RAFT/MADIX copolymerization of vinyl acetate and 5,6-benzo-2-methylene-1,3- dioxepane

Article abstract of DOI:10.1002/pola.26976

The synthesis of well-defined degradable poly(vinyl acetate) analogues is achieved by RAFT copolymerization of 5,6-benzo-2-methylene-1,3-dioxepane (BMDO) and vinyl acetate (VAc) using methyl (ethoxycarbonothioyl)sulfanyl acetate (MEA) as controlling agent. Several monomer mixtures with low BMDO contents (<30 mol %) are employed to prepare different copolymers. In all the cases, the evolution of molar masses and the dispersity values (<1.26) confirm the controlled feature of the polymerization. The livingness of the obtained chains is demonstrated by successful chain extension experiments with VAc, although the presence of dead chains is also shown. The introduction of ester groups into the main chain of these P(VAc-co-BMDO) copolymers allows their degradation when treated with a mixture of KOH/MeOH in reflux during 2.5 h.

Full text of DOI:10.1002/pola.26976

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RAFT/MADIX Copolymerization of Vinyl Acetate
and 5,6-Benzo-2-methylene-1,3-dioxepane
Giovanna Gomez d’Ayala,1 Mario Malinconico,1 Paola Laurienzo,1 Antoine Tardy,2
Yohann Guillaneuf,2 Muriel Lansalot,3 Franck D’Agosto,3 Bernadette Charleux3
1Istituto di Chimica e Tecnologia dei Polimeri, Via Campi Flegrei, 34 80078 Pozzuoli (NA), Italy
2
ꢀ
Aix-Marseille Universite, CNRS, Institut de Chimie Radicalaire ICR UMR 7273, 13397 Marseille, France
3
ꢀ
ꢀ ꢀ
Universite de Lyon, Univ. Lyon 1, CPE Lyon, CNRS UMR 5265, Laboratoire de Chimie Catalyse Polyme`res et Procedes (C2P2),
Equipe LCPP Bat 308F, 43 Bd du 11 Novembre 1918, F-69616 Villeurbanne, France
Correspondence to: G. Gomez d’Ayala (E-mail: giovanna.gomez@ictp.cnr.it); F. D’Agosto (E-mail: dagosto@lcpp.cpe.fr)
Received 2 September 2013; accepted 4 October 2013; published online 29 October 2013
DOI: 10.1002/pola.26976
ABSTRACT: The synthesis of well-defined degradable poly(vinyl
acetate) analogues is achieved by RAFT copolymerization of
5,6-benzo-2-methylene-1,3-dioxepane (BMDO) and vinyl acetate
(VAc) using methyl (ethoxycarbonothioyl)sulfanyl acetate
(MEA) as controlling agent. Several monomer mixtures with
low BMDO contents (<30 mol %) are employed to prepare dif-
ferent copolymers. In all the cases, the evolution of molar
masses and the dispersity values (<1.26) confirm the con-
trolled feature of the polymerization. The livingness of the
obtained chains is demonstrated by successful chain extension
experiments with VAc, although the presence of dead chains is
also shown. The introduction of ester groups into the main
chain of these P(VAc-co-BMDO) copolymers allows their degra-
dation when treated with a mixture of KOH/MeOH in reflux
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during 2.5 h. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part
A: Polym. Chem. 2014, 52, 104–111
KEYWORDS: copolymerization; cyclic ketene acetal; degradable
polymer; reversible addition fragmentation chain transfer; vinyl
acetate
backbone. A recent article by Agarwal3 reviews the concepts
and the progress in the field of free radical polymerization
of cyclic ketene acetals for the synthesis of polyesters based
materials. Among the different cyclic ketene acetal mono-
mers, 5,6-benzo-2-methylene-1,3-dioxepane (BMDO, Scheme
2) has been shown to polymerize via free radical polymer-
ization by successive ring opening reactions only. First homo-
polymerized by Bailey et al.,1 BMDO has then been
copolymerized with methyl methacrylate (MMA),4,5 styrene
(S),6 N-isopropylacrylamide (NIPAM),7,8 methacrylic acid
(MAA),9 n-butyl acrylate (BA),10 poly(ethylene glycol) meth-
