RAFT polymerization of a novel allene-derived asymmetrical divinyl monomer: A facile strategy to alkene-functionalized hyperbranched vinyl polymers with high degrees of branching

Article abstract of DOI:10.1002/pola.26693

Hyperbranched vinyl polymers with high degrees of branching (DBs) up to 0.43 functionalized with numerous pendent allene groups have been successfully prepared via reversible addition fragmentation chain transfer polymerization of a state-of-art allene-derived asymmetrical divinyl monomer, allenemethyl methacrylate (AMMA). The gelation did not occur until high monomer conversions (above 90%), as a result of the optimized reactivity difference between the two vinyl groups in AMMA. The branched structure was confirmed by a combination of a triple-detection size exclusion chromatography (light scattering, refractive index, and viscosity detectors) and detailed ¹H NMR analyses. A two-step mechanism is proposed for the evolution of branching according to the dependence of molecular weight and DB on monomer conversion. Controlled radical polymerization proceeds until moderate conversions, mainly producing linear polymers. Subsequent initiation and propagation on the polymerizable allene side chains as well as the coupling of macromolecular chains generate numerous branches at moderate-to-high monomer conversions, dramatically increasing the molecular weight of the polymer. AMMA was also explored as a new branching agent to construct poly(methyl methacrylate)-type hyperbranched polymers by its copolymerization with methyl methacrylate. The DB can be effectively tuned by the amount of AMMA, showing a linear increase trend. The pendent allene groups in the side chains of the copolymers were further functionalized by epoxidation and thiol-ene chemistry in satisfactory yields.

Full text of DOI:10.1002/pola.26693

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RAFT Polymerization of a Novel Allene-Derived Asymmetrical Divinyl
Monomer: A Facile Strategy to Alkene-Functionalized Hyperbranched
Vinyl Polymers with High Degrees of Branching
1
,2
1,2
3
1
Youmei Bao, Guorong Shen, Xiaohui Liu, Yuesheng Li
1
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun 130022, China
2
University of the Chinese Academy of Sciences, Changchun Branch, Changchun 130022, China
3
School of Materials Science and Engineering, Key Laboratory of Hollow Fiber Membrane Material and Processes of Ministry of
Education, Tianjin Polytechnic University, Tianjin 300387, China
Correspondence to: Y. Li ( E-mail: ysli@ciac.jl.cn)
Received 16 October 2012; accepted 13 March 2013; published online
DOI: 10.1002/pola.26693
ABSTRACT: Hyperbranched vinyl polymers with high degrees
of branching (DBs) up to 0.43 functionalized with numerous
pendent allene groups have been successfully prepared via
reversible addition fragmentation chain transfer polymeriza-
polymerizable allene side chains as well as the coupling of
macromolecular chains generate numerous branches at mod-
erate-to-high monomer conversions, dramatically increasing
the molecular weight of the polymer. AMMA was also
explored as a new branching agent to construct poly(methyl
methacrylate)-type hyperbranched polymers by its copoly-
merization with methyl methacrylate. The DB can be effec-
tively tuned by the amount of AMMA, showing a linear
increase trend. The pendent allene groups in the side chains
of the copolymers were further functionalized by epoxidation
tion of
a state-of-art allene-derived asymmetrical divinyl
monomer, allenemethyl methacrylate (AMMA). The gelation
did not occur until high monomer conversions (above 90%),
as a result of the optimized reactivity difference between the
two vinyl groups in AMMA. The branched structure was con-
firmed by a combination of a triple-detection size exclusion
chromatography (light scattering, refractive index, and vis-
and thiol-ene chemistry in satisfactory yields. V
2013 Wiley
C
1
cosity detectors) and detailed H NMR analyses. A two-step
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013,
00, 000–000
mechanism is proposed for the evolution of branching
according to the dependence of molecular weight and DB on
monomer conversion. Controlled radical polymerization pro-
ceeds until moderate conversions, mainly producing linear
polymers. Subsequent initiation and propagation on the
KEYWORDS: allene; functionalized; hyperbranched; living radical
polymerization (LRP); radical polymerization; reversible addi-
tion fragmentation chain transfer (RAFT)
INTRODUCTION Dendritic and hyperbranched structures are
of particular interest in polymer science because of their
promising use in various industrial applications owing to
inherent specific characteristics when compared with linear
polymerization (SCVP) from a monomer called AB* inimer
(initiator monomer), where A stands for a vinyl group and
1
0–12
B* is an initiating site.
A general feature of SCVP is the
unequal reactivities between chain growth of the vinyl
group and step growth through the initiating site, which
results in strong dependence of degrees of branching (DBs)
on the reactivity difference between A* and B* end
1
–5
counterparts.
For example, their unusual globular and
highly branched structure endow them more compactness
while providing a high density of functional sites on the pe-
riphery. Considering that vinyl polymers are important syn-
thetic materials, it will undoubtedly bring some particular
material properties when three-dimensional branched
architecture is introduced into vinyl polymers, especially
1
3
groups. Meanwhile, tremendous efforts were required to
design and synthesize the expensive and tailored inimers
with special functional groups. Alternatively, free radical
copolymerizations of a monofunctional vinyl monomer with
a multifunctional vinyl comomoner have also been per-
6
–9
those reactive polymers with functional groups.
