Amino-functionalized (meth)acryl polymers by use of a solvent-polarity sensitive protecting group (Br-t-BOC)

Article abstract of DOI:10.3762/bjoc.12.26

We describe the synthesis of bromo-tert-butyloxycarbonyl (Br-t-BOC)-amino-protected monomers 2-((1-bromo-2-methylpropan-2-yl)oxycarbonylamino)ethyl (meth)acrylate 3a,b. For this purpose, 2-isocyanatoethyl (meth)acrylate 1a,b was reacted with 1-bromo-2-methylpropan-2-ol (2a). The free radical polymerization of (Br-t-BOC)-aminoethyl (meth)acrylates 3a,b yielded poly((Br-t-BOC)-aminoethyl (meth)acrylate) 6a,b bearing protected amino side groups. The subsequent solvolysis of the Br-t-BOC function led to the new polymers poly(2-aminoethyl (meth)acrylate) 8a,b with protonated free amino groups. The monomers and the resulting polymers were thoroughly characterized by 1H NMR, IR, GPC and DSC methods. The kinetics of the deprotection step was followed by 1H NMR spectroscopy. The solvent polarity and neighboring group effects on the kinetics of deprotection are discussed.

Full text of DOI:10.3762/bjoc.12.26

Amino-functionalized (meth)acryl polymers by use of a
solvent-polarity sensitive protecting group (Br-t-BOC)
Helmut Ritter*, Monir Tabatabai and Markus Herrmann
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 245–252.
Institute of Organic Chemistry and Macromolecular Chemistry,
Heinrich-Heine-University Duesseldorf, Universitaetsstraße 1,
D-40225 Duesseldorf, Germany
doi:10.3762/bjoc.12.26
Received: 25 November 2015
Accepted: 25 January 2016
Published: 10 February 2016
Email:
Helmut Ritter* - h.ritter@uni-duesseldorf.de
Associate Editor: P. J. Skabara
*
Corresponding author
©
2016 Ritter et al; licensee Beilstein-Institut.
Keywords:
License and terms: see end of document.
amino group protection; bromo-tert-butyloxycarbonyl; deprotection;
free radical polymerization; (meth)acryl polymers; neighboring group
effects; solvent polarity
Abstract
We describe the synthesis of bromo-tert-butyloxycarbonyl (Br-t-BOC)-amino-protected monomers 2-((1-bromo-2-methylpropan-2-
yl)oxycarbonylamino)ethyl (meth)acrylate 3a,b. For this purpose, 2-isocyanatoethyl (meth)acrylate 1a,b was reacted with 1-bromo-
2
-methylpropan-2-ol (2a). The free radical polymerization of (Br-t-BOC)-aminoethyl (meth)acrylates 3a,b yielded poly((Br-t-
BOC)-aminoethyl (meth)acrylate) 6a,b bearing protected amino side groups. The subsequent solvolysis of the Br-t-BOC function
led to the new polymers poly(2-aminoethyl (meth)acrylate) 8a,b with protonated free amino groups. The monomers and the result-
ing polymers were thoroughly characterized by 1H NMR, IR, GPC and DSC methods. The kinetics of the deprotection step was fol-
lowed by 1H NMR spectroscopy. The solvent polarity and neighboring group effects on the kinetics of deprotection are discussed.
Introduction
Amino groups are important functionalities in polymer chem- in application by certain restrictions on the deprotection condi-
istry, e.g., for hardening various epoxy resins [1]. However, tions. Therefore, there is a continued interest in developing new
they easily react in an undesired side reaction with electron- protecting groups which can be cleaved by different mecha-
poor double bonds of (meth)acrylates [2]. Therefore, for the nisms. Keeping this in mind, the bromo-tert-butyloxycarbonyl
synthesis of amino-containing (meth)acrylic monomers and (Br-t-BOC) group represents the first known solvent-polarity
polymers suitable amino-protecting groups are required. Clas- sensitive amino-protecting group. As shown in Figure 1, this
sical protecting groups such as ammonium salts, F-MOC, Z- or group is stable in nonpolar solvents because of high activation
t-BOC, respectively are readily available. Regarding to this energy and easily decomposes in a more polar environment
aspect, we published some papers about polymer protecting because of reduced activation energy. This effect is a result of
groups about three decades ago [3-7]. However, they are limited an increased polarity of the transition state in comparison to the
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Beilstein J. Org. Chem. 2016, 12, 245–252.
