Radical addition-coupling polymerization with various nitroso compounds

Article abstract of DOI:10.1002/pola.27061

Four nitroso esters were prepared by oxidation of 4,4-dimethyl dihydro-1,3-oxazine or 4,4-dimethyl-2-oxazoline with two equiv of m-chloroperoxybenzoic acid. All of them can be applied in radical addition-coupling polymerization to produce periodic polymer together with introduction of ester group at side chain. Compared with 2-methyl-2- nitrosopropane, 2-nitroso-2-methyl-4-acetoxypentane and 2-methyl-2-nitrosopropyl hexanoate present good stability at high temperature up to 70°C and can result polymer with high molecular weight.

Full text of DOI:10.1002/pola.27061

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Radical Addition-Coupling Polymerization with Various Nitroso
Compounds
Junjie Li, Qi Wang
Key Laboratory of Macromolecular Synthesis and Functionalization (Ministry of Education), Department of Polymer Science &
Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China
Correspondence to: Q. Wang (E-mail: wangq@zju.edu.cn)
Received 26 July 2013; accepted 29 November 2013; published online 20 December 2013
DOI: 10.1002/pola.27061
ABSTRACT: Four nitroso esters were prepared by oxidation of
4,4-dimethyl dihydro-1,3-oxazine or 4,4-dimethyl-2-oxazoline
with two equiv of m-chloroperoxybenzoic acid. All of them can
be applied in radical addition-coupling polymerization to pro-
duce periodic polymer together with introduction of ester
high temperature up to 70 ꢀC and can result polymer with high
C
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molecular weight. 2013 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2014, 52, 810–815
group
at
side
chain.
Compared
with
2-methyl-2-
KEYWORDS: atom transfer radical polymerization; functionaliza-
tion of polymers; nitroso ester; radical addition-coupling poly-
merization; step-growth polymerization
nitrosopropane, 2-nitroso-2-methyl-4-acetoxypentane and 2-
methyl-2-nitrosopropyl hexanoate present good stability at
nitro compound or photodecompose to carbon-centered radi-
cal and nitric oxide. Methods for the preparation of C-nitroso
compounds have been tentatively reviewed.6,7 For the syn-
thesis of aliphatic nitroso alkanes, oxidation of amine with-
out a-hydrogen under mild condition is an easy method. For
one-pot synthesis of aliphatic nitroso compounds containing
ester group, oxidation of 4,4-dimethyl dihydro-1,3-oxazine or
4,4-dimethyl 2-oxazoline with m-chloroperoxybenzoic acid
(MCPBA) is a convenient method.8,9
INTRODUCTION Radical addition-coupling polymerization
(RACP) has been proposed for synthesis of periodic polymer
with regular monomer sequence.1–3 As shown in Scheme 1,
the polymerization involves consecutive addition of carbon-
centered radical to N@O of C-nitroso compound followed by
cross-coupling of carbon-centered radical and in situ formed
nitroxyl radical, which produces copolymers with high
molecular weight and unimodal molecular weight distribu-
tion from saturated and unsaturated monomers. The key
step of RACP is the formation of intermediate nitroxyl radi-
cal, which is stable enough to take cross-coupling reaction
with relatively active carbon-centered radical based on per-
sistent radical effect.4 The N@O double bond plays an essen-
tial role in RACP. 2-Methyl-2-nitrosopropane (MNP) as a
simple and easily-made or commercially available nitroso
compound is capable of trapping radical efficiently. But its
low boiling point and stability limits its application in RACP
at relative high temperature. Other commericially available
nitroso compounds, such as nitrosobenzene (PhNO), are not
suitable for RACP in the presence of Cu(I)/ligand, because
the reaction of nitrosobenzene with Cu(I) complexes of the
tetradentate ligand tris(2-dimethylaminoethyl)amine (Me6T-
ren) has been reported.5 Therefore, exploring the suitable
nitroso compound is an important issue in RACP.
With respect to various dibromides successfully used in
RACP, only MNP as nitroso compound has been used effi-
ciently in RACP.1–3 As an important component in RACP,
screening on suitable nitroso compound for RACP at differ-
ent temperature in various solvents and for different dibro-
mides will broaden the versatility of this synthetic method.
In this article, we report synthsis of some aliphatic nitroso
esters and their application in RACP. Using nitroso ester, not
only the RACP at various temperatures can be explored but
also the possibility of incorporation of functional group at
the side chain of the polymer can be realized as well.