Origi-
1
4–18
nally, Fr ꢀe chet et al. developed a one-pot synthetic route to
formed to prepare hyperbranched polymers.
Unfortu-
hyperbranched vinyl polymers, self-condensing vinyl
nately, the methodology showed limitations in the synthesis
Additional Supporting Information may be found in the online version of this article.
2013 Wiley Periodicals, Inc.
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of well-defined branched structures despite the use of chain
transfer agents (CTAs), because of its easy gelation at high
monomer conversion and concentration originated from
the equal reactivity of the multiple vinyl groups in the
asymmetric divinyl monomer and highly branched polymer.
Additionally, RAFT technique is a widely used living/
controlled polymerization method, including as a powerful
tool to obtain soluble polymers from (a)symmetric divinyl
1
5
30,40–42
comonomer.
In addition, applications of the resultant
monomers.
Wooley and coworkers announced a facile
hyperbranched vinyl polymers synthesized from the two
methods mentioned above were limited by lack of func-
tional groups.
approach to synthesize well-defined linear polymers bearing
pendent vinyl functionalities via selective RAFT polymeriza-
tion. Using asymmetrical divinyl monomers with
a
fluorinated styrene group, preferential polymerizations to-
ward high-selectivity styrene group were achieved, and the
well-defined linear polymers with multiple pendent alkene
or cycloalkenyl groups were obtained. However, there are
rather rare allene-derived, especially branched, polymers
being synthesized by RAFT polymerization technique
till now.
Recently, we have noticed the interesting radical polymeriza-
tion behaviors of a vinyl monomer containing two vinyl
groups with different reactivities (reactivity ratio, r
1
/r
2
) to-
1
9–30
ward radical polymerization.
This kind of monomers are
termed as asymmetric divinyl monomers or AB-type mono-
mers, where A represents the vinyl group with higher reac-
tivity and B is the other lower-reactive vinyl group. When
the reactivity difference between the two vinyl groups (r
1
/
In this contribution, we designed and synthesized a state-of-
art allene-derived asymmetrical divinyl monomer, allene-
methyl methacrylate (AMMA). Further RAFT polymerization
behaviors of this particular monomer were investigated.
Because of the two polymerization pathways of allene group
and relatively low reactivity difference between methacrylate
and allene groups (r /r 5 54), enhanced branching degrees
are expected for the resultant reactive polymers, substan-
tially decorated with abundant allene groups, which are
potential for further functionalization through various inter-
esting reactions related to allene species and alkenes. More-
over, we attempted to modulate the branching degree of
hyperbranched poly(methyl methacrylate) (PMMA) via
copolymerization of methyl methacrylate (MMA) with AMMA
added as a branching agent. The hyperbranched architec-
tures and evolution process were confirmed by kinetic
r
2
) is too large, linear polymers with intact B as pendent
groups are obtained. On the contrary, radical polymerization
of an AB-type monomer bearing two vinyl groups with simi-
lar reactivity (also called symmetric divinyl monomer) lead
to crosslinked materials. Most attractively, moderate reactiv-
ity difference between the two vinyl groups in an AB-type
monomer favors highly soluble polymers with highly
branched structures through controlled/living radical poly-
merization (LRP). We have well designed a series of AB-type
asymmetric divinyl monomers consisting of two vinyl groups
with moderate reactivity difference toward radical polymer-
1
2
1
9,21,22
ization.
By using an important controlled/LRP tech-
nique of reversible addition fragmentation transfer (RAFT),
we have obtained a series of hyperbranched vinyl polymers
with low to moderate DBs. When compared with the afore-
mentioned two methods, this technique can yield soluble
hyperbranched vinyl polymers at much higher monomer
conversions, in addition, retaining plentiful alkene groups for
accessible postmodification to functional materials. It has
been proved that DBs of the polymers increased with declin-
ing r /r ratio. However, syntheses of hyperbranched poly-
experiments and characterized by combination of triple-
1
detection size exclusion chromatography (tri-SEC) and
H
NMR techniques. Accordingly, a polymerization mechanism
for the formation of branchings was proposed. Finally, the
allene pendants in the side chains were postfunctionalized
by epoxidation and thiol-ene chemistry.
1
2
mers with further enhanced DB at high monomer conversion
by this method were still limited because of the much easier
occurrence of crosslinking at low r /r ratio, which is also a
EXPERIMENTAL
Materials
1
2
great challenge to the molecular design.
MMA (99%) was purchased from Aldrich and purified by
passing through a column filled with basic alumina to
remove the inhibitors and antioxidants. Triethylamine was
refluxed over CaH for 24 h and distilled before use. Toluene
2
and diethyl ether was purified by an Mbraun solvent purifi-
cation system (M. Braun, Germany). 2,2-Azodiisobutyronitrile
Allene derivatives containing cumulated double bonds, as
special vinyl monomers, can be regarded as isomers of
propargyl derivatives. Their polymerization behaviors have
3
1–36
been investigated in detail.
workers developed a living coordination polymerization of
allene derivatives using [(p-allyl)NiOCOCF ] /PPh catalytic
For example, Endo and co-
(
AIBN) was recrystallized from methanol and stored in re-
frigerator before use. 2-(2-Cyanopropyl)dithiobenzoate
CPDB) was synthesized according to the literature proce-
3
2
3
system, which generally produced the polymers with both
(
3
2–36
4
3
1,2- and 2,3-polymerized units.
