Stereospecific free radical and RAFT polymerization of bulky silyl methacrylates for tacticity and molecular weight controlled poly(methacrylic acid)

Article abstract of DOI:10.1021/ma202155b

A series of silyl methacrylates [CH₂=C(CH₃)CO ₂SiR₃] with varying silyl group bulkiness [R ₃Si: Me₃Si, Et₃Si, Me₂tBuSi, iPr₃Si, Ph₂tBuSi, Ph

Full text of DOI:10.1021/ma202155b

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ARTICLE
pubs.acs.org/Macromolecules
Stereospecific Free Radical and RAFT Polymerization of Bulky Silyl
Methacrylates for Tacticity and Molecular Weight Controlled
Poly(methacrylic acid)
Kenji Ishitake,† Kotaro Satoh,† Masami Kamigaito,*,† and Yoshio Okamoto‡,§
†Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603,
Japan
‡Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
§College of Material Science and Chemical Engineering, Harbin Engineering University, Nantong St. Harbin, P. R. China
S Supporting Information
b
ABSTRACT: A series of silyl methacrylates [CH2dC(CH3)CO2SiR3] with
varying silyl group bulkiness [R3Si: Me3Si, Et3Si, Me2tBuSi, iPr3Si, Ph2tBuSi,
Ph3Si, and (Me3Si)3Si] were synthesized and radically polymerized to effi-
ciently give soluble polymers with the exception of the highly bulky
tris(trimethylsilyl)silyl methacrylate (TTMSSMA), which resulted in insoluble
polymers. All the polymers can easily be converted into poly(methacrylic acid)
(PMAA) via acid- or fluoride-induced deprotection of the silyl groups and further
into poly(methyl methacrylate) (PMMA) via methylation with trimethylsilyl-
diazomethane for the analysis of molecular weight and tacticity. The tacticity was
dependent on the bulkiness of the silyl substituents; the isotacticity increased
with increasing bulkiness. Thus, a series of PMAAs and PMMAs with various
tacticities ranging from syndiotactic-rich (rr = 74%; Me2tBuSi) to atactic (mr = 50%; iPr3Si) and highly isotactic [mm = 93%;
(Me3Si)3Si] enchainment were obtained by conventional radical polymerization of silyl methacrylates followed by simple
postreactions. The high isotacticity and insolubility of poly(TTMSSMA) suggested the formation of helical polymers as in the
polymerization of similarly bulky triarylmethyl methacrylate. Reversible additionꢀfragmentation chain-transfer (RAFT) polym-
erization also worked for these silyl methacrylates, which resulted in well-defined polymers with controlled molecular weights and
various tacticities. RAFT polymerization was further applied to the synthesis of novel stereoblock polymers, such as stereo-triblock
PMAA and PMMA that consisted of syndiotactic-rich, atactic, and isotactic stereogradient segments.
’ INTRODUCTION
transfer polymerization,6 where the silylated propagating species
induces a kind of chemoselective propagation reaction by
diminishing the side reactions. However, silyl groups have rarely
been used for stereoselective or stereospecific polymerizations, in
which their steric properties can be used for stereochemical
control, except for a few reports on the free-radical polymeriza-
tion of 2-silyl-substituted 1,3-butadiene derivatives, where the
alkoxysilyl substituents affect the microstructure of the resulting
polydienes.7
In vinyl polymerizations, including radical polymerizations,
bulkiness in monomers often plays an important role in dictating
the stereochemistry of the resulting polymers.8 A variety of alkyl
methacrylates with different substituents have been synthesized
and radically polymerized under various conditions to produce a
series of poly(alkyl methacrylate)s with various tacticities, ran-
ging from syndiotactic-rich to highly isotactic enchainment. The
tacticity is mainly governed by the bulkiness of the substituents.9
Silyl groups are among the most frequently used protective
groups for alcoholic protons and can similarly be used for
protecting carboxylic acidic protons in their silyl ester forms.1
