino-1-propyne (≥98%), 3-aminopropyl bro-
mide hydrobromide (≥98%), N-Boc-1,2-diaminoethane (≥98%), Boc-
hydrazide (≥98%), bis(thiobenzoyl) disulfide (≥90%), 2-cyanopro-
pan-2-yl benzodithioate (≥97%), 4-cyano-4-(thiobenzoylthio)-
pentanoic acid (CTA-COOH, (≥97%)), 4-(dimethylamino)pyridine
(DMAP, (≥99%)), N,N′-dicyclohexylcarbodiimide (DCC, (≥99%)),
N-ethyl-N′-(3-(dimethylamino)propyl)carbodiimide hydrochloride
(EDC, (≥98%)), 2,4,6-trinitrobenzenesulfonic acid (TNBSA), N,N-
diisopropylethylamine (DIPEA, ≥ 99%), dimethyl sulfoxide (DMSO,
≥ 99%), tert-butyl alcohol (t-BuOH, ≥ 99%), dichloromethane
(DCM) and silica gel 60 were purchased from Sigma-Aldrich, Czech
Republic. All other chemicals and solvents were of analytical grade.
The solvents were dried and purified by conventional procedures and
distilled before use.
1
16.45. H NMR 300 MHz (DMSO, 296 K), ppm: 0.84−0.89 (m,
12H), 1.69 (s, 6H), 1.97−2.03 (m, 4H), 2.17−2.20 (m, 4H), 2.28−
2.34 (m, 2H), 3.62 (s, 6H), 4.13−4.19 (m, 2H), 8.29−8.31 (d, 2H).
ESI−MS: m/z = 529.00 (M−Na)+.
The procedure described above for the synthesis of azo initiator
INI-(Val-OMe)2 was used for the synthesis of all other azo initiators
containing various functional groups (leucyl methyl ester, azide,
propargyl, thiazolidin-2-thione, -NH-Boc and -NHNH-Boc).
Synthesis of 4,4′-Azobis(4-cyanopentanoylleucine methyl ester)
(INI-(Leu-OMe)2). The INI-(Leu-OMe)2 was prepared by the reaction
of INI-(COOH)2 (2.0 g; 7.14 mmol) with Leu-OMe.HCl (2.85 g, 15.7
mmol) in the presence of DIPEA (2.82 mL, 16.4 mmol) and EDC
(3.61 g; 18.8 mmol). Yield was 1.4 g (37%). HPLC analysis gave a
single peak at 220 nm with a retention time of 10.19 min. Anal: Calcd/
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1
found: C, 58.41/58.5; H, 7.92/8.34; N, 15.72/15.87. H NMR 300
MHz (DMSO, 296 K), ppm: 0.82−0.89 (m, 12H), 1.48−1.50 (m,
4H), 1.52−1.61 (m, 8H), 1.68 (s, 6H), 2.12−2.14 (m, 2H), 3.61 (s,
6H), 4.22−4.29 (m, 2H), 8.37−8.39 (d, 2H). ESI−MS: m/z = 556.92
(M−Na)+.
dichloromethane (10 mL) in the presence of DIPEA (0.27 mL, 1.57
mmol) and CTA-COOH (0.4 g, 1.43 mmol) was added to the
solution. Then EDC (0.39 g, 2.05 mmol) was added in several
portions and reaction mixture was stirred at room temperature for 3 h.
Reaction mixture was extracted 3 times with distilled water (15 mL)
and with 2 wt % solution of NaHCO3 in water. Organic layer was
dried with Na2SO4. Pure product was obtained after purification on
Silicagel column using ethyl acetate as a mobile phase. The red colored
fraction was separated and ethyl acetate was evaporated. The red oily
product was obtained. Yield: 0.34 g (47%). HPLC analysis gave a
single peak at 305 nm with a retention time of 9.26 min. 1H NMR 300
MHz (DMSO, 296 K) ppm: 1.19−1.94 (m, 3H), 1.83 (s, 3H), 2.33−
2.35 (m, 2H), 2.37−2.40 (m, 2H), 3.53 (s, 3H), 4.17−4.19 (m, 1H),
7.40−7.44 (m, 4H), 7.60−7.62 (m, 2H), 7.82−7.84 (m, 4H), 8.41 (m,
1H); ESI−MS: m/z = 387.00 (M−Na)+.
The procedure described above for the synthesis of chain transfer
agent CTA-Ala-OMe was used for the synthesis of all other CTAs
containing various functional groups (azide, propargyl, -NH-Boc and
−NHNH-Boc). The structures of synthesized azo initiators and chain
transfer agents are shown in the Supporting Information (Figure 1S).