Figure 1: Schematic representation of a deprotection taking the relatively high polarity of the transition state and the solvent polarity into account: high
activation energy in nonpolar solvents and low activation energy in polar solvents.
starting molecule (Figure 1). In contrast to the classical t-BOC Results and Discussion
protecting group, the Br-t-BOC group can be easily removed The Br-t-Boc-protected monomers 2-((1-bromo-2-methyl-
without pH adjustments. Since the published papers from L. A. propan-2-yl)oxycarbonylamino)ethyl acrylate (3a) and 2-((1-
Carpino [8-10] and a first practical application in peptide syn- bromo-2-methylpropan-2-yl)oxycarbonylamino)ethyl methacry-
thesis [11] this protecting group was quasi forgotten.
late (3b) were synthesized by the reaction of 1-bromo-2-methyl-
propan-2-ol (2a) with 2-isocyanatoethyl (meth)acrylate (1a,b)
In the present work we report new Br-t-Boc-protected (Scheme 1). The success of the reaction can be easily shown by,
meth)acrylic monomers and their polymerization through free e.g., the disappearance of the N=C=O peak of 1a at 2260 cm−1
radical polymerization. The kinetics of Br-t-BOC solvolysis of and the appearance of an N–H peak at 3350 cm−1 in the IR
the monomers and polymers in a polar solvent are described. spectra. Pure samples of 3a,b could be isolated by column chro-
(
Scheme 1: Synthesis of Br-t-Boc-protected monomers 3a,b and homopolymers 6a,b.
246

Beilstein J. Org. Chem. 2016, 12, 245–252.
matography and the 1H NMR spectra of 3a and 3b are shown in corresponding amine hydrobromide (4a,b) and the cyclic
Figure 2 and Figure 3.
carbonate 4,4-dimethyl-1,3-dioxolan-2-one (5). As explained
above, the protecting group undergoes a polar solvent driven
After dissolving monomers 3a and 3b in a polar solvent, they intramolecular nucleophilic displacement (Figure 1). In
show the above mentioned self-cleaving process yielding the contrast, in nonpolar solvents such as benzene, toluene, chloro-
Figure 2: 300 MHz 1H NMR spectrum of 3a in CDCl3.
Figure 3: 300 MHz 1H NMR spectrum of 3b in CDCl3.
247

Beilstein J. Org. Chem. 2016, 12, 245–252.
form (CHCl3) or dichloromethane (CH2Cl2) 3a,b are very the cleaving process is coupled with cyclization to create the
stable, whereas in polar protic solvents such as ethanol (EtOH) cyclic carbonate 5, the homopolymer could be self-deactivated
or methanol (MeOH) the cleaving takes place very fast due to competing hydrogen-bond interactions of the urethane
(
Scheme 2). This deprotecting process can be directly followed carbonyl group with neighboring groups. Since the comonomer
using 1H NMR spectroscopy.
should act as diluting agent a better access of the carbonyl
group to the bromo-substituted carbon atom to fulfill the
Carpino assumed that, changing one or both of the methyl nucleophilic intramolecular displacement could be expected
groups to bulkier substituents would significantly increase the (Scheme 4).
cleaving process [10]. Thus, we attempted to replace one
methyl group with a phenyl substituent simply by the reaction The kinetic studies of 3a, 6a and 7a are shown in Figure 4. Ad-
of 1a with 1-bromo-2-phenylpropan-2-ol (2b). Although spec- ditionally, Figure 5 shows the 1H NMR spectra of homopoly-
troscopic examinations gave evidence for the successful forma- mer 6a and deprotected polymer 8a.
tion of the expected carbamate 3c, only the corresponding
amine hydrobromide 4a was isolated as main product. This The calculated t0.5 and t0.3 values of deprotection for 3a,b, 6a,b
means that the amino-protecting group is extremely sensitive to and 7a,b are summarized in Table 1.
decomposition. A third type of monomer was synthesized by
reaction of 1a with 1-bromo-2-methylbut-3-en-2-ol (2c)
Table 1: Half-times (t0.5) and t0.3 of deprotection for 3a,b, 6a,b and
yielding 2-((1-bromo-2-methylbut-3-en-2-yl)oxycarbonyl-
amino)ethyl acrylate (3d, Scheme 3). Although monomer 3d is
stable in nonpolar solvents, the cleaving process could not be
followed by 1H NMR spectroscopy because of rapid decompo-
sition in a polar solvent.