Various silyl groups have been prepared and adapted for the
realization of selective deprotection reactions of silyl ethers
because the reactivity of the SiꢀO bond is strongly affected by
the steric and electronic properties of the silyl moiety. However,
silyl groups were not employed to induce diastereoselective
reactions until an extremely bulky tris(trimethylsilyl)silyl group,
known as the hypersilyl, sisyl, or supersilyl group,2 was used for
diastereoselective [2 + 2] cyclizations and Mukaiyama aldol
reactions of the silyl enol ethers by Yamamoto et al.3
Even in polymer chemistry, silyl groups have been extensively
used as protecting groups for the functional moieties (OH, NH2,
CHO, COCH3, COOH, and CtCH) of various monomers and
initiators, mainly in ionic polymerizations,4,5 where these func-
tional groups induce termination or chain-transfer reactions
with the ionic propagating species. In addition, silyl groups
have also been employed as part of the initiating system for
inducing living or controlled polymerization, such as in group
Received: September 23, 2011
Revised:
October 29, 2011
Published: November 16, 2011
r
2011 American Chemical Society
9108
dx.doi.org/10.1021/ma202155b Macromolecules 2011, 44, 9108–9117
|

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Scheme 1. Stereospecific RAFT Polymerization of Various Silyl Methacrylates for Stereocontrolled and Molecular Weight
Controlled Poly(methacrylic acid) and Its Ester
Specifically, usual alkyl methacrylates, such as methyl methacrylate
(MMA), form predominantly syndiotactic polymers (rr ∼ 65%),
and the syndiotacticity gradually decreases with increasing bulki-
ness of the pendent groups. Upon further increasing the bulki-
ness,10,11 an extremely bulky methacrylate, triphenylmethyl (trityl)
methacrylate (TrMA), gives highly isotactic (mm = 64ꢀ98%)
polymers, even in common organic solvents such as toluene, at
60 °C, probably due to the helical conformation imposed by the
highly bulky substituent. Although Kitayama and Hatada reported
stereospecific anionic polymerization of trimethylsilyl methacry-
late at low temperature,12 there have been no studies focusing
on the stereospecific radical polymerization of a series of various
silyl methacrylates with different bulkiness. In addition, trimethyl-
silyl and its related silyl methacrylates have been synthesized13ꢀ16
and homo- or copolymerized radically or anionically, which were
particularly directed to photoresist applications as precursors of
PMAA moieties.17,18
In this study, we investigated the radical polymerization of a
series of silyl methacrylates [CH2dC(CH3)CO2SiR3] with vary-
ing degrees of bulkiness in their silyl substituent [R3Si: Me3Si,
Et3Si, Me2tBuSi, iPr3Si, Ph2tBuSi, Ph3Si, and (Me3Si)3Si] and
examined the effect of stereospecificity on the radical polymer-
ization (Scheme 1). We focused our attention on changing the
tacticity of poly(methacrylic acid)s from syndiotactic-rich to
highly isotactic enchainment by radically polymerizing a series
of silyl methacrylates and simply deprotecting them. Among the
various silyl methacrylates, particular attention was paid to the
synthesis and polymerization of the novel bulky monomer super-
silyl methacrylate (TTMSSMA), which is a different type of
bulky methacrylate from the series of triarylmethyl methacrylates
previously listed that undergoes a similar isospecific radical poly-
merization. Furthermore, we investigated reversible additionꢀ
fragmentation chain transfer (RAFT) polymerizations19 of these
silyl methacrylates for simultaneous control of the molecular weight
andthetacticity. Thesynthesisofstereoblockpoly(methacrylic acid)
(PMAA) and poly(methyl methacrylate) (PMMA) by block copo-
lymerization of silyl methacrylates with different substituents and
subsequent simple postreactions was also investigated.