Synthesis of Dithiobenzoic Acid 3-(3-Azidopropylcarbamoyl)-1-
cyano-1-methylpropyl Ester (CTA-N3). The CTA-N3 was prepared by
the reaction of CTA-COOH (0.2 g, 0.72 mmol) with 3-azido-
propylamine (0.079 g, 0.79 mmol) in the presence of EDC (0.180 g,
0.95 mmol). Pure product was obtained after purification on silica gel
column using ethyl acetate as a mobile phase. The red colored fraction
was separated and ethyl acetate was evaporated. The red oily product
was obtained. Yield: 0.34 g (47%). HPLC analysis gave a single peak at
305 nm with a retention time of 10.12 min. 1H NMR 300 MHz
(DMSO, 296 K), ppm: 1.54−1.59 (m, 2H), 1.83 (s, 3H), 2.30−2.32
(m, 2H), 2.34−2.40 (m, 2H), 3.02−3.05 (m, 2H), 3.21−3.26 (m, 2H),
7.40−7.44 (m, 4H), 7.60−7.62 (m, 2H), 7.79−7.84 (m, 4H), 8.00 (t,
1H). ESI−MS: m/z = 384.08 (M−Na)+.
Synthesis of N-(3-Azidopropyl)-4-[3-(3-azidopropylcarbamoyl)-1-
cyano-1-methylpropylazo]-4-cyano-4-methylbutyramide (INI-
(N3)2). 3-Azido-propylamine was synthesized by refluxing of 3-
aminopropyl bromide hydrobromide with sodium azide as described
earlier.35
The INI-(N3)2 was prepared by reaction of INI-(COOH)2 (1.0 g,
3.61 mmol) with 3-azido-propylamine (0.8 g, 7.9 mmol) in the
presence of EDC (1.8 g, 9.2 mmol). Yield: 1.1 g (52%). HPLC
analysis: two symmetrical peaks at 220 nm with retention times of 9.1
and 9.16 min. Anal.: Calcd/found: C, 48.64/49.04; H, 6.35/5.87; N,
1
37.81/37.53%. H NMR 300 MHz (DMSO, 296 K), ppm: 1.64−1.68
(m,6H), 2.07−2.09 (m,4H), 2.19−2.21 (m, 4H), 2.24−2.28 (m, 4H),
3.06−3.12 (m, 4H), 3.32−3.36 (m, 4H), 8.03(t, 2H). ESI−MS: m/z =
466.67 (M−Na)+.
Synthesis of 4-Cyano-4-(1-cyano-3-ethynylcarbamoyl-1-methyl-
propylazo)-N-ethynyl-4-methylbutyramide (INI-(propargyl)2). The
INI-(propargyl)2 was prepared by the reaction of INI-(COOH)2 (4 g,
0.0143 mol) with 3-amino-1-propyne (1.92 mL, 0.030 mol) in the
presence of EDC (1.8 g, 9.2 mmol). The final product was obtained by
recrystallization from acetone/diethyl ether (1:1). Yield: 3.5 g (69%).
HPLC analysis gave two symmetrical peaks at 220 nm with retention
times of 8.37 and 8.46 min. Anal.: Calcd/found C, 61.00/60.96; H,
6.26/5.96; N, 23.71/23.48. 1H NMR 300 MHz (DMSO, 296 K), ppm:
1.68 (s, 6H), 2.29 (s, 2H), 2.48 (t, 4H), 3.09 (t, 4H), 3.85 (m, 4H),
8.42−8.46 (m, 2H). ESI−MS: m/z = 376.92 (M−Na)+.
Synthesis of N′-[4-Cyano-4-(1-cyano-1-methyl-3-methylcarba-
moylpropylazo)-4-methylbutyryl]hydrazinecarboxylic acid tert-
Butyl Ester (INI-(NHNH-Boc)2). The INI-(NHNH-Boc)2 was prepared
by the reaction of INI-(COOH)2 (1.0 g, 3.61 mmol) with Boc-
hydrazide (1.05 g, 7.9 mmol) in the presence of EDC (1.67 g, 8.7
mmol). The final product was obtained by recrystallization from ethyl
acetate/diethyl ether (1:1). Yield: 1.1 g (61%). HPLC analysis gave
two peaks at 220 nm with retention times of 9.12 and 9.22 min. Anal.:
Calcd/found: C, 51.96/51.48; H, 7.13/7.02; N, 22.03/20.44. 1H NMR
300 MHz (DMSO, 296 K), ppm: 1.38 (s, 18H), 1.68 (s, 6H), 2.20−
2.25 (m, 4H), 2.25−2.32 (m, 4H), 8.73 (s, 2H), 9.67 (s, 2H). ESI−
MS: m/z = 531.00 (M-Na)+.