RESULTS AND DISCUSSION
Synthesis of Nitroso Esters
Two methods are used to prepare aliphatic nitroso ester. As
illustrated in Scheme 2, dihydrooxazine was first prepared
by acid-catalyzed reaction of diol and acetonitrile,10 which
Nitroso compound is not so stable during preparation and
storage, because it is subjected to be further oxidized to
Additional Supporting Information may be found in the online version of this article.
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Cu/PMDETA for 1 h. The polymerization temperature varies
ꢀ
from 25 to 70 C. MNP was also used as the control. Figure 2
is the GPC curves of the polymer prepared by RACP using
MNP as nitroso compound. At 25 ꢀC, polymer with high
number-average molecular weight (Mn) can be obtained. The
Mn slightly increased when the polymerization temperature
ꢀ
increased from 25 to 60 C. When the temperature is further
increased to 70 ꢀC, the Mn greatly decreased. The main reason
is that MNP is volatile and easy to photodecompose to tert-
butyl radical and nitric oxide.12
The nitroso esters prepared in this article were applied in
RACP at different temperatures. Figure 3 is the GPC curves
of polymers prepared by RACP using NMAP at different tem-
peratures. It can be found that as the temperature increased
ꢀ
from 25 to 50 C, the Mn of polymer increased from 3.49 3
104 g/mol to 7.01 3 104 g/mol; and remained almost con-
stant as the temperature increased from 50 to 70 ꢀC. The
polydispersity indexes of the polymers are around 1.5. This
suggests that NMAP is better than MNP for high temperature
polymerization and results polymer with high Mn.
SCHEME 1 Mechanism of radical addition-coupling polymer-
ization and the structure of nitroso esters.
was further oxidized by 2 equiv. of MCPBA.8 The colorless
crystals of the dimer can be obtained upon standing of its
petroleum ether saturated solution at 220 ꢀC. The nitroso
ester, 2-nitroso-2-methyl-4-acetoxypentane (NMAP), was
characterized by NMR, ELEM. ANAL (EA), and mass spectrum
(MS) (see Supporting Information). The nitroso compound
usually forms dimer in both solid state and solution, while
in the 1H NMR spectrum of NMAP, only one group of peaks
were found, which suggested no dimer was formed in the
solution. Combined with the results of EA and MS, it is clear
that the nitroso ester was prepared.
The other three nitroso esters are also applied in RACP and
the results are summarized in Table 1. The monomer conver-
sion for all polymerizations using DMDBT and various
nitroso esters is higher than 95%, which indicates these
nitroso esters are efficient in RACP. For MNPB, the Mn of the
polymer decreased with the polymerization temperature. For
ꢀ
MNPH, the lowest Mn was obtained at 60 C and the highest
one at 70 ꢀC, For MNPU, the Mn of polymer obtained at 25
ꢀC is higher than those obtained at 50–70 ꢀC. Among the
three nitroso esters, MNPH can be applied to produce the
polymer with the highest Mn at various temperatures. The
possible reason is related to the chain length of the carboxyl
unit. The short chain of carboxyl unit, such as MNPB, might
result low stability of MNPB at high temperature, which
leads to low Mn of polymer; the long chain analogs, such as
MNPU, might lead to the low mobility and activity of nitroso
ester, which also results low Mn at various temperatures.
The other method is oxidation of 2-oxazoline with 2 equiv. of
MCPBA,9 which was prepared by acid-catalyzed reaction
between aliphatic carboxylic acid and 2-amino-2-methyl-1-pro-
panol, as illustrated in Scheme 3.11 Several nitroso esters, such
as 2-methyl-2-nitrosopropyl butanoate (MNPB), 2-methyl-2-
nitrosopropyl hexanoate (MNPH), 2-methyl-2-nitrosopropyl
undecanoate (MNPU), were prepared by this method with vari-
ous carboxylic acids. The nitroso esters were characterized by
NMR, ELEM. ANAL, and mass spectrum. The 1H NMR spectrum of
MNPH and the assignment were presented in Figure 1. It is
The 1H NMR spectrum and its assignment of polymer pre-
pared by RACP between DMDBT and MNAP at 70 ꢀC were
illustrated in Figure 4. The ester group of NMAP can be
detected in the spectrum in addition to the ester of dibro-
mide. The molar ratio of two monomer units was calculated
1
clear that both monomer and dimer can be detected in the H
NMR spectra. As the element analysis result fits well with the
theoretic value, it is convinced that nitroso compound instead
of nitro compound is obtained. The mass spectra also proved
the formation of monomer and dimer of this compound.