Allene derivatives with
dures. But-3,4-Dien-1-ol was prepared by the literature
method.
3
7
38
4
4–47
various substituents such as allenyl ethers, phenylallene,
Other reagents were used as received without
3
9
and N-allenylpyrrolidone have been found to undergo radi-
further purification.
cal polymerizations. All studies indicated that both 1,2- and
2
,3-polymerization products were present in various poly-
Measurements
1
merization results of allene derivatives. Therefore, the attrac-
tive polymerization tendency of allene derivatives may be of
great significance to construct a characteristic AB-type
H NMR spectra were recorded on a Bruker 300-MHz spec-
1
3
trometer. C NMR spectra were recorded with a Bruker
600 spectrometer operating at 150 MHz. In all spectra,
2
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CDCl was used as a solvent and tetramethylsilane was used
H NMR (300 MHz, CDCl ): d (ppm): 5.30 (s, @CHA),
3
3
as an internal standard. FTIR spectra were recorded on a
Bio-Rad FTS-135 spectrophotometer. Elemental analysis was
determined with an Elementar Analysensysteme GmbH Vari-
oEL (Germany) elemental analyzer. Weight-average molecular
2 2
4.91(s, @CH ), 4.70–4.16 (br, AOCH A), 3.12 (m, ACHA in
the side chain), 2.31–2.08 (br, ACH A in the side chain),
2
2.08–1.19 (br, ACH A in the main chain), 1.19–0.72 (br,
2
1
3
ACH3). C NMR (100 MHz, CDCl3): d (ppm): 210.1
(ACH@C@CH ), 177.2 (ACH OCOA), 128.7–124.7 (ACH@C
weights (M s) and polydispersity indices (PDIs) of the
w
2
2
(
1
co)polymers were measured by tri-SEC containing a Waters
525 separation module (Waters Corp.) connected with
in the internal double bond), 85.7 (ACH@C@CH ), 77.3
2
(ACH@C@CH ), 63.6–62.1 (ACH OCOA), 55.3–51.9 (ACH A
2
2
2
M302 triple-detector array (Viscotek Corp., Houston, TX), a
in the backbone), 46.0–44.4 (quaternary carbon in the back-
ꢀ
combination of light scattering [LS angle, 7 low angle light
bone), 18.2–15.9 (ACH in the backbone).
3
ꢀ
scattering (LALS) and 90 right-angle light scattering (RALS),
Copolymerization of MMA and AMMA
laser wavelength, k 5 670 nm], refractive index (RI, k 5 670
nm), and viscosity detectors [IV-differential pressure (DP)].
Two mixed-bed SEC columns (GMHHR-M with molecular
A typical copolymerization reaction of MMA and AMMA was
as follows. MMA (1.0 g, 10.0 mmol), AMMA (138.2 mg,
1.0 mmol), CPDB (22.1 mg, 0.1 mmol), AIBN (16.4 mg,
0.1 mmol), and toluene (1 mL) with the predetermined ratio
of [MMA]:[AMMA]:[AIBN]:[CPDB] 5 100:10:1:1 were mixed
in a Schlenk polymerization tube. The mixture was degassed
by conventional freezing–thawing technique to remove resid-
ual oxygen and charged with nitrogen before sealed. The sys-
6
weight range <1.0 3 10 Da, and GMHHR-H with molecular
5
weight range <1.0 3 10 Da; Viscotek Corp.) were used. For
the SEC analyses, HPLC-grade tetrahydrofuran was used as
ꢀ
the mobile phase at 25 C (flow rate: 1 mL/min). The analy-
ses were based on narrow polystyrene standards.
Synthesis of AMMA
ꢀ
tem was then maintained at 60 C for the desired reaction
The asymmetrical methacrylate-type divinyl monomer was
prepared via an oxa-acylation reaction reaction. But-3,4-dien-
time. Finally, the polymerization was ceased by liquid nitro-
gen. The produced viscous mixture was diluted with ethyl
acetate, and a large amount of methanol was added to sedi-
mentate the product. The resultant polymers were filtered
and dried in vacuum at room temperature. The monomer
conversions were determined gravimetrically.
1
-ol (2.88 g, 41.1 mmol), triethylamine (8.6 mL, 61.6 mmol),
and diethyl ether (10 mL) were combined in a predried
0-mL flask with a mechanical stirrer. Methacryloyl chloride
5
(
4 mL, 61.6 mmol) in 20 mL of Et O was added dropwise to
2
ꢀ
the mixture with stirring at 220 C over 1 h. The resultant
white suspension was maintained at room temperature over-
night to ensure a complete reaction. After filtration, the fil-
trate was washed with 10 wt % NaOH solution, saturated
NaCl, and distilled water several times. The organic layer
was collected and dried over anhydrous magnesium sulfate.