Synthesis of Dithiobenzoic Acid 1-Cyano-1-methyl-3-prop-2-
ynylcarbamoylpropyl Ester (CTA-propargyl). CTA-propargyl was
prepared by the reaction of CTA-COOH (0.4 g, 1.43 mmol) with 3-
amino-1-propyne (0.087 g, 1.58 mmol) in the presence of EDC (0.392
g, 2.05 mmol). Pure product was obtained after crystallization from
mixture ethyl acetate/hexane (1/1 v/v). Yield: 0.18 g (40%). HPLC
analysis gave a single peak at 305 nm with a retention time of 11.22
1
min. H NMR 300 MHz (DMSO, 296 K), ppm:1.90 (s, 3H), 2.43−
Synthesis of (2-{4-[3-(2-tert-Butoxycarbonylaminoethylcarbamo-
yl)-1-cyano-1-methylpropylazo]-4-cyano-4-methylbutyrylamino}-
ethyl)carbamic Acid tert-Butyl Ester (INI-(NH-Boc)2). The INI-(NH-
Boc)2 was prepared by the reaction of INI-(COOH)2 (0.8 g, 2.85
mmol) with N-Boc-1,2-diaminoethane (1.0 g, 6.2 mmol) in the
presence of EDC (1.3 g, 6.85 mmol). The final product was obtained
by recrystallization from ethyl acetate/diethyl ether (1:1). Yield: 1.0 g
(62%). HPLC analysis gave two peaks at 220 nm with retention times
of 9.26 and 9.36 min. Anal.: Calcd/found: C, 55.30/55.45; H, 7.85/
7.66; N, 19.84/19.51. 1H NMR 300 MHz (DMSO, 296 K), ppm: 1.36
(s, 18, H), 1.67 (s, 6H), 2.17−2.19 (m, 4H), 2.24−2.28 (m, 4H),
2.95−2.99 (m, 4H), 3.02−3.05 (m, 4H), 6.76−6.80 (t, 2H), 7.96−8.00
(t, 2H). ESI−MS: m/z = 587.00 (M−Na)+.
Synthesis of Dithiobenzoic Acid 1-Cyano-1-methyl-4-oxo-4-(2-
thioxothiazolidin-3-yl)butyl Ester (CTA-TT). The CTA-TT was
synthesized according to the literature.36 A solution of INI-(TT)2
(0.95 g, 1.96 mmol) and bis(thiobenzoyl) disulfide (0.5 g, 1.63 mmol)
in ethyl acetate (250 mL) was heated at 80 °C for 6 h. After cooling,
the reaction mixture was concentrated and purified by column
chromatography (silica gel 60; 200 μm) using ethyl acetate/hexane
(1/1, v/v) as eluent. Yield: 0.44 g (56%). HPLC analysis gave a single
peak at 305 nm with a retention time of 9.96 min. 1H NMR 300 MHz
(DMSO, 296 K), ppm: 1.84 (s, 3H), 2.45−2.51 (m, 2H), 3.30−3.31
(m, 2H), 3.42−3.44 (m, 2H), 4.42−4.44 (m, 2H), 7.41−7.44 (m, 4H),
7.60−7.62 (m, 2H), 7.81−7.83 (m, 4H). ESI−MS: m/z = 380.92 (M−
Na)+.
2.45 (m, 2H), 2.46−2.49 (m, 2H), 3.10−3.11 (t, 1H), 3.85−3.88 (m,
2H), 7.48−7.53 (m, 4H), 7.66−7.71 (m, 2H), 7.89−7.92 (m, 4H),
8.46−8.50 (t, 1H). ESI−MS: m/z = 339.00 (M−Na)+.
Synthesis of Dithiobenzoic Acid 3-(2-tert-Butoxycarbonylami-
noethylcarbamoyl)-1-cyano-1-methylpropyl Ester (CTA-NH-Boc).
The CTA-NH-Boc was prepared by the reaction of CTA-COOH
(0.37 g, 1.34 mmol) with N-Boc-1,2-diaminoethane (0.24 g, 1.47
mmol) in the presence EDC (0.31 g, 1.6 mmol). Pure product was
obtained after purification on silica gel column using ethyl acetate/
hexane (3/1 v/v) as a mobile phase. Yield: 0.28 g (48%). HPLC
analysis gave a single peak at 305 nm with a retention time of 10.08
min. 1H NMR 300 MHz (DMSO, 296 K), ppm: 1.37 (s, 9H), 1.90 (s,
3H), 2.35−2.40 (m, 2H), 2.41−2.46 (m, 2H), 2.96−3.00 (m, 2H),
3.04−3.08 (m, 2H), 6.77−6.80 (t, 1H), 7.48−7.53 (m, 4H), 7.66−7.85
(m, 2H), 7.89−7.92 (m, 4H), 8.03 (t, 1H). ESI−MS: m/z = 444.00
(M−Na)+.