7a,b.
Compound
t0.3 [min]
t0.5 [min]
3a
3b
6a
6b
7a
7b
84
180
180
276
264
192
204
102
162
159
114
120
Next, classical free radical polymerizations of 3a,b were carried
out in toluene as solvent at 50 °C using 2,2'-azobis(4-methoxy-
2
,4-dimethylvaleronitrile) (V-70) as initiator yielding homo-
polymers 6a,b. This initiator is known to have a low decompo-
sition temperature of 30 °C in toluene. Additionally, copoly- As assumed above the molecular dispersed monomer shows the
mers of 3a and 3b, respectively with N,N-dimethylacrylamide highest reactivity followed by the copolymer. The solvolysis of
(
7) were synthesized yielding 7a,b. It was assumed that, since the protecting group in the homopolymer is relatively slow
Scheme 2: Stability of 3 in different solvents.
Scheme 3: Synthesized derivative monomers 3c,d.
248

Beilstein J. Org. Chem. 2016, 12, 245–252.
Scheme 4: a) Self deactivation of homopolymer 6a,b due to competing hydrogen interactions in comparison to b) copolymer 7a,b.
Figure 4: Kinetic studies of the deprotection of 3a, 6a and 7a.
because of neighboring group formed retarding hydrogen bonds mild conditions in polar solvents, some sensitive components,
(
Scheme 4).
e.g., drugs, proteins, DNA may be present without being
affected.
Conclusion
It can be concluded from the above described results that the
Experimental
bromo-tert-butyloxycarbonyl group has a certain potential for Methods
the preparation of amino group containing functional 1H NMR measurements were performed using a Bruker Avance
(
meth)acryl polymers. Since the deprotection takes place under 300 operating at 300 MHz at room temperature. Fourier trans-
249

Beilstein J. Org. Chem. 2016, 12, 245–252.
Figure 5: 300 MHz 1H NMR spectra of a) 6a in CDCl3 and b) 8a in DMSO-d6.
formation infrared spectroscopy (FTIR) was performed on a 15 °C min−1. The Tg values are reported as the average of five
Nicolet 6700 FTIR spectrometer equipped with a diamond measurements using the midpoint method. The THF–GPC
single bounce ATR accessory. The measurements were per- System compromises a Scharmbeck SFD degasser (Gastor
formed in the range of 400–3000 cm−1 at room temperature. BG12), a FLOW pump (Intelligent PUMP AL-12) and a
Differential scanning calorimetry (DSC) was performed using a Scharmbeck SFD (Model S5200) sampler. A Waters 486 Turn-
Mettler Toledo DSC 822 instrument equipped with a sample able Absorbance Detector and a Scharmbeck SFD RI 2000
robot TSO801RO. The Apparatus was controlled over a temper- detector were used for detection. A set of columns packed with
ature range between −50 °C and 350 °C at a heating rate of porous styrene–divinylbenzene-copolymer beads was used for
250

Beilstein J. Org. Chem. 2016, 12, 245–252.
separation of the analytes (MZ Analysentechnik GmbH, (diamond) [cm−1]: 3442 (s, νOH), 3027 (m, νC-H), 2978 (m,
1
1
× guard column 100 Å, 3 × columns with 10,000, 1,000 und νC-H), 1602 (w, νC=C), 1493 (w, νC=C), 1446 (m, ν-CH2-), 1374
00 Å). The system was calibrated with polystyrene standards (s, νOH tert-alcohol).
with a molecular range from 575 g/mol to 3,114,000 g/mol.
THF was used as eluent at a flow rate of 1 mL min−1.