’ EXPERIMENTAL SECTION
Materials. α,α-Azobis(isobutyronitrile) (AIBN) (Kishida, >99%)
was purified by recrystallization from methanol. 2,20-Azobis(4-methoxy-
2,4-dimethylvaleronitrile) (V-70) (Wako, >95%) was purified by wash-
ing dry acetone and drying under reduced pressure. Trimethylsilyl
methacrylate (TMSMA) (Aldrich) and 1,2,3,4-tetrahydronaphthalene
(Wako, 97%) were distilled from calcium hydride under reduced
pressure before use. Toluene was distilled over sodium benzophenone
ketyl and bubbled with dry nitrogen over 15 min just before use. Cumyl
dithiobenzoate (CDB) was synthesized according to the literature.20
Trimethylsilyldiazomethane (Aldrich, 2.0 M in Et2O), tetrabutylammo-
nium fluoride (TBAF) (Aldrich, 1.0 M in THF), sodium methacrylate
(Aldrich, 99%), tert-butyldimethylsilyl chloride (TCI, >98%), triethyl-
silyl chloride (TCI, >97%), triisopropylsilyl chloride (TCI, >95%), tert-
butyldiphenylsilyl chloride (TCI, >95%), triphenylsilyl chloride (TCI,
>95%), tris(trimethylsilyl)silyl chloride (Aldrich, 97%), phenothiazine
(Kishida, 98%), and 2,6-di-tert-butyl-p-cresol (TCI, >99%) were used as
received.
Synthesis of Silyl Methacrylates. A series of silyl methacryl-
ates were synthesized by a simple reaction between sodium methacry-
late and silyl chlorides in the presence of 2,6-di-tert-butyl-p-cresol or
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ARTICLE
phenothiazine as an inhibitor for radical polymerization in dry THF as
follows. The reaction was carried out by the use of a syringe technique
under a dry nitrogen atmosphere in an oven-dried glass tube equipped
with three-way stopcocks. Each silyl methacrylate was synthesized as
follows.
After stirring at ambient temperature for an additional 72 h, the solu-
tion was evaporated, washed with n-hexane, and filtrated to remove
sodium chloride. After evaporation, the crude product was obtained
and purified by distillation from calcium hydride under reduced
pressure. TBDPSMA was obtained as colorless liquid (20.1 g, 70%, bp
120 °C/40 Pa). 1H NMR (CDCl3, rt): δ 1.10 (s, 9H, SiPh2C(CH3)3),
1.96 (dd, 3H, CH2dCꢀCH3), 5.64 (m, 1H, cis CH2dCꢀCH3), 6.25
(m, 1H, trans CH2dCꢀCH3), 7.32ꢀ7.42 (m, 6H, m, p-ArH),
7.62ꢀ7.68 (d, 4H, o-ArH). 13C NMR (CDCl3, rt): δ 18.7 (CH2d
CꢀCH3), 19.4 (SiPh2C(CH3)3), 27.1 (SiPh2C(CH3)3), 126.7 (CH2d
CꢀCH3), 127.9, 130.1, 132.0, and 135.5 (phenyl), 137.7 (CH2d
CꢀCH3), 166.6 (CO2SiPh2C(CH3)3).
tert-Butyldimethylsilyl Methacrylate (TBDMSMA). TBDMSMA
was synthesized by the reaction between the sodium methacrylate and tert-
butyldimethylsilyl chloride (TBDMS-Cl).21 Sodium methacrylate (16.8 g,
0.155 mol) was dispersed in dry THF (86.0 mL) in the presence of 2,6-di-
tert-butyl-p-cresol (34.2 mg, 0.155 mmol). Into the suspension, 2.78 M
TBDMS-Cl solution (51.0 mL, 0.142 mol) was added dropwise at 0 °C over
a period of 30 min under stirring. After stirring at ambient temperature for an
additional 24 h, the solution was evaporated, washed with n-hexane, and
filtrated to remove sodium chloride. After evaporation, the crude product
was obtained and purified by distillation from calcium hydride under reduced
pressure. TBDMSMA was thus obtained as colorless liquid (15.6 g, 55%, bp
52 °C/533 Pa). 1H NMR (CDCl3, rt): δ 0.30 (s, 6H, Si(CH3)2C(CH3)3),
0.96 (s, 9H, Si(CH3)2C(CH3)3), 1.92 (dd, 3H, CH2dCꢀCH3), 5.56 (m,
1H, cis CH2dCꢀCH3), 6.09 (m, 1H, trans CH2dCꢀCH3). 13C NMR
(CDCl3, rt): δ ꢀ4.8 (Si(CH3)2C(CH3)3), 17.8 (Si(CH3)2C(CH3)3), 18.5
(CH2dCꢀCH3), 25.7 (Si(CH3)2C(CH3)3), 126.1 (CH2dCꢀCH3),
137.9 (CH2dCꢀCH3), 167.7 (CO2Si(CH3)2C(CH3)3).