1
by H NMR spectra (see Supporting Information). For all the
polymers prepared by nitroso esters, the molar ratio of two
units is close to 1:1. This indicates the obtained polymers
have the regular alternative monomer sequence as reported
in our previous paper.1,3 Only theꢀmolar ratio of two units of
polymer prepared by MNP at 70 C is slightly different from
1:1. If MNP volatilized or decomposed, the real monomer
RACP Using Nitroso Esters
Equivalent dimethyl 2,13-dibromotetradecanedioate (DMDBT)
and nitroso compound were polymerized in the presence of
SCHEME 2 Synthesis route of NMAP.
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SCHEME 3 Synthesis route of 2-methyl-2-nitrosopropyl alkyl ester.
ratio [DMDBT]:[MNP] will be higher than 1:1. The RACP fol-
lows the step-growth polymerization mechanism, and the Mn
of the polymer is affected by the monomer ratio. As reported
in our previous paper, the less amount of MNP than dibro-
mide leads to low Mn of polymer.1,3 As a result, the polymer
composition in terms of [DMDBT]:[MNP] is higher than 1:1,
which is evidenced by the 1.06:1 for polymer obtained by
polymer can be obtained at 25 and 70 ꢀC. In this case, the
functional group was introduced at the side chain of polymer
using nitroso ester as monomer.
ꢀ
MNP at 70 C.
The NMAP and MNPH exhibit better performance than
MNP at high temperature in RACP. The reason can be
explained in two aspects. On one hand, the relatively high
boiling point and stability of NMAP and MNPH prevent
them from volatilization and decomposition even at high
temperature, which enable the concentration of nitroso
compound to remain the same as the feed ratio. On the
other hand, the equilibrium between monomer and dimer
shifts to monomer if the temperature increases. More
nitroso compound exists in monomer state in solution at
higher temperature if the monomer of nitroso compound
is stable. As the dibromide is the same, the higher Mn of
polymer prepared by NMAP and MNPH at 70 ꢀC suggests
that they are stable enough to achieve high efficient con-
centration of monomeric nitroso compound at 70 ꢀC.
The other dibromide, 1,4-bis(1-bromoethyl)benzene (BBEB),
was also used in RACP with MNPH. As shown in Table 1,
FIGURE 2 GPC curves of polymer prepared by RACP between
DMDBT and MNP.
FIGURE 3 GPC curves of polymer prepared by RACP between
FIGURE 1 1H NMR spectrum (CDCl3, 400 MHz) of MNPH.
DMDBT and NMAP.
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TABLE 1 Polymer Prepared by Radical Addition-Coupling Polymerization Between Dibromo Monomers and Nitroso Esters Under
Different Temperatures
Polymer
Runa
Nitroso Compound
MNP
Dibromide
DMDBT
Temp.b (ꢀC)
Yield (%)c
Mn (103)d
PDId
[Dibromide]/[Nitroso]e
1
25
50
60
70
25
40
50
60
70
25
50
60
70
25
50
60
70
25
70
25
50
60
70
99.7
98.6
97.2
99.0
97.9
97.9
98.3
99.1
98.5
98.3
98.3
97.2
97.4
98.1
98.0
97.4
98.2
99.1
–
59.2
77.1
71.7
18.7
34.9
55.2
70.1
71.1
79.5
59.1
31.7
10.4
10.4
71.3
77.5
59.8
83.9
9.4
1.63
1.71
1.67
1.43
1.55
1.43
1.61
1.63
1.44
1.62
1.52
1.42
1.49
1.74
1.85
1.63
1.80
1.92
2.22
1.41
1.42
1.40
1.43
1.00
2
–
3
–
4
1.06
5
NMAP
DMDBT
–
6
–
7
–
8
–
9
1.00
1.01
–
10
11
12
13
14
15
16
17
18
19
20
21
22
23
MNPB
MNPH
DMDBT
DMDBT
–
1.01
–
–
–
1.00
0.99
–
BBEBf
7.2
MNPU
DMDBT
97.6
97.2
96.9
97.2
17.1
11.9
11.7
13.3
1.01
–
–
1.00
a
d
[Nitroso compound]0/[dibromide]0/[Cu]0/[PMDETA]0 5 1.05/1/2/2.2, 1 h,
THF 5 0.3 mL.