1-Bromo-2-methylbut-3-en-2-ol (2c)
Into a stirring solution of N-bromosuccinimide (NBS, 20 g,
Materials
0.12 mol) in 1/3 THF and 2/3 H2O, isoprene was added in
Commercial reagents and solvents were purchased from Sigma- 3 small portions of 5 mL (0.05 mol) at 0 °C. Stirring was
Aldrich, Merck and Fluka. If not stated otherwise all chemicals continued for an additional hour. THF was evaporated and the
were of analytical grade and were used as received without any remaining aqueous mixture was extracted several times with
further purification. 2-Isocyanatoethyl acrylate and (meth)acry- diethyl ether. The extract was dried over magnesium sulfate and
late were purchased from Shōwa Denkō K.K.
the solution was evaporated. The remaining oil was distilled to
give 2c as a pure colorless liquid: yield 7.41 g (37.5%);
Chloroform-d (99.8 atom % D) was obtained from Deutero bp 62 °C at 20 mbar. 1H NMR (300 MHz, CDCl3) δ [ppm] 5.88
GmbH (Germany). All solvents were dried by standard (dd, 1H, -CH=CHH), 5.34–5.16 (m, 2H, -CH=CH2), 3.44 (s,
methods. Column chromatography was performed using Acros 2H, -CH2-Br), 2.32 (s, 1H, -OH), 1.40 (s, 3H, -CH3); IR
Organics silica gel 60 (230–400 mesh) and thin layer chroma- (diamond) [cm−1]: 3412 (s, νOH), 2980 (m, νC-H), 2928 (m,
tography (TLC) of the products using Merck silica 60 F254 νC-H), 1644 (m, νC=C), 1453 (m, ν-CH2-), 1371 (s, νOH).
plates.
General information for the preparation of the
1-Bromo-2-methylpropan-2-ol (2a)
monomers
In a 500 mL three-necked flask with a reflux condenser tert-bu- All syntheses were carried out under an argon atmosphere at
tanol (100 g, 1.35 mol) was heated under reflux. A second room temperature using dibutyltin dilaurate (DBTL) as catalyst.
5
00 mL single-necked flask was equipped with N-bromosuccin-
2-((1-Bromo-2-methylpropan-2-yl)oxy-
imide (NBS, 50 g 0.28 mol) in a mixture of THF/H2O 1:2. To
the boiling tert-butanol 5 portions of 5 mL each concentrated carbonylamino)ethyl acrylate (3a)
sulfuric acid were added dropwise every 15 min. The forming To a stirring solution of 2-isocyanatoethyl acrylate (1a, 8 g,
isobutene was bubbled into the NBS solution under vigorous 0.056 mol) and 2 mol % DBTL in 25 mL toluene, 1-bromo-2-
stirring. After 45 min, all of the NBS disappeared and stirring methylpropan-2-ol (2a, 7.7 g, 0.05 mol) was added. After 24 h
was continued for an additional hour. THF was evaporated and the solvent was evaporated and the crude product was purified
the remaining aqueous mixture was extracted several times with by column chromatography using n-hexane/ethyl acetate 1:1 to
diethyl ether. The extract was dried over magnesium sulfate and give the pure product 3a: yield 5.39 g (32%). 1H NMR
the solution was evaporated. The remaining oil was distilled to (300 MHz, CDCl3) δ [ppm] 6.32 (dd, 3J = 17.3 Hz, 2J = 1.5 Hz,
give 2a as a pure colorless liquid: yield 22 g (51%); bp 55 °C at 1H, -CH=CHH), 6.04 (dd, 3J = 17.3 Hz, 3J = 10.4 Hz, 1H,
2
5 mbar; 1H NMR (300 MHz, CDCl3) δ [ppm] 3.39 (s, 2H, -CH=CH2), 5.76 (dd, 3J = 10.4 Hz, 2J = 1.5 Hz, 1H,
CH2-Br); 2.43 (s, 1H, -OH); 1.30 (s, 6H, -CH3); IR (diamond) -CH=CHH), 5.22 (s, 1H, -NH-), 4.13 (t, 3J = 5.4 Hz, 2H,
cm−1]: 3380 (s, νOH), 2976 (m, νC-H), 2931 (m, νC-H), 2872 -OCH2), 3.67 (s, 2H, -CH2-Br), 3.34 (q, 3J = 5.6 Hz, 2H, -NH-
-
[
(
m, νC-H), 1465 (m, ν-CH2-), 1379 (s, νOH).