After evaporation of the solvent, the orange mixture was dis-
tilled under reduced pressure to yield a colorless liquid.
Yield: 4.8 g (84%).
1
3
H NMR (300 MHz, CDCl ): d (ppm): 7.94–7.85 (d, ortho
@CHA in the benzene ring of the CPDB unit), 7.57–7.50 (t,
meta @CHA in the benzene ring of the CPDB unit), 7.42–
7.33 (t, para @CHA in the benzene ring of the CPDB unit),
5.38–5.22 (m, @CHA in the allene unit), 4.90 (s, @CH
the allene unit), 4.51 (s, ACH OCOA), 3.68 (s, AOCH in the
MMA unit), 2.89 (s, ACHA in the allene unit), 2.56–2.42 (m,
ACH A in the allene unit), 2.11–1.71 (br, ACH A in the
backbone), 1.52–1.19 (br, ACH in the CPDB unit), 1.12–0.85
in
2
2
3
2
2
3
1
1
3
H NMR (300 MHz, CDCl ): d (ppm): 6.08 (q, 1H), 5.52 (q,
3
(m, ACH3 in the backbone of MMA and AMMA). C NMR
100 MHz, CDCl ): d (ppm): 209.8 (ACH@C@CH ), 177.5
1
2
2
H), 5.33–5.23 (q, 1H), 4.82–4.79 (m, 2H), 4.62–4.58 (m,
(
3
2
1
3
H), 1.90 (t, 3H). C NMR (100 MHz, CDCl ): d (ppm):
3
(ACH OCOA), 85.1 (ACH@C@CH ), 76.3 (ACH@C@CH ),
2 2 2
08.85, 166.25, 124.82, 85.56, 75.75, 61.47, 17.42. ELEM.
5
3.9 (ACH OCOA), 51.2 (AOCH in the MMA unit), 44.4 (the
2 3
ANAL. Calcd. for C H O : C, 69.54; H, 7.30; O, 23.16. Found:
8
10
2
quaternary carbon in the backbone), 42.0 (ACHA in the
C, 69.43; H, 7.36; O, 23.21.
backbone), 18.2, 15.9 (ACH in the backbone).
3
Homopolymerization of AMMA
Epoxidation of the Allene Pendants in the Copolymers
The pendent allene end groups in the resultant hyper-
branched copolymers can be directly epoxidized by the fol-
A typical homopolymerization reaction of AMMA was as fol-
lows. AMMA (1.0 g, 7.2 mmol), CPDB (31.8 mg, 0.072 mmol),
AIBN (11.9 mg, 0.072 mmol), and toluene (1 mL) with the
predetermined ratio of [M]:[I]:[CTA] 5 100:1:3 were mixed in
a Schlenk polymerization tube. The mixture was degassed by
conventional freezing–thawing technique to remove residual
oxygen and charged with nitrogen before sealed. The system
lowing steps: 0.2 g of the polymers (M 5 28.6 kDa,
w
PDI 5 2.14; Entry 3 in Table 2), 0.5 g of m-chloropreoxyben-
zoic acid (CPBA, 3.0 mmol), and 5 mL of toluene were mixed
ꢀ
in a flask. The system was sealed and heated at 70 C for 24
h. Toluene was then removed under reduced pressure, and
methanol was used to wash out the residual CPBA. After fil-
tration and vacuum drying, a white epoxidized product was
obtained.
ꢀ
was then maintained at 80 C for the desired reaction time.
The polymerization was ceased by liquid nitrogen. The pro-
duced viscous mixture was diluted with ethyl acetate and
precipitated in a large amount of methanol. The resultant
polymers were filtered and dried in vacuum at room temper-
ature. The monomer conversions were determined gravimet-
rically. Yield: 0.96 g (96%).
1
H NMR (300 MHz, CDCl
): d (ppm): 4.90 (ACOOCH
), 4.52 (ACH@COCH ), 2.32
solution): 805 (mCAOAC).
A in
3
2
allene unit), 4.63 (ACH@COCH
(ACOCH
2
2
2
1
A). FTIR (cm , CH
Cl
2
2
2
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The synthetic route to the AMMA monomer in this contribu-
tion is depicted in Scheme 1. First, but-3,4-dien-1-ol was
4
4–47
obtained according to the literature procedures,
follow-
ing two sequential steps in the presence of BuLi/HCHO and
LiAlH , respectively. The subsequent acylation reaction of
4
but-3,4-dien-1-ol with methacryloyl chloride afforded the
1
desired monomer. The H NMR spectra of the product clearly
confirmed the structure of the monomer, where the charac-
teristic peaks a and b located at 4.79–4.82 and 5.23–5.33
ppm, respectively, were attributed to the allene protons. The
1
3
other protons are also clearly assigned in Figure 1. C NMR
Fig. 2) further proved the exact structure of the expected
(
asymmetrical divinyl monomer (ADM). More direct evidence
was given by the elemental analysis. In combination of the
elemental analysis and H NMR as well as C NMR techni-
ques, the structure of the monomer was firmly established,
indicating successful preparation of the desired monomer.