Synthesis of Dithiobenzoic Acid 4-(N′-tert-Butoxycarbonylhydra-
zino)-1-cyano-1-methyl-4-oxobutyl Ester (CTA-NHNH-Boc). The
CTA-NHNH-Boc was prepared by the reaction of CTA-COOH
(0.4 g, 1.43 mmol) with Boc-hydrazide (0.21 g, 1.57 mmol) in the
presence of EDC (0.36 g, 1.89 mmol). Pure product was obtained
after purification on silica gel column using ethyl acetate/hexane (5/1
v/v) as a mobile phase. Yield: 0.31 g (55%). HPLC analysis gave a
1
single peak at 305 nm with a retention time of 10.08 min. H NMR
300 MHz (DMSO, 296 K), ppm: 1.39 (s, 9H), 1.90 (s, 3H), 2.38−
2.41 (m, 2H), 2.43−2.44 (m, 2H), 7.48−7.53 (m, 4H), 7.66−7.71 (m,
2H), 7.88−7.94 (m, 4H), 8.76 (t, 1H), 9.71 (t, 1H). ESI−MS: m/z =
415.90 (M−Na)+.
S y n t h e s i s o f 2 - ( 4 - C y a n o - 4 - m e t h y l - 4 -
thiobezoylsulfanylbutyrylamino)propionic Acid Methyl Ester (CTA-
Ala-OMe). The Ala-OMe·HCl (0.22 g, 1.57 mmol) was dissolved in
C
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Table 1. Characteristics of pHPMAs Synthesized by RAFT Polymerization of HPMA: Influence of the Ratio of HPMA:CTA on
a
the Structure of the α-End Chain Group
HPMA:CTA:INI, mol Ala:Val monomer mixture, mol % Ala:Val polymer composition, mol % Mn Ala/Val, g/mol Mn SEC, g/mol
Đ
conversion, %
100:2:1
200:2:1
300:2:1
400:2:1
500:2:1
600:2:1
50:50
50:50
50:50
50:50
50:50
50:50
75.6:24.4
73.6:26.4
73.1:26.9
70.7:29.3
72.0:28.0
70.5:29.5
7 900
13 600
20 600
27 800
33 600
42 100
8 300
15 300
23 200
29 900
36 600
47 700
1.11
1.10
1.07
1.12
1.04
1.03
66.1
68.8
73.8
57.3
80.7
73.4
a
CTA-Ala-OMe:INI-(Val-OMe)2 = 2:1, t-BuOH, 70 °C, 16 h.
Synthesis and Characterization of Polymers. The RAFT
(CTA-Ala-OMe) and initiators 4,4′-azobis(4-cyanopentanoyl-
valine methyl ester or leucine methyl ester) (INI-(Val-OMe)2
or INI-(Leu-OMe)2) were synthesized and used for RAFT
polymerization of HPMA. Amino acids Val, Leu, and Ala were
used as effective labels of INI a CTA residues incorporated into
polymer chain at its α-end. Amino acid analysis was found to be
sufficiently sensitive method for the analysis of the α-end
structure and functionality of the polymer chain. The influence
of the HPMA:CTA-Ala-OMe ratio on the structure of the α-
end chain group was studied using a constant 2:1 ratio of CTA-
Ala-OMe:INI-(Val-OMe)2 which was found to be optimal to
achieve high yield of polymerization and narrow molecular
weight distribution. The results are summarized in Table 1, and
the SEC record of polymers is presented in Figure 1. These
polymerization of HPMA was studied in t-BuOH at temperature range
60−80 °C for 2−16 h. The ratio of monomer (M):CTA was in a range
from 100:1 to 600:1 and the ratio of CTA:INI was in the range from
2:1 to 2:0.3. Example of polymerization (ratio M:CTA:INI =
200:2:1):HPMA (0.3 g, 2.1 mmol), CTA-Ala-OMe (7.64 mg, 2.09
× 10−2 mmol) and INI-(Val-OMe)2 (5.3 mg, 1.05 × 10−2 mmol) were
dissolved in t-BuOH (2.33 mL). The solution was inserted into the
ampule and bubbled with argon for 10 min. Ampule was sealed and
polymerization was carried out at 70 °C for 16 h. Polymer was isolated
by precipitation into the mixture acetone/diethyl ether (3:1), filtered
off and dried in vacuum. Yield: 0.259 g (84.2%). The ω-end
dithiobenzoate group (DTB) was removed by a method described by
Perrier.11
Synthesis of Polymer with Thiazolidine-2-thione Group at the
Chain α-End. Two HPMA polymers containing the TT reactive group
at the chain α-end were prepared by RAFT polymerization. The ratio
of M:CTA-TT:INI = 400:2:1 was used. To compare the influence of
azo initiator, polymerizations were initiated with INI-(TT)2 or INI-
(COOH)2.
HPMA (0.15 g, 1.05 mmol), CTA-TT (1.99 mg, 5.24 × 10−6
mmol) and INI-(TT)2 (1.26 mg, 2.62 × 10−6 mmol) were dissolved in
t-BuOH (1.16 mL). The solution was inserted into the ampule and
bubbled with argon for 10 min. Ampule was sealed and polymerization
was carried out at 70 °C for 16 h. Polymer was isolated by
precipitation into the mixture acetone diethyl ether (3:1), filtered off
and dried in vacuum. Yield: 0.043 g (29%).