Triphenylsilyl Methacrylate (TPSMA). Sodium methacrylate
(10.1 g, 0.102 mol) was dispersed in dry THF (56.8 mL) in the presence
of phenothiazine (20.3 mg, 10.2 mmol). Into the suspension, 0.832 M
triphenylsilyl chloride solution (102 mL, 0.849 mol) was added drop-
wise at 0 °C over a period of 30 min under stirring. After stirring at
ambient temperature for an additional 48 h, the solution was evaporated,
washed with n-hexane and ethyl acetate, and filtrated to remove sodium
chloride. After evaporation, the obtained crude product was purified by
recrystallization in n-hexane (20.3 g, 70%). And then the crude TPSMA
(12.0 g) product was purified by column chromatography on silica gel
with CHCl3 as an eluent. TPSMA was obtained as white solid (7.71 g,
64%). 1H NMR (CDCl3, rt): δ 2.00 (m, 3H, CH2dCꢀCH3), 5.67 (m,
1H, cis CH2dCꢀCH3), 6.31 (m, 1H, trans CH2dCꢀCH3), 7.32ꢀ7.42
(m, 9H, m, p-ArH), 7.63ꢀ7.69 (d, 6H, o-ArH). 13C NMR (CDCl3, rt): δ
18.5 (CH2dCꢀCH3), 127.2 (CH2dCꢀCH3), 128.1, 130.7, 132.4, and
135.8 (phenyl), 137.4 (CH2dCꢀCH3), 167.0 (CO2SiPh3).
Triethylsilyl Methacrylate (TESMA). Synthesis of TESMA was
conducted in a similar way to that of TBDMSMA by replacing TBDMS-
Cl with triethylsilyl chloride (TES-Cl). Sodium methacrylate (17.9 g,
0.166 mol) was dispersed in dry THF (92.6 mL) in the presence of 2,6-
di-tert-butyl-p-cresol (36.6 mg, 0.166 mmol). Into the suspension, 2.70
M TES-Cl solution (25.3 mL, 0.149 mol) was added dropwise at 0 °C
over a period of 30 min under stirring. After stirring at ambient
temperature for an additional 18 h, the solution was evaporated, washed
with n-hexane, and filtrated to remove sodium chloride. After evapora-
tion, the crude product was obtained and purified by distillation from
calcium hydride under reduced pressure. TESMA was obtained as color-
Tris(trimethylsilyl)silyl Methacrylate (TTMSSMA). Sodium
methacrylate (7.32 g, 67.7 mmol) was dispersed in dry THF (37.7 mL)
in the presence of phenothiazine (135 mg, 0.677 mmol). Into the
suspension, 0.834 M tris(trimethylsilyl)silyl chloride solution (67.8 mL,
56.4 mmol) was added dropwise at 0 °C over a period of 30 min under
stirring. After stirring at ambient temperature for an additional 2 h, the
solution was evaporated, washed with n-hexane, and filtrated to remove
sodium chloride. After evaporation, the crude product was obtained
purified by distillation with calcium hydride under reduced pressure.
TTMSSMA was obtained as colorless liquid (11.0 g, 56%, bp 106 °C/
133 Pa). 1H NMR (CDCl3, rt): δ 0.22 (s, 27H, Si(Si(CH3)3)3), 1.91 (m,
3H, CH2dCꢀCH3), 5.55 (m, 1H, cis CH2dCꢀCH3), 6.00 (m, 1H,
trans CH2dCꢀCH3). 13C NMR (CDCl3, rt): δ 0.01 (Si(Si(CH3)3)3),
18.9 (CH2dCꢀCH3), 125.5 (CH2dCꢀCH3), 137.6 (CH2dCꢀCH3),
169.3 (CO2Si(Si(CH3)3)3).