CH2), 1.44 (s, 6H, -CH3); IR (diamond) [cm−1]: 3358 (m,
νN-H), 2982 (m, νC-H), 2936 (m, νC-H), 1708 (w, νC=O), 1635 (w,
νC=C), 1618 (w, νC=Ct), 1516 (s, νN-H), 983 (s, νCH=CH2), 809
1
-Bromo-2-phenylpropan-2-ol (2b)
Into a stirring solution of N-bromosuccinimide (NBS, 32 g, (s, νCH=CH2).
0
0
.18 mol) in 1/3 THF and 2/3 H2O, α-methylstyrene (10 mL,
2-((1-Bromo-2-methylpropan-2-yl)oxy-
.15 mol) were added. After 2 h of vigorous stirring, THF was
evaporated and the aqueous solution was extracted several times carbonylamino)ethyl methacrylate (3b)
with diethyl ether. The extract was dried over magnesium To a stirring solution of 2-isocyanatoethyl methacrylate (1b,
sulfate and the solution was evaporated. The crude product was 6 g, 0.039 mol) and 2 mol % DBTL in 25 mL toluene, 1-bromo-
purified by column chromatography using n-hexane/ethyl 2-methylpropan-2-ol (2a, 5.8 g, 0.038 mol) was added. After
acetate 1:1 to give the desired colorless liquid 2b: yield 21.35 g 24 h the solvent was evaporated and the crude product was puri-
(
66%). 1H NMR (300 MHz, CDCl3) δ [ppm] 7.57–7.54 (m, 2H, fied by column chromatography using n-hexane/ethyl acetate
ArH), 7.38–7.32 (m, 2H, ArH), 7.27–7.22 (m, 1H, ArH), 5.40 1:1 to give the pure product 3b: yield 5.3 g (43%). 1H NMR
s, 1H, -OH), 3.78 (s, 2H, -CH2-Br), 1.66 (s, 3H, -CH3); IR (300 MHz, CDCl3) δ [ppm] 6.12–6.11 (m, 1H, -C(CH3)-CHH),
(
251

Beilstein J. Org. Chem. 2016, 12, 245–252.
5
.59–5.57 (m, 1H, -C(CH3)=CHH), 4.95 (s, 1H, -NH-), 4.21 (t, (89%). 1H NMR (300 MHz, CDCl3, rt) δ [ppm] 6.12–5.27 (m,
3
J = 5.4 Hz, 2H, -OCH2), 3.75 (s, 2H,-CH2-Br), 3.44 (m, 2H, 1H), 4.12–3.92 (m, 2H), 3.85–3.65 (m, 2H), 3.55–3.27 (m, 2H),
-
NH-CH2-), 1.93 (s, 3H, -CH3), 1.53 (s, 6H, -CH3); IR 1.90–1.72 (m, 3H), 1.68–1.60 (m, 2H), 1.58–1.52 (m, 6H); IR
(
diamond) [cm−1]: 3365 (m, νN-H), 2978 (m, νC-H), 2929 (m, (diamond) [cm−1]: 3353 (m, νN-H), 2978 (m, νC-H), 2934 (m,
νC-H), 1708 (w, νC=O), 1637 (w, νC=C), 1520 (s, νN-H), 943 (s, νC-H), 1715 (w, νC=O), 1521 (s, νN-H), 1455 (s, νC-H (R-CH3));
νCH=CH2), 814 (s, νCH=CH2). DSC: Tg = 125.9 °C; GPC (THF): = 8900 g/mol,
000 g/mol, D = 1.48.
=
6
2-((1-Bromo-2-methylbut-3-en-2-yl)oxy-
carbonylamino)ethyl acrylate (3d)
References
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Macromol. Chem. Phys. 2014, 215, 421–425.
0
.03 mol) and 2 mol % DBTL in 25 mL toluene, 1-bromo-2-
doi:10.1002/macp.201300734
methylbut-3-en-2-ol (2c, 5 g, 0.03 mol) was added. After 24 h
the solvent was evaporated and the crude product was purified
by column chromatography using n-hexane/ethyl acetate 1:1 to
give the pure product 3d: yield 1.7 g (17%). 1H NMR
2.
Mather, B. D.; Viswanathan, K.; Miller, K. M.; Long, T. E.