SCHEME 1 Synthetic route to allene-functionalized hyper-
branched vinyl polymers via RAFT polymerization of a novel
allene-derived asymmetrical divinyl monomer of allenemethyl
methacrylate (AMMA).
1
13
Synthesis and Characterization of Polymers via RAFT
Polymerization of AMMA
Hydroxy Functionalization of the Copolymers by Thiol-Ene
Chemistry
Hydroxy (OH)-functionalization of the vinyl pendants in the
polymers can be achieved by UV-initiated thiol-ene addition
reaction. As a typical example, 0.2 g of the copolymers
Theoretically, successful RAFT process for a given monomer
depends on the structures of both Z and R groups of the
CTA. In this study, CPDB was used as the CTA to mediate the
RAFT radical polymerization of AMMA because of its widely
5
0–52
(
1
(
Entry 3 in Table 2), 1.0 g of 2-mercaptoethanol (12.8 mmol),
0.0 mg of 2,2-dimethoxy-2-phenyl-acetophenone (DMPA)
0.039 mmol), and 2 mL of toluene were mixed in a vial. The
reported control over MMA and its derivatives.
The
hyperbranched structure of the resultant polymers can be
confirmed by tri-SEC with triple detectors including RI, light-
ꢀ
ꢀ
system was radiated under a UV bench lamp (365 nm, 6 W)
for 4 h. Then, the product was precipitated in excess petro-
leum ether and dried in vacuum at room temperature. A
light pink OH-functionalized product was obtained.
scattering [two angles, 7 (LALS) and 90 (RALS)], and vis-
cosity detectors (IV-DP). As shown in Figure 3, the SEC elu-
tion curves exhibited broad peaks with shoulders
corresponding to polymer species with different MWs.
Importantly, the higher MW indicated by light-scattering de-
1
H NMR (300 MHz, CDCl ): d (ppm): 3.85 (ACH SCH
2
3
2
tector than RI strongly supported the hyperbranched struc-
CH OH), 3.21 (ACH SCH CH OH), 2.82 (ACH SCH CH OH).
22,53,54
2
2
2
2
2
2
2
ture of the polymers.
The results were further verified
by the MW versus retention volume curves in Figure 4.
When compared with linear PMMA reference, the resulting
hyperbranched polymers had much higher MWs at the same
RESULTS AND DISCUSSION
Design and Synthesis of the Allene-Derived
Asymmetrical Divinyl Monomer
Allenes are a special class of compounds containing 1,2-pro-
padiene functional group, where the central sp-hybridized
carbon forms one r-bond and one p-bond with each of the
2
two terminal sp -hybridized carbons. Numerous investiga-
tions on polymerization methods of allenes have revealed
their interesting 1,2- and 2,3-polymerization pathways in the
same polymerization system. It promoted us to design an
allene-derived asymmetrical divinyl monomer with suitable
reactivity difference between the two vinyl groups of the
monomer, expecting to obtain novel hyperbranched vinyl
polymers with enhanced DBs and numerous allene func-
tional groups. Consequently, an AB-type monomer bearing
one methacrylate and one allene groups was designed and
synthesized with r /r 5 54. The r /r was quantitatively cal-
1
2
1
2
culated on the basis of the (Q, e) values from Alfrey-Price
theory, that is, r 5 (Q /Q ) exp[2e (e 2 e )] and r 5 (Q /
1
1
2
1
1
2
2
2
1
Q ) exp[2e (e 2 e )], where the (Q, e) values are cited from
FIGURE 1 H NMR spectra of the (A) AMMA monomer and (B)
1
2
2
1
3
1,48,49
the Polymer Handbook.
3
hyperbranched polymer in CDCl .
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3
FIGURE 2 C NMR spectra of the AMMA monomer.
retention time because of their smaller hydrodynamic
2
2,55
volumes.
Furthermore, the compact structures of the hyperbranched
polymers were confirmed by their lower intrinsic viscosities
FIGURE 4 Molecular weight versus retention volume curves
for linear PMMA and hyperbranched polymers under various
reaction temperatures.
(
IVs) than the linear PMMA. More accurately, a lower slope,
namely, Mark-Houwink exponent a, was obtained from the
plot of log IV versus log M of the resultant polymers in com-
parison with linear PMMA reference. The typical a values
listed in Figure 5 were usually below 0.5 and much
lower than that of linear PMMA (0.72), reflecting the
compact hyperbranched structures of the obtained
found, indicating that the methacrylate vinyl groups were
completely polymerized. Additionally, the broad peaks a and
b located at 4.90 and 5.29 ppm were attributed to the pro-
tons of double bond in remained allene groups, including the
totally unreacted allene unit as well as the double bonds in
1
6,22,54,56,57
polymers.
Conclusively, these results mentioned
1
,2- and 2,3-polymerization products. The methylene origi-
above powerfully established that the polymers with highly
branched structures were successfully prepared through
RAFT polymerization of AMMA mediated by CPDB.
nally located at 4.50 ppm (peak c) in the spectra of the
monomer [Fig. 1(A)] shifted to 4.19 ppm [peak c in Fig.