1
less liquid (17.9 g, 60%, bp 77 °C/666 Pa). H NMR (CDCl3, rt):
δ 0.76ꢀ0.84 (q, 6H, Si(CH2CH3)3), 0.93ꢀ1.03 (t, 9H, Si(CH2CH3)3),
1.93 (dd, 3H, CH2dCꢀCH3), 5.58 (m, 1H, cis CH2dCꢀCH3), 6.11
(m, 1H, trans CH2dCꢀCH3). 13C NMR (CDCl3, rt): δ 4.7
(Si(CH2CH3)3), 6.7 (Si(CH2CH3)3), 18.5 (CH2dCꢀCH3), 126.1
(CH2dCꢀCH3), 137.7 (CH2dCꢀCH3), 167.7 (CO2Si(CH2CH3)3).
Triisopropylsilyl Methacrylate (TIPSMA). Synthesis of TES-
MA was conducted in a similar way to that of TBDMSMA by replacing
TBDMS-Cl with triisopropylsilyl chloride (TIPS-Cl). Sodium metha-
crylate (12.3 g, 0.114 mol) was dispersed in dry THF (65.4 mL) in the
presence of 2,6-di-tert-butyl-p-cresol (20.5 mg, 0.114 mmol). Into the
suspension, 2.87 M TIPS-Cl solution (36.3 mL, 0.104 mol) was added
dropwise at 0 °C over a period of 30 min under stirring. After stirring at
ambient temperature for an additional 25 h, the solution was evaporated,
washed with n-hexane, and filtrated to remove sodium chloride. After
evaporation, the crude product was obtained and purified by distillation
from calcium hydride under reduced pressure. TIPSMA was obtained as
colorless liquid (14.1 g, 57%, bp 78 °C/533 Pa). 1H NMR (CDCl3, rt):
δ 1.04ꢀ1.09 (s, 18H, Si(CH(CH3)2)3), 1.21ꢀ1.39 (m, 3H, Si(CH-
(CH3)2)3), 1.95 (dd, 3H, CH2dCꢀCH3), 5.59 (m, 1H, cis
CH2dCꢀCH3), 6.14 (m, 1H, trans CH2dCꢀCH3). 13C NMR
(CDCl3, rt): δ 12.1 (Si(CH(CH3)2)3), 17.9 (Si(CH(CH3)2)3), 18.6
(CH2dCꢀCH3), 126.1 (CH2dCꢀCH3), 137.8 (CH2dCꢀCH3),
167.4 (CO2Si(CH(CH3)2)3).
General Procedure for Conventional Radical Polymeriza-
tion. Polymerization was carried out by the syringe technique under dry
argon or nitrogen in sealed glass tubes. A typical example for polymer-
ization of TBDMSMA with AIBN in toluene is given below. In a 50 mL
round-bottomed flask were placed toluene (4.65 mL), TBDMSMA
(1.61 mL, 7.00 mmol), and 1,2,3,4-tetrahydronaphthalene (0.39 mL) as
an internal standard and toluene solutions of AIBN (0.35 mL of 100 mM
solution in toluene) at room temperature. The total volume of the
reaction mixture was 7.0 mL. Immediately after mixing, aliquots (1.0 mL
each) of the solution were distributed via a syringe into baked glass
tubes, which were then sealed by flame under a nitrogen atmosphere.
The tubes were immersed in thermostatic oil bath at 60 °C. In pre-
determined intervals, the polymerization was terminated by the cooling
of the reaction mixtures to ꢀ78 °C. Monomer conversion was deter-
1
mined from the concentration of residual monomer measured by H
tert-Butyldiphenylsilyl Methacrylate (TBDPSMA). Synthesis
of TBDPSMA was conducted in a similar way to that of TBDMSMA by
replacing TBDMS-Cl with tert-butyldiphenylsilyl chloride (TBDPS-Cl).
Sodium methacrylate (10.6 g, 98.1 mmol) was dispersed in dry THF
(55.0 mL) in the presence of phenothiazine (0.195 g, 98.1 mmol). In to
the suspension, 2.88 M TBDPS-Cl solution (30.7 mL, 88.4 mmol)
was added dropwise at 0 °C over a period of 30 min under stirring.
NMR with 1,2,3,4-tetrahydronaphthalene as an internal standard (30 h,
93%). The quenched reaction solutions were evaporated to dry to give
poly(TBDMSMA).