HPMA (0.15 g, 1.05 mmol), CTA-TT (1.99 mg, 5.24 × 10−6
mmol) and INI-(COOH)2 (0.58 mg, 2.62 × 10−6 mmol) were
dissolved in t-BuOH (1.16 mL). The solution was inserted into the
ampule and bubbled with argon for 10 min. The ampule was sealed
and polymerization was carried out at 70 °C for 16 h. Polymer was
isolated by precipitation into the mixture acetone/diethyl ether (3:1),
filtered off and dried in vacuum. Yield: 0.023 g (15%). The DTB ω-
end group was removed by the Perrier method using AIBN.11 The
polymers were characterized by molecular weight, molecular weight
distribution, and content of TT groups.
Figure 1. SEC chromatograms of pHPMAs prepared by RAFT
polymerization: Influence of the ratio of HPMA:CTA on the
molecular weight. CTA-Ala-OMe:INI-(Val-OMe)2 = 2:1, t-BuOH,
70 °C, 16 h.
Synthesis of Polymer with Amino Group at the Chain α-End.
Two HPMA polymers containing the amino group at the chain α-end
were prepared by the same method as described above in the presence
of CTA-NH-Boc/INI-(COOH)2 and CTA-NH-Boc/INI-(NH-Boc)2.
The Boc protecting group was removed by TFA and the content of
amino group was determined by TNBSA.
results show that the ratio of HPMA:CTA-Ala-OMe controls
the molecular weight of the resulting pHPMA, which is in
agreement with the theory of RAFT polymerization.1,37,38
Molecular weight distributions (polydispersities, Đ) of all the
polymers were narrow, with Đ equal or below 1.22 (see Table 1
− 5). The amino acid analysis of pHPMA proved that 70 to
75% of the pHPMA chains were at the α-end terminating in a
residue coming from CTA (containing Ala) and that 25 to 30%
of pHPMA chains were at the α-end terminating in a residue
coming from INI (containing Val).
The number-average molecular weight Mn determined by
SEC is in close agreement with the Mn calculated from the
content of Ala and Val in the pHPMA.
To verify whether the structure of the INI and the CTA:INI
ratio influences the RAFT polymerization and composition of
the α-end chain group of pHPMAs, the properties of the
pHPMAs obtained by RAFT polymerization using a combina-
RESULTS AND DISCUSSION
■
The synthesis of semitelechelic polymers based on HPMA with
well-defined functional groups at the α-end of the chain by
RAFT polymerization was studied. t-BuOH was selected as the
polymerization solvent accommodating best the requirements
for RAFT polymerization of HPMA. We focused on the effect
of the structure of the azo initiator, composition of initiation
system and composition of polymerization mixture on the
structure and functionality of the α-end of the polymer chain of
pHPMA during RAFT polymerization. The chain transfer agent
4-cyano-4-(thiobenzoylthio)pentanoylalanine methyl ester
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Table 2. Characteristics of pHPMAs Synthesized by RAFT Polymerization of HPMA: Influence of the Ratio of CTA-Ala-
a
OMe:INI-(Val-OMe)2 on the Structure of the α-End Chain Group
M:CTA:INI, mol Ala:Val monomer mixture, mol % Ala:Val polymer composition, mol % Mn Ala/Val, g/mol Mn SEC, g/mol
Đ
conversion, %
200:2:1
50:50
72.4:27.6
76.6:23.4
82.2:17.8
88.5:11.5
15 200
14 800
13 900
10 900
15 600
15 000
14 600
11 200
1.22
1.19
1.13
1.10
84.2
79.3
70.9
49.4
200:2:0.8
200:2:0.5
200:2:0.3
55.6:44.4
66.7:33.3
76.9:23.1
a
HPMA:CTA-Ala-OMe = 200:2, t-BuOH, 70 °C, 16 h.
Table 3. Characteristics of pHPMAs Synthesized by RAFT Polymerization of HPMA: Influence of the Ratio of CTA-Ala-
a
OMe:INI-(Leu-OMe)2 on the Structure of the α-End Chain Group
M:CTA:INI, mol Ala:Leu monomer mixture, mol % Ala:Leu polymer composition, mol % Mn Ala/Leu, g/mol Mn SEC, g/mol
Đ
conversion, %
200:2:1
50:50
67.8:32.2
72.5:27.5
78.5:21.5
86.7:13.3
16 200
14 700
14 400
13 000
17 100
16 300
16 000
13 000
1.16
1.14
1.09
1.05
87.8
85.5
74.7
54.6
200:2:0.8
200:2:0.5
200:2:0.3
55.6:44.4
66.7:33.3
76.9:23.1
a
HPMA:CTA-Ala-OMe = 200:2, t-BuOH, 70 °C, 16 h.