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(
300 MHz, CDCl3) δ [ppm] 6.43 (dd, 3J = 17,3 Hz, 2J = 1.5 Hz,
H, -CH-CHH ), 6.12 (dd, 3J = 17.3 Hz, 2J = 10.4 Hz, 2H, -CH-
CH2), 5.86 (dd, 3J = 10.4 Hz, 2J = 1.5 Hz, 1H, -CH-CHH),
.81–5.74 (m, 1H, -CH=CH2), 5.06 (s, 1H, -NH-), 4.58–4.46
4
.
.
Rehse, H.; Ritter, H. Makromol. Chem. 1989, 190, 697–706.
doi:10.1002/macp.1989.021900403
1
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745–755. doi:10.1002/macp.1991.021920401
(
-
(
m, 2H, -CH=CH2), 4.24 (t, 2H, -OCH2), 4.01–3.98 (m, 2H, 6. Gormanns, M.; Ritter, H. Tetrahedron 1993, 49, 6965–6974.
CH2-Br), 3.50 (q, 2H, -NH-CH2), 1.73 (s, 3H, -CH3); IR
diamond) [cm−1]: 3348 (m, νN-H), 2951 (m, νC-H), 2869 (m,
doi:10.1016/S0040-4020(01)87972-2
7
.
.
Gormanns, M.; Ritter, H. Macromolecules 1994, 27, 5227–5228.
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νC-H), 1705 (w, νC=O), 1636 (w, νC=C), 1615 (w, νC=C), 1528 (s,
νN-H), 983 (s, νCH=CH2), 808 (s, νCH=CH2).
8
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Schmitz, E. J. Org. Chem. 1970, 35, 3291–3295.
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General information for the preparation of the
9
1
1
.
Carpino, L. A. Acc. Chem. Res. 1973, 6, 191–198.
doi:10.1021/ar50066a003
polymers and copolymers
All polymerizations were carried out in toluene (75 wt %) at
0.Carpino, L. A.; Rice, N. W.; Mansour, E. M. E.; Triolo, S. A.
J. Org. Chem. 1984, 49, 836–842. doi:10.1021/jo00179a017
1.Ohnishi, T.; Sugano, H.; Miyoshi, M. Bull. Chem. Soc. Jpn. 1972, 45,
5
0 °C using 5 mol % 2,2'-azobis(4-methoxy-2,4-dimethyl-
valeronitrile) (V-70) as initiator. After 24 h the polymerizations
were stopped and the solutions were precipitated by pouring
into n-hexane. The obtained polymers were filtered off and
dried under vacuum.
2603–2607. doi:10.1246/bcsj.45.2603
License and Terms
Poly-(2-((1-bromo-2-methylpropan-2-yl)oxy-
This is an Open Access article under the terms of the
Creative Commons Attribution License
carbonylamino)ethyl acrylate) (6a)
To a mixture of 3a (0.35 g, 1.19 mmol) in 0.7 g toluene, V-70
(http://creativecommons.org/licenses/by/2.0), which
(
5 mol %, 18.35 mg) dissolved in 0.35 g toluene was added.
Polymer 6a was obtained as a colorless solid: yield 0.28 g
80%). 1H NMR (300 MHz, CDCl3) δ [ppm] 6.04–5.45 (m,
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
(
1
2
H), 4.33–3.97 (m, 2H), 3.90–3.60 (m, 2H), 3.55–3.24 (m, 2H),
.53–2.12 (m, 1H), 2.09–1.66 (m, 2H), 1.61–1.54 (m, 6H); IR
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(
diamond) [cm−1]: 3359 (m, νN-H), 2955 (m, νC-H), 2930 (m,
νC-H), 1698 (w, νC=O), 1516 (s, νN-H); DSC: Tg = 93.1 °C; GPC
THF): = 9300 g/mol, = 4400 g/mol, D = 2.11.
(http://www.beilstein-journals.org/bjoc)
(
The definitive version of this article is the electronic one
which can be found at:
Poly-(2-((1-bromo-2-methylpropan-2-yl)oxy-
doi:10.3762/bjoc.12.26
carbonylamino)ethyl methacrylate) (6b)
To a mixture of 3b (0.38 g, 1.23 mmol) in 0.76 g toluene, V-70
(
5 mol %, 19.02 mg) dissolved in 0.38 g toluene was added.
Polymer 6b was obtained as a colorless solid: yield 0.34 g
252