General Procedure for RAFT Polymerization. RAFT poly-
merization was carried out by the syringe technique under dry argon or
nitrogen in sealed glass tubes. A typical example for polymerization of
9110
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Table 1. Free Radical Polymerization of Various Silyl Methacrylatesa
poly(silyl methacrylate)
converted PMMAf
convc (%)
Mn
Mw/Mn
Tge (°C)
Mn
Mw/Mn
rr/mr/mmg
Tge (°C)
d
d
d
d
entry
silyl group
temp (°C)
time (h)
1
2b
TMS
60
20
60
80
60
20
60
80
60
60
20
40
60
80
30
72
87
92
93
89
88
88
68
34
80
93
93
90
66
8
28 200
46 100
30 600
17 000
30 900
28 000
18 000
8 300
2.42
2.47
2.77
2.39
2.42
2.00
2.08
1.99
2.99
4.17
n.d.h
n.d.h
n.d.h
n.d.h
90
140
146
137
74
26 700
38 800
23 500
13 600
26 700
20 600
11 300
6 500
2.41
2.82
3.13
2.53
2.41
2.08
2.21
1.91
2.37
2.41
2.96
2.94
3.10
63.7/31.6/4.7
73.5/23.5/3.0
67.4/29.3/3.3
63.7/31.7/4.6
59.5/36.2/4.3
29.1/49.0/21.9
31.4/50.6/18.3
32.5/49.7/17.8
35.9/49.9/14.2
46.6/44.9/8.5
8.4/9.5/82.1
124
126
122
118
120
100
102
95
TBDMS
TBDMS
TBDMS
TES
3
30
4
24
5
6b
30
TIPS
580
80
115
87
7
TIPS
8
TIPS
14
91
9
TBDPS
TPS
145
30
23 400
32 800
n.d.h
n.d.h
n.d.h
131
161
16 200
40 400
50 700
92 000
34 900
107
116
56
10
11b
12
13
14
TTMSS
TTMSS
TTMSS
TTMSS
280
80
4.3/6.2/89.5
58
56
1.4/5.2/93.4
54
56
n.d.h
a [Silyl methacrylate]0 = 1.0 M, [AIBN]0 = 5.0 mM, in toluene. b [Silyl methacrylate]0 = 1.0 M, [V-70]0 = 5.0 mM, in toluene. c By 1H NMR. d By SEC
using PMMA standard. e By DSC. f PMMA converted from the poly(silyl methacrylate). g By 13C NMR. h Insoluble polymer.
TBDMSMA with AIBN in the presence of CDB is given below: in a
50 mL round-bottomed flask were placed toluene (3.98 mL) as an
internal standard, TBDMSMA (1.38 mL, 6.00 mmol), and toluene
solutions of AIBN (0.30 mL of 100 mM solution in toluene) and CDB
(0.34 mL of 178 mM solution in toluene) at room temperature. The
total volume of the reaction mixture was 6.0 mL. Immediately after
mixing, the solution was evenly charged in six glass tubes, and the tubes
were sealed by flame under a nitrogen atmosphere. The tubes were
immersed in thermostatic oil bath at 60 °C. In predetermined intervals,
the polymerization was terminated by the cooling of the reaction
mixtures to ꢀ78 °C. Monomer conversion was determined from the
concentration of residual monomer measured by 1H NMR with toluene
as an internal standard (32 h, 96%). The quenched reaction solutions
were evaporated to dry to give poly(TBDMSMA).
Transformation of Poly(silyl methacrylate) into Poly-
(methacrylic acid) and Poly(methyl methacrylate). The ob-
tained poly(silyl methacrylate)s were converted into poly(methacrylic
acid) (PMAA) and poly(methyl methacrylate) (PMMA). A typical
example for poly(TBDMSMA) by acid hydrolysis of the silyl group is
as follows: a portion of the obtained poly(TBDMSMA) (216 mg, Mn =
30 600, Mw/Mn = 2.77) was dispersed in CH3OH (20 mL) containing a
small amount of hydrochloric acid (11 M, 1 mL), and the solution was
refluxed for 24 h. After concentrating it by evaporation, the product was
washed with Et2O and dried in vacuo at room temperature overnight to
give the PMAA (75.5 mg, 98% yield). For poly(TMSMA) and poly-
(TESMA), hydrolysis reaction was continued for 24 h. For poly-
(TIPSMA) and poly(TBDPSMA), hydrolysis reaction was continued
for 72 h.