Table 4. Characteristics of pHPMAs Prepared by RAFT Polymerization of HPMA: Effect of Temperature on the Structure of
a
the α-End Chain Groups of pHPMAs
temperature, °C Ala:Val monomer mixture, mol % Ala:Val polymer composition, mol % Mn Ala/Val, g/mol Mn SEC, g/mol
Đ
conversion, %
60
70
80
66.7:33.3
66.7:33.3
66.7:33.3
89.6:10.4
82.2:17.8
79.0:21.0
13 600
13 900
10 200
15 160
14 600
11 040
1.12
1.13
1.17
58.2
70.9
46.8
a
HPMA:CTA-Ala-OMe = 200:2, CTA-Ala-OMe:INI-(Val-OMe)2 = 2:0.5, t-BuOH, 16 h.
Table 5. Characteristics of pHPMAs Prepared by RAFT Polymerization of HPMA: Influence of the Polymerization Time on the
a
Structure of the α-End Chain Group
polymerization time, h Ala:Val monomer mixture, mol % Ala:Val polymer composition mol % Mn Ala/Val, g/mol Mn SEC, g/mol
Đ
conversion, %
2
3
66.7:33.3
66.7:33.3
66.7:33.3
66.7:33.3
66.7:33.3
95.5:4.5
91.7:8.3
88.2:11.8
86.5:13.5
82.2:17.8
4 800
6 100
4 710
7 040
1.08
1.04
1.10
1.10
1.13
7.2
24.1
39.7
52.3
70.9
4
8 700
9 470
6
11 300
13 900
12 940
14 600
16
a
HPMA:CTA-Ala-OMe = 200:2, CTA-Ala-OMe:INI-(Val-OMe)2 = 2:0.5, t-BuOH, 70 °C.
tion of CTA-Ala-OMe/INI-(Val-OMe)2 or CTA-Ala-OMe/
INI-(Leu-OMe)2 was studied; the results are presented in
Table 2 and Table 3.
RAFT polymerization, rising from 10.4 (60 °C) to 21.0% (80
°C). This increase can be attributed to the higher concentration
of radicals resulting from a higher rate of decomposition of the
azo initiator at higher temperatures. The direct relationship
between the conversion and temperature culminates at the
polymerization temperature 70 °C; an increase in the
polymerization temperature to 80 °C led to a decrease in the
conversion. This limit may be attributable to increased rates of
termination reactions or the instability of the CTA at higher
temperatures. The influence of the polymerization time on
conversion and the structure of the α-end chain groups of
pHPMAs prepared at constant ratios of HPMA:CTA-Ala-OMe
and CTA:INI-(Val-OMe)2 is shown in Table 5.
The results summarized in Table 5 show a significant
influence of the polymerization time and conversion on the
structures of the α-end chain groups of pHPMAs. After 2 h of
polymerization (conversion 7%) 95% of pHPMA chains
contained a CTA residue (Ala) at the α-end, while at much
higher conversion achieved after 16 h of polymerization
(conversion 71%) the content of CTA residues (detected as
Ala) decreased to 82.2%. The Mn increased with increasing
polymerization time and conversion and when plotted against
conversion, the obtained linear dependence indicated that the
The results summarized in Table 2 and Table 3 demonstrate
that the use of azo initiators with different structures (INI-(Val-
OMe)2 or INI-(Leu-OMe)2) under the same conditions of
RAFT polymerization of HPMA give pHPMAs with com-
parable characteristics. Decreasing the ratio of CTA:INI from
2:1 to 2:0.3 decreases the percentage of pHPMA α-end chains
terminated in an INI residue containing Val or Leu (decreasing
from 32.2 to 11.5%) but also simultaneously decreases the
conversion of polymerization. The constant ratio of HPMA:
CTA-Ala-OMe controls the molecular weight of the pHPMA.
The small decrease in the molecular weight of the pHPMAs can
be attributed to the decrease in conversion.2,39 The Mn
determined by SEC is in good agreement with the Mn
calculated from the content of the Ala and Val or Ala and
Leu amino acids in the pHPMAs.
The effect of temperature on the structure of the α-end chain
groups of pHPMAs at a constant ratio of HPMA:CTA-Ala-
OMe and CTA-Ala-OMe:INI-(Val-OMe)2 is shown in Table 4.
The percentage of the α-end chain groups terminating with
INI (Val residues) increased with increasing temperature of the
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Scheme 1. Modified Scheme of CTA Activation/Initialization Process
Figure 2. ESI−MS spectrum of the polymerization mixture HPMA:CTA-Ala-OMe = 100:2, CTA-Ala-OMe:INI-(Leu-OMe)2 = 2:1, t-BuOH,
polymerization time 50 min at 70 °C.
CTA provides adequate control over the polymerization
process. Mn determined by SEC was again in good agreement
with the Mn calculated from the content of Ala and Val in the
pHPMA.