0
1(B)] after 2,3-polymerization of the allene group, whereas it
More information about the hyperbranched structures of the
polymers can be acquired from H NMR analyses [Fig. 1(B)].
First, a series of broad peaks in the region of 0.85–2.08 ppm
were attributed to the protons in the polymer backbone, and
no peak related to the double bond of methacrylate can be
was almost unchanged in 1,2-polymerization product. Sev-
eral peaks in the range of 1.5–4.0 ppm are most interesting.
The only peak g at 3.11 ppm in the region of 3.0–4.0 ppm
was identified as the methine group in the geminal olefin
unit, which was probably generated by continuous insertion
1
FIGURE 5 Mark-Houwink plots of hyperbranched polymers
ꢀ
ꢀ
ꢀ
synthesized at 80 C (W), 70 C (᭹), and 60 C (᭡) correspond-
ing to samples of Entries 3, 5, and 6 in Table 1 and linear
PMMA (᭺). The values beside the curves represent their Mark-
Houwink exponent a values.
FIGURE 3 Tri-SEC curves for typical hyperbranched polymers
obtained under the conditions of [AMMA]:[CPDB] 5 100:2 and
ꢀ
polymerization temperature5 80 C.
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SCHEME 2 Proposed branching mechanism of AMMA radical homopolymerization.
of two or more allene units at the late stage of the polymer-
ization because of very low content of methacrylate vinyl
group and relative much higher content of allene unit. The
other peaks around 2.0 ppm, for example, peak f and prob-
ably some peaks about 2.0 ppm overlapping with some pro-
tons from the polymer backbone, indicated the branching
types of allene–methacrylate repeating units, where the
allene vinyl group can be 1,2- or 2,3-polymerized.
TABLE 1 Kinetic Data of the RAFT Homopolymerization of
AMMA
a
b
w
b
c
1
c
2
c
Entry Time (h) Yield (%)
M
(kDa) PDI DB
DB
DB
d
d
1
2
3
4
5
6
7
8
1
14
24
42
60
82
90
92
95
–
–
0.02 0.03 0.05
1.5
2
2.1
8.5
1.19 0.04 0.05 0.09
1.66 0.08 0.09 0.17
1.72 0.15 0.11 0.26
2.54 0.15 0.15 0.30
2.53 0.18 0.18 0.36
2.81 0.22 0.18 0.40
3.08 0.19 0.24 0.43
3
12.9
38.8
1
6
The H NMR data were then used to determine the DB of
the polymers, which is one of the most useful parameters
for the characterization of hyperbranched polymers. Consid-
ering that the DB defined by Fr ꢀe chet and Frey were not ap-
plicable to this system, we quantitatively calculated the DBs
by determining the content of reacted allene groups. Never-
theless, two parts of DBs should be considered due to the
fact that both of the two double bonds in allene group can
participate in the polymerization (Scheme 2), that is, DB1
9
61.7
12
23
78.4
117.0
a
[AMMA] 5 7.2 mol/L; [AMMA]:[AIBN]:[CPDB] 5 100:1:4; solvent:
toluene.
b
M
n
s, M
w
s, and PDIs were measured by tri-SEC.
c
1
The degrees of branching (DBs) were calculated by H NMR.
d
n w
M and M were too low to be measured by tri-SEC.
6
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poly(allenemethyl methacrylate) (PAMMA) was formed at
this time with polymethacrylate as the polymer backbone
functionalized with numerous unreacted allene side chains.
As an asymmetrical divinyl monomer, AMMA contains one
quite reactive methacrylate double bond and one relatively
less reactive allene group (r /r 5 54). Because of the ini-
1
2
tially high concentration of the methacrylate group, preferen-
tial propagation occurred in the methacrylate units to form a
nearly linear macromolecular main chain with almost intact
allene pendent groups at the early stage.
With the polymerization proceeding, the monomer conver-
sion kept rising. The concentration of the methacrylate group
significantly decreased, whereas the allene groups remained
in the initially high level. Therefore, there is a great opportu-
nity for allene groups to be initiated and propagated despite
their relatively less reactivity, leading to the generation of
branching. The continuous propagation in the branching sites
and the coupling of the newly formed macroradicals eventu-
ally formed the polymers with highly branched structure,
which simultaneously resulted in considerably increased
MWs (Entries 5–8 in Table 1) and broadened MWDs. Thus,
the branching was mainly generated at the later stage of the
polymerization of AMMA, yielding highly branched polymers
with high MWs at the end of the polymerization.
FIGURE 6 Plots of M
w
(W) and DB (᭹) of the resultant poly-
mers with monomer conversion.
and DB2 originated from the 2,3 and 1,2-polymerization of
the allene group, respectively.