PMAA thus obtained was dissolved in 10 mL of a toluene/CH3OH
mixture (4/1 vol), and then an Et2O solution of trimethylsilyldiazo-
methane (2.0 M, 2.50 mL) was added. After 12 h, the methylation was
quenched by adding a small amount of acetic acid. The mixture was
washed with distilled water and evaporated to dryness under reduced
pressure and then vacuum-dried to give PMMA (84.8 mg, 85% yield, Mn
= 23 500, Mw/Mn = 3.13).
In contrast, the silyl groups in poly(TPSMA) and poly(TTMSSMA)
were deprotected by TBAF. A typical example for poly(TTMSSMA) by
deprotection of the silyl group followed by methylation with trime-
thylsilyldiazomethane is as follows: a portion of the obtained poly-
(TTMSSMA) (494 mg) was freeze-dried with benzene, and an THF
solution of TBAF (1.0 M, 5.00 mL) was added dropwise at 0 °C over a
period of 10 min under stirring. The solution was stirring at 40 °C for an
additional 24 h. The deprotection was quenched by adding methanol
(5.00 mL). After concentrating it by evaporation, the product was
dissolved in 20 mL of a toluene/CH3OH mixture (4/1 vol), and then an
Et2O solution of trimethylsilyldiazomethane (2.0 M, 5.00 mL) was
added. After 12 h, the methylation was quenched by adding a small
amount of acetic acid. The mixture was washed with distilled water,
evaporated to dryness under reduced pressure, and then vacuum-dried
to give PMMA containing a small amount of the residual silyl fluoride
(159 mg, Mn = 34 900, Mw/Mn = 3.10).
Polymer samples for NMR analysis were fractionated by preparative
size-exclusion chromatography (SEC) (column: Shodex K-2002) to be
free from low molecular weight compounds without loss of MMA
oligomers if present.
Measurements. Monomer conversion was determined from the
concentration of residual monomer measured by 1H NMR spectroscopy
with 1,2,3,4-tetrahydronaphthalene or toluene as an internal standard.
1H NMR spectra for monomer conversion were recorded in CDCl3 at
25 °C on a Varian Mercury 300 spectrometer, operating at 300 MHz. 1H
and 13C NMR spectra of the obtained polymers were recorded in CDCl3
at 55 °C on a JEOL ECS-400 spectrometer, operating at 400 and 100
MHz for 1H and 13C, respectively. The triad tacticity of the polymer was
determined by the area of the α-methyl protons at 0.8ꢀ1.3 ppm in the
1H NMR spectrum or carbonyl CdO carbons at 175ꢀ180 ppm in the
13C NMR spectrum of the side chain.
The number-average molecular weight (Mn) and weight-average
molecular weight (Mw) of the product polymers were determined by
SEC in CHCl3 at 40 °C on two polystyrene gel columns [Shodex K-805
(poresize: 20ꢀ1000 Å; 8.0mm i.d. ꢁ 30 cm) ꢁ 2; flow rate 1.0mL/min]
connected to Jasco PU-2080 precision pump and a Jasco RI-2031
detector. The columns were calibrated against 8 standard PMMA
samples (Shodex; Mp = 875ꢀ1 950 000; Mw/Mn = 1.02ꢀ1.09).
The glass-transition temperature (Tg: midpoint of the transition) of
the polymers was recorded on Q200 differential scanning calorimetry
(TA Instruments Inc.). Certified indium and sapphire were used for
temperature and heat flow calibration. For poly(silyl methacrylate)s
and PMMA, samples were first heated to 180 at 10 °C/min, equilibrated
at this temperature for 5 min, and cooled to ꢀ50 at 10 °C/min.
After being held at this temperature for 5 min, the sample was then
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dx.doi.org/10.1021/ma202155b |Macromolecules 2011, 44, 9108–9117