Polymerization temperature.
Number-averaged molecular weight (Mn in 103 g/mol) and polydisper-
V
sity index (PDI) of polymer measured by gel permeation chromatography.
e
b
Mole ratio of dibromo monomer unit to nitroso compound unit of
c
Polymer yield calculated by the integrity area of original GPC curve of
polymer determined by 1H NMR.
f
crude polymer.
Mn and PDI of polymers were measured without further purification.
EXPERIMENTAL
on a PL GPC220 equipped with two PLgel 5 mm MIXED-C
columns using polystyrene standards and THF as the eluent
Materials
1
ꢀ
at a flow rate of 1.0 mL/min at 40 C. H NMR spectra were
recorded at room temperature by a Bruker DMX 400 MHz
spectrometer using tetramethylsilane as the internal stand-
ard and deuterated chloroform as the solvent. Elemental
analyses were performed with Flash EA1112 (Thermo Finni-
gan). ESI-MS (m/z) was performed using a LCQ Deca XP Max
(Thermo Finnigan).
(6)22-Methyl-2,4-pentanediol (98%, Alfa Aesar), m-chloro-
peroxybenzoic acid (70–75%, Alfa Aesar), 2-amino-2-methyl-
1-propanol (>97%, Fluka), butyric acid (>99%, Alfa Aesar),
hexanoic acid (>98%, Alfa Aesar), undecanoic acid (98%,
Alfa Aesar), copper powder (3.25–4.75 mm, 99.9%, alfa), and
N,N,N0,N00,N00-pentamethyl diethylenetriamine (PMDETA, 98%,
Alfa). Tetrahydrofuran (THF) was distilled from sodium ben-
zophenone. Synthesis of dimethyl 2,13-dibromotetradecane-
dioate (DMDBT) and 1,4-bis(1-bromoethyl)benzene (BBEB)
was reported in our previous article.1 2-Methyl-2-
nitrosopropane dimer (MNP) was synthesized according to
procedures previously reported in the literature.13 All other
reagents were used as received without further purification.
Synthesis of Nitroso Compounds
2-Nitroso-2-Methyl-4-Acetoxypentane
2-Nitroso-2-methyl-4-acetoxypentane (NMAP) was synthe-
sized according to published literature.8,10 The yield of
NMAP is 16.7%.
1H NMR (400 MHz, CDCl3)
d (ppm): 1.10 (6H, d,
Characterization Methods
Number average (Mn) and molecular weight distributions
were determined by gel permeation chromatograph (GPC)
CH3ACACH3), 1.21 (3H, d, CH3ACH), 1.88 (3H, s,
CH3AC@O), 2.17, 2.81 (2H, dd, ACH2A), 4.90 (1H, m,
ACHAO). 13C NMR (400 MHz, CDCl3) d (ppm): 20.54
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2-Methyl-2-Nitrosopropyl Butyrate
The yield of MNPB is 4.6%.
1H NMR (monomer, 400 MHz, CDCl3) d (ppm): 0.89 (3H, t,
CH3ACH2), 1.15 (6H, s, CH3ACACH3), 1.56 (2H, m,
CH3ACH2A), 2.19 (2H, t, ACH2AC@O), 4.81 (2H, s,
ACH2AO). 1H NMR (dimer, 400 MHz, CDCl3) d (ppm): 0.94
(3H, t, CH3ACH2), 1.59 (6H, s, CH3ACACH3), 1.63 (2H, m,
CH3ACH2A), 2.29 (2H, t, ACH2AC@O), 4.58 (2H, s,
ACH2AO). ESI (m/z): 195.86 (100) [M1Na]1, 481.83
(89.69) [3M22NO1Na]1, 338.97 (70.92) [2M2NO1Na]1,
368.76 (54.61) [2M1Na]1. Anal. calcd. for C15H29NO3: C,
55.47; H, 8.73; N, 8.09. Found: C, 52.70; H, 8.34; N, 7.64.
Radical Addition-Coupling Polymerization
Polymerization Procedure
In a typical procedure, nitroso compound (0.144 mmol),
dibromo monomer (0.138 mmol), PMDETA (57 mL, 0.275
mol), and THF (0.3 mL) were added to a 10 mL Schlenk
flask equipped with a stirring bar. After three freeze–pump–
thaw cycles, copper powder (19.4 mg, 0.302 mmol) was
FIGURE 4 1H NMR spectrum (CDCl3, 400 MHz) of polymer pre-
pared by RACP between DMDBT and NMAP at 70 ꢀC.