Ic1Ic0 22Ib
DB15
;
(1)
Ic1Ic0
Ic1Ic0 2Ia
Ic1Ic0
DB25
;
(2)
(3)
Proposed Mechanism for the Formation of Branched
Architecture
DB 5DB 11DB 2;
According to the above information, the formation mecha-
nism of branched structure was proposed as follows. At the
early-to-middle stage of the polymerization (corresponds to
conversions from 0 to 60%), a controlled radical polymeriza-
tion process was conducted as evidenced by a linear evolu-
tion of the molecular weight with conversion. The initiation
and chain growth mainly occurred on the methacrylate dou-
ble bond (Step 1 in Scheme 2), possibly along with negligible
irregular chain growth of the allene group (Step 2) and ini-
tiation of the pendent vinyl group (Step 3). Almost linear
polymers with numerous polymerizable moieties in the side
chains were correspondingly produced at this stage. As the
polymerization proceeded (>60% monomer conversion), the
pendent vinyl groups were initiated and propagated, finally
generating large amounts of branches (Steps 3 and 4). Mean-
while, the low-molecular-weight propagation chains, created
by controlled radical polymerization during the first stage,
coupled with each other. As a result, the molecular weight
sharply increased and the dispersity greatly broadened. This
coupling can also occur at lower conversion; however, the
effect on the polymer properties, especially on its molecular
weight, is more significant in the later stage. Coupling gener-
ates large compact structure of the resultant polymers.
where Ic1Ic0 represented the total integrations of the methyl-
ene group adjacent to the carboxyl group (peaks c and c ), and
0
Ib and Ia represented the integration of peaks b and a corre-
sponding to the remained internal and terminal double bonds,
respectively. Noticeably, the highest DB value turned out to be
as high as 0.43, which is much higher than those produced by
the polymerizations of 1-(3-butenyloxymethyl)24-vinylben-
zene (DB 5 0.16) and 2-(5-norbornene)methyl methacrylate
(DB 5 0.17–0.35). The significant enhancement of DB in this
contribution was probably ascribed to two factors. The first
one is the appreciably lowered reactivity difference in the
monomer. The other one is that the two vinyl groups in the
allene moiety provided more chances for branching.
Kinetic Investigations on AMMA Homopolymerization
To investigate the evolution of branched architecture, kinetic
experiments were carried out under optimized conditions.
The results are summarized in Table 1.
As shown in Figure 6, the evolution of absolute weight-aver-
age molecular weight determined by tri-SEC was plotted as a
function of monomer conversion. The linear increase of MW
with conversion was not observed for the whole polymeriza-
tion process. Nevertheless, the MW linearly increased with
monomer conversion in the initial 3 h up to 60% monomer
conversion, whereas the molecular weight distributions
Synthesis of PMMA-Type Hyperbranched Polymer with
Tunable Branching Degree by RAFT Copolymerization of
MMA with AMMA
(
MWDs) were relative narrow, which agreed with the typical
Highly branched polymethacrylates have recently offered
new perspectives in lithographic performance and drug
characteristics of a controlled/LRP. In combination with the
low DBs at this stage, it is convincing that slightly branched
5
8–63
delivery.
The control of branching remains yet
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TABLE 2 Copolymerization Results of MMA and AMMA
a
ꢀ
b
w
b
b
b
c
c
c
Entry [MMA]:[AMMA] T ( C) Time (h) Yield (%) M
(kDa) PDI [IV] (dL/g) M-Ha xDB
1
/(10 1 x) (%) xDB
2
/(10 1 x) (%) DB (%)
1
2
3
4
5
6
7
8
10:1
10:2
10:3
10:4
10:5
10:1
10:1
10:2
80
80
80
80
80
70
60
60
22
22
22
21
16
23
26
23
96
96
96
94
93
96
93
94
15.1
27.1
28.6
38.9
61.2
16.0
21.5
17.8
1.20 0.31
2.12
0.49
0.46
0.45
0.48
0.73
2.33
3.23
4.57
5.33
1.27
0
1.82
2.83
4.15
3.14
3.00
0.27
0.91
1.67
2.55
5.16
7.38
7.71
8.33
1.54
0.91
4.00
2.14 0.23
2.54
2.67 0.21
1.49
1.80 0.40
1.72
2.33
a
[
AMMA] 5 10.0 mol/L; [AMMA]:[AIBN] 5 100:1; [MMA]:[CPDB] 5 100:1;
solvent: toluene.
b
M
n
s, M
w
s, PDIs, intrinsic viscosity ([IV]), and Mark-Houwink exponent
a were measured by tri-SEC.
c
1
DBs were calculated by H NMR.
challenging and requires fundamental investigation for new
applications. Therefore, a series of PMMA-type polymers
with tunable DBs have been prepared via copolymerization
of MMA with AMMA as a branching agent. Copolymerization
experiments with different monomer feed ratios ([MMA]:
than RI detector supported the branching structure of the
copolymer. Additionally, it was further verified by the higher
MWs of the copolymers than those of the linear PMMA at
the same retention volume (see Supporting Information Fig.
S2) and low inherent viscosities as well as low a values of
the resultant polymers.
[
>
AMMA]) were out of crosslinking (monomer conversions
90%). The typical copolymerization results are listed in
1
The branched structure can be further confirmed by the H
Table 2.