The obtained results can be explained by the CTA
activation/initialization process, which was described in detail
by Lowe.7 The modified scheme of the CTA activation/
initialization process adopted from Lowe’s paper is shown in
Scheme 1.
radical, or it may add to the RAFT agent CTA-Ala-OMe. The
B(Leu)-M• radical can either add a new monomer and start to
propagate or add to the RAFT agent CTA-Ala-OMe. Given the
inherently high chain transfer constants of most RAFT agents,
it is unlikely that more than a few monomers will add to the
generated radical before adding to the RAFT agent CTA-Ala-
OMe. Assuming that this forward fragmentation is favored, a
new RAFT agent CTA-M-B(Leu) is formed, as well as the
radical R(Ala)•. Provided that R(Ala)• is a good reinitiating
species, R(Ala)• will add a new monomer and initiate the
growth of the polymer chain R(Ala)-M•.
In the first step, the monomer (M) molecule adds to the
primary radical B(Leu)• formed by decomposition of the azo
initiator INI-(Leu-OMe)2 to form the B(Leu)-M• adduct
F
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Figure 3. ESI−MS spectrum of the polymerization mixture HPMA:CTA-Ala-OMe = 100:2, CTA-Ala-OMe:INI-(Leu-OMe)2 = 2:1, t-BuOH, after
50 min at 70 °C.
The second option for the primary radical B(Leu)• that
forms from the azo initiator INI-(Leu-OMe)2 is its direct
addition to the RAFT agent CTA-Ala-OMe to yield the
intermediate radical. This reaction is reversible, and fragmenta-
tion in the forward direction is the preferred pathway. To favor
fragmentation, the formation of the new RAFT agent CTA-
Leu-OMe and the radical R(Ala)• occurs, and ideally, R(Ala)•
will again add the first monomer unit to initiate the growth of a
new polymer chain R(Ala)-M•. This CTA activation/
initialization process, with its many reversible steps, is fairly
complex, and the formation of new RAFT agents CTA-M-Leu-
OMe and CTA-Leu-OMe, which differ in their structures from
CTA-Ala-OMe, influences the final structure of the α-end of
the resulting polymer chain.
To confirm the formation of either CTA-M-Leu or CTA-
Leu-OMe during the CTA activation/initiation process, we
performed the RAFT polymerization of HPMA in the presence
of CTA-Ala-OMe initiated with INI-(Leu-OMe)2 (HPMA:C-
TA-Ala-OMe = 100:2, CTA-Ala-OMe:INI-(Leu-OMe)2 = 2:1,
t-BuOH, 70 °C) for 50 min and then analyzed the
polymerization mixture by mass spectrometry (MS LCQ
Fleet, Thermo Fisher Scientific). In Figure 2, the ESI−MS
spectrum is shown, where the peak with m/z = 386.98
corresponds to [CTA-Ala-OMe + Na]+ and the peak with m/z
= 428.99 corresponds to the newly formed RAFT agent [CTA-
Leu-OMe + Na]+.
1388.20, 1531.26, 1674.32, and 1818.36, which correspond to a
pHPMA with the structure [Ala-OMe-(HPMA)n-DTB+Na]+
where n = 2−10, peaks with m/z = 650.90, 793.86, 936.86,
1079.89, 1222.95, 1366.05, and 1509.06, which correspond to a
pHPMA with the structure [Ala-OMe-(HPMA)n-DTB+H]+
where n = 2−8, and peaks with m/z = 857.07, 1001.10,
1144.20, and 1287.23, which correspond to a pHPMA with the
structure [Leu-OMe-(HPMA)n-DTB+Na]+ where n = 3−6.
On the basis of these results, it appears that the formation of
RAFT agents with different structures during the polymer-
ization can be avoided by using a CTA and an INI with the
same functional group. For this purpose, a series of CTAs and
azo initiators bearing various functional groups (azide,
propargyl, thiazolidin-2-thione, -NH-Boc and -NHNH-Boc)
were synthesized and used for the synthesis of semitelechelic α-
end-functionalized pHPMAs by RAFT. The dependence of the
Mn on the ratio of monomer:CTA for a series of CTA and azo
initiators bearing the same functional groups was measured and
is shown in Figure 4.
Figure 4 shows the dependence of the Mn of semitelechelic
α-end-functionalized pHPMAs on the ratio of monomer:CTA
prepared by RAFT polymerization using CTAs and INIs
bearing the same functional groups. The resulting Mn of the
semitelechelic pHPMA depends not only on the ratio of
monomer:CTA but also on the structure of the functional
group of the CTA and INI when the same functional group was
used for both, INI and CTA. The combination CTA-
COOH:INI-(COOH)2 (i.e., both agents have COOH func-
The ESI−MS spectrum presented in Figure 3 shows the
peaks with m/z = 673.00, 816.00, 959.05, 1102.10, 1245.16,
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functionality values in the case of semitelechelic polymers with
TT end groups.
CONCLUSION
■
A number of new azo initiators and RAFT chain transfer agents
containing various functional groups were synthesized.