ꢀ
added under N2. The flask was reacted at 25 C for 1 h. The
mixture was diluted with THF and purified by passing
through a neutral alumina column. The polymer solution
was concentrated and dried under vacuum at 40 C to yield
(CH3ACH), 21.08 (CH3AC@O), 21.30, 21.95 (CH3ACACH3),
43.18 (ACH2A), 67.36 (ACA), 97.92 (OACHACH3), 170.29
(AC@O). ESI (m/z): 185.80 (100) [M1Na]1, 368.55 (21.25)
[2M1Na]1, 165.63 (3.96) [M2NO1Na]1. Anal. calcd. For
C8H15NO3: C, 55.47; H, 8.73; N, 8.09. Found: C, 55.63; H,
8.75; N, 7.99.
ꢀ
polymer as a colorless tacky gum.
Purification of Polymer Prepared by RACP
To a solution (2%, m/m) of crude polymer in acetone in a 45
ꢀC water bath, methanol/water (7/3, v/v) was added drop-
wise until the solution just became turbid. This turbid liquid
was slowly cooled to room temperature and then kept at 4 ꢀC
over night. Oil-like product appeared at the bottom, which
was separated and dried under vacuum at 45 ꢀC to afford
final product. Polymer yield was calculated based on the area
corresponding to the fractions of monomer and polymer in
the GPC curve of crude polymer before purification.
2-Methyl-2-nitrosopropyl alkyl esters were synthesized
according to the published procedure.9,11 Undecanoic acid,
hexanoic acid, and butyric acid were used.
2-Methyl-2-Nitrosopropyl Undecanoate
The yield of MNPU is 5.3%.
1H NMR (400 MHz, CDCl3) d (ppm): 0.88 (3H, t, CH3ACH2),
1.14 (6H, s, CH3ACACH3), 1.25 (14H, m, CH3A (CH2)7A),
1.52 (2H, m, ACH2ACH2AC@O), 2.20 (2H, t, ACH2AC@O),
4.79 (2H, s, ACH2AO). ESI (m/z): 293.89 (100) [M1Na]1,
535.02 (47.87) [2M2NO1Na]1, 564.19 (35.26) [2M1Na]1,
241.02 (12.92) [M2NO]1, 512.32 (11.93) [2M2NO]1,
775.87 (9) [3M22NO1Na]1. Anal. calcd. for C15H29NO3: C,
66.38; H, 10.77; N, 5.16. Found: C, 65.85; H, 10.85; N, 5.08.
CONCLUSIONS
In summary, we report radical addition-coupling polymeriza-
tion using various nitroso esters with dibromide at different
temperatures. The nitroso esters can be polymerized at 70
ꢀC and produce polymer with high molecular weight and
regular monomer sequence in addition to introducing ester
group at side-chain of polymer.
2-Methyl-2-Nitrosopropyl Hexanoate
The yield of MNPH is 5.1%.
ACKNOWLEDGMENT
1H NMR (monomer, 400 MHz, CDCl3) d (ppm): 0.88 (3H, t,
CH3ACH2), 1.15 (6H, s, CH3ACACH3), 1.25 (6H, m, CH3A
(CH2)3A), 2.20 (2H, t, ACH2AC@O), 4.80 (2H, s, ACH2AO).
1H NMR (dimer, 400 MHz, CDCl3) d (ppm): 0.88 (3H, t,
CH3ACH2), 1.53 (6H, m, CH3A (CH2)3A), 1.59 (6H, s,
CH3ACACH3), 2.32 (2H, t, ACH2AC@O), 4.58 (2H, s,
ACH2AO). ESI (m/z): 424.45 (100) [2M1Na]1, 223.85
(87.19) [M1Na]1, 394.95 (80.72) [2M2NO1Na]1, 239.88
(8.85) [M1K]1, 371.64 (5.95) [2M2NO]1. Anal. calcd. for
Financial support from National Natural Science Foundation of
China (21174123) is appreciated.
REFERENCES AND NOTES
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2 C. Y. Zhang, Q. Wang, J. Polym. Sci. Polym. Chem. 2011, 49,
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C15H29NO3: C, 59.68; H, 9.52; N, 6.96. Found: C, 59.64; H,
3 C. Y. Zhang, J. Ling, Q. Wang, Macromolecules 2011, 44,
9.50; N, 6.79.
8739–8743.
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