NMR spectra of the copolymers (Fig. 8). As assignments of
PAMMA in the homopolymerization section, the characteris-
tic peaks of the MMA and AMMA units can be clearly
observed. No peaks originated from the double bond of the
methacrylate (d 5 5.25 and 5.90 ppm) were identified in the
spectra, indicating that the methacrylate vinyl group in
AMMA has been polymerized completely; in other words, all
of the AMMA comonomers have been incorporated into the
polymer. Although the two new peaks g and f shown in Fig-
ure 1(B) were totally overlapped with the signals from the
The MW of the resultant polymer increased and MWD broad-
ened with increasing amount of AMMA (Entries 1–5 in
Table 2). Higher AMMA feed ratio provided more branching
sites and coupling points, resulting in higher MW and DB
results.
Branching Structure Characterization of Copolymer
First, the branched structure of the obtained PMMA poly-
mers was characterized by multidetection SEC. As illustrated
in Figure 7, higher MW measured by light-scattering detector
0
backbone of the polymer, the peak c clearly demonstrated
the presence of branching in the copolymer. In the mean-
time, the copolymers were decorated with numerous pend-
ent allene groups, endowing them with potential applications
to further chemical modification and postpolymerization.
1
H NMR spectra favored the determination of DBs for the
copolymers following the same formulas (1) and (2).
FIGURE 7 Tri-SEC curves for the typical hyperbranched copoly-
1
mers (Entry 2 of Table 2).
FIGURE 8 H NMR spectra of the copolymer in CDCl
3
.
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all cases investigated, the AMMA comonomer was totally
incorporated into the copolymer. At low AMMA content, the
polymers were formed with a low density of the branching
points, and as a result, branching sites were easily triggered
because of low steric hindrance, whereas it became difficult
due to bulky steric hindrance at high AMMA content, finally
resulting in a loss-of-controlled DB.
Postfunctionalization of the Hyperbranched PMMA-Type
Polymer
Hyperbranched PMMA-type polymers with numerous pend-
ent allene groups were successfully obtained through RAFT
copolymerization of monomers MMA and AMMA. These
pendants can be further converted to other functional
groups. First, epoxidation reaction was performed by CPBA
ꢀ
under 70 C. From the FTIR spectra of the functionalized
FIGURE 9 Dependence of the DB of the copolymer on
polymers (see Supporting Information Fig. S5), the character-
istic peak at 812 cm for the epoxy group arises, whereas
the peaks at 2027 cm for allene group and 843 cm for
CAH of the terminal double bond are totally undetected.
Likewise, as shown in its H NMR spectra (Fig. 10), the sig-
nals from the terminal double bond disappear, and corre-
spondingly new peaks
characteristic of the epoxy group. These results indicate that
the reaction can be conducted quantitatively on the terminal
double bond and hardly on the internal double bond because
of the bulky steric effect and higher reactivity of the terminal
double bond.
[
AMMA]:[MMA] ratio.
2
1
2
1
21
Nevertheless, the molar fraction x of the AMMA comonomer
was necessary to be considered in the determination of the
integral DB values (Formula 4) due to the fact that the re-
sultant products were the copolymers of MMA and AMMA.
As a typical example, x 5 2 when [MMA]:[AMMA] 5 10:2.
1
c and d emerged, which are
xDB1
xDB2
DB 5
1
:
(4)
x 1 10 x 1 10
The DB of the copolymer was tunable with the AMMA como-
nomer feed ratio, showing evident increase with increasing
AMMA content, reaching 8.33% at [MMA]:[AMMA] 5 10:5
Additionally, we transformed the terminal double bonds in
the allene unit to hydroxyl group by widespread thiol-ene
chemistry. It has been shown that it was easy to undergo the
addition reaction of the terminal double bonds with 2-mer-
captoethanol, resulting in hydroxyl-functionalized PMMA.
(
Fig. 9). At the beginning, the DB increased linearly with the
amount of AMMA comonomer till [MMA]:[AMMA] 5 10:3 and
then deviated from this linear relationship with further
increased AMMA content. Considering that AMMA was added
as a branching agent, it is reasonable that high AMMA dos-
age definitely led to elevated DB. As mentioned before, for
1
The H NMR spectra of the products is shown in Figure 11.
New peaks e and c located at 2.85–2.70 ppm and 3.92–3.76
1
1
FIGURE 10 H NMR spectra of (A) PMMA-type copolymer and
FIGURE 11 H NMR spectra of (A) PMMA-type copolymer and
(
B) epoxidation product in CDCl
3
.
3
(B) thiol-ene product in CDCl .
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ppm, respectively, correspond to the methylene protons of 2-
mercaptoethanol moiety in the product. In combination with
another new peak a in the region of 5.90–5.59 ppm, which
is attributed to the remained double bond after thiol-ene
reaction of the terminal double bond in allene group, the
successful functionalization of the PMMA-type copolymer via
thiol-ene chemistry can be strongly confirmed. Nevertheless,
the residual signals (peak “*” in Fig. 11) for the terminal
double bond indicate incomplete conversion, that is, only
about 70% conversion was reached, which is also ascribed
to the serious steric effect.
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