RAFT polymerization of HPMA using CTAs and INIs
containing various functional groups allows for reasonable
control of the molecular weights of the resulting polymers and
generates polymers with narrow molecular weight distributions
that terminate in functional groups that are useful for future
syntheses of well-defined polymer drug carriers or for
preparation of polymer-coated drug and gene delivery vectors.
The temperature, composition of the initiation/chain transfer
system and conversion of the reaction significantly influence the
functionality of the α-end of the resulting polymer chain. The
use of RAFT polymerization controlled by CTAs and INIs
containing the same functional group enables the synthesis of
semitelechelic polymers with very narrow molecular weight
distributions and functional groups situated at the α-end of the
polymer chain with functionality values very close to 1. Our
recent results show that the conclusions mentioned above are
fully applicable also to the synthesis of a significant number of
HPMA copolymers.30,31 This finding provides an opportunity
for the design and synthesis of new well-defined HPMA
copolymer carriers of various structures and architectures
(linear, block and multiblock, grafted, micellar, star architec-
tures) that are suitable for use as drug and gene delivery carriers
or polymer−protein/glycoprotein conjugates.
Figure 4. Dependence of the Mn on the ratio of HPMA:CTA. CTAs
and INIs with the same functional groups were used. CTA:INI = 2:1,
□
t-BuOH, 70 °C, 16 h. Key: ( ) CTA/AIBN; (Δ) CTA-COOH/INI-
■
◇
▲
(COOH)2; ( ) CTA-NH-Boc/INI-(NH-Boc)2; ( ) CTA-TT/INI-
●
(TT)2; ( ) CTA-propargyl/INI-(propargyl)2; ( ) CTA-NHNH-
○
Boc/INI-(NHNH-Boc)2; ( ) CTA-N3/INI-(N3)2.
tional group) gave a semitelechelic pHPMA with a relatively
low Mn. The combination of CTAs and INIs with Ala-OMe,
-NH-Boc, -NHNH-Boc, -azide, and TT functional groups
enables the synthesis of semitelechelic pHPMAs with well-
defined structures of their α-end groups over a broad range of
molecular weights that depend on the monomer:CTA ratio.
The results summarized in Table 6 show the comparison of a
pHPMA prepared by RAFT polymerization in the presence of a
CTA and an INI bearing the same TT functional group and a
pHPMA prepared in the presence of CTA-TT and INI-
(COOH)2 groups, i.e., CTA and INI with diverse functional
groups.
The functionality F, calculated as the ratio of Mn(SEC)/
Mn(TT), for the pHPMA prepared by RAFT polymerization
using CTA-TT/INI-(COOH)2 (diverse groups) was 0.72,
while the functionality of the pHPMA prepared by RAFT
polymerization using CTA-TT/INI-(TT)2 (for both the same
TT group) was 0.96. A functionality value close to one means
that all of the polymer chains contain in average one TT
reactive group at the α-end. Similar results were obtained for
the pHPMAs prepared by RAFT polymerization using CTA-
NH-Boc/INI-(COOH)2 and CTA-NH-Boc/INI-(NH-Boc)2,
which demonstrates the advantages of using CTAs and INIs
containing the same functional groups. In this case, the absolute
value of functionality was affected by error of indirect
determination of NH2 groups with TNBSA method (see
characterization methods above). Important is the difference
between values of CTA-NH-Boc/INI-(COOH)2 and CTA-
NH-Boc/INI-(NH-Boc)2, which is very similar to difference of
ASSOCIATED CONTENT
■
S
* Supporting Information
Structures of synthesized azo initiators and chain transfer
agents (Figure 1S). This material is available free of charge via
the Internet at http://pubs.acs.org/
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: subr@imc.cas.cz. Telephone: +420 296 809 389. Fax:
+420 296 809 410.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Grant Agency of Czech
Republic (Grant No. P301/12/1254) and by the Ministry of
Education, Youth and Sports of the Czech Republic (Grant No.
CZ.1.07/2.3.00/30.0029).
Table 6. Characteristics of Polymers Prepared by RAFT Polymerization of HPMA: Influence of the Structure of CTA and INI
a
on the Structure of the α-End Chain Group of the pHPMA
CTA/INI
Mn SEC, g/mol
Mn TT, g/mol
conversion, %
F TT
CTA-TT/INI-(COOH)2
CTA-TT/INI-(TT)2
CTA/INI
12 260
17 100
15.0
0.72
0.96
14 350
14 880
29.0
Mn SEC, g/mol
Mn NH2, g/mol
conversion, %
F NH2
CTA-NH-Boc/INI-(COOH)2
CTA-NH-Boc/INI-(NH-Boc)2
12 400
14 400
14 000
12 300
27.0
30.0
0.89
1.18
a
HPMA:CTA:INI = 400:2:1, t-BuOH, 70 °C, 16 h.
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(35) Park, K. D.; Morieux, P.; Salomeu, C.; Cotten, S. W.; Reamtong,
̈
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dx.doi.org/10.1021/ma400042u | Macromolecules XXXX, XXX, XXX−XXX