acrylate (PEGMA)11,12 to produce the corresponding degrad-
able vinylic polymers.
INTRODUCTION The free radical polymerization of vinylic
monomers leads to the formation of polymers with carbon–
carbon link between the monomer units. In 1982, Bailey
et al.1 showed that ester links could also be incorporated
into the polymer backbone by free radical ring opening poly-
merization of specific monomers such as cyclic ketene ace-
tals (CKAs). These monomers can indeed undergo radical
addition on their C5C double bond that subsequently leads
to propagation either by ring opening or ring retaining or by
a combination of both depending especially on their struc-
ture (Scheme 1).
When the propagation step occurs only by successive radical
ring opening reactions, the produced polymer is a polyester
that can thus be obtained under much less demanding condi-
tions than those required by the techniques usually
employed to synthesize this type of polymers such as poly-
condensation, ionic, and coordination ring opening polymer-
izations.2 Free radical copolymerization of cyclic ketene
acetals with vinylic monomers has thus rapidly been fore-
seen as a powerful tool to produce degradable polymers by
taking advantage of the presence of ester groups along the
Since the advent of controlled/living free-radical polymeriza-
tion (CRP) based on reversible deactivation of the propagat-
ing radicals,13 the design of complex macromolecules is
much simpler and a myriad of macromolecular architectures
has been made accessible under very simple synthetic condi-
tions. Combining the best attributes of CRP with those of
free radical ring opening polymerization should thus lead to
degradable polymers with controlled and narrowly
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Free radical polymerization of cyclic ketene acetals: possible propagation routes.
EXPERIMENTAL
distributed molar masses, along with potentially original and
switchable properties. In this regard, most of the works
dedicated to CRP involving BMDO6–8,10–12,14–16 was essen-
tially focused on atom transfer radical polymerization
(ATRP)4,6,7,10–12,15–17 and performed at rather high tempera-
Materials
Vinyl acetate (VAc, Aldrich, 99 %), was distilled over CaH2
under vacuum. 2,20-Azobisisobutyronitrile (AIBN, Merck, 98
%), used as initiator, was purified by recrystallization in
ethanol. Toluene was degassed by argon passage and puri-
fied with an SPS800 MBraun solvent purification system.
5,6-Benzo-2-methylene-1,3-dioxepane (BMDO) has been syn-
thesized according to a slightly modified procedure originally
proposed by Agarwal and coworkers.6 Dimethyl orthophtha-
late (Acros, 99 %), LiAlH4 (Aldrich, 95 %), chloroacetalde-
hyde dimethylacetal (Acros, 97 %), p-toluenesulfonic acid
(Acros, 99%), potassium tert-butylate (t-BuOK, Aldrich, 95
%), methyl bromoacetate (Sigma, 97 %), and O-ethyl xanthic
acid potassium salt (Sigma, 96 %) were used as received.
ꢀ
ture (around or above 100 C), although very recent reports
on NMP seem to be promising.14b,c For example, block
copolymers composed of a PBMDO segment and a polysty-
rene (PS), poly(methyl methacrylate) (PMMA), or poly(-
methyl acrylate) segment could be formed.16 Only two
papers report on the reversible addition–fragmentation chain
transfer (RAFT)18 radical polymerization of BMDO either
homopolymerized16 or copolymerized with NIPAM.8 The
RAFT homopolymerization of BMDO was depicted to be suc-
cessful (molar mass dispersity [Ð] values around 1.3) when
conducted in THF in sealed vials at 120 ꢀC in the presence
of 1-ethoxy(carbonyl)prop-1-yl dithiobenzoate (EPDTB) as a
chain transfer agent. Conversions ranged between 16 and 66
% after polymerization times up to 52 h. The use of NIPAM
as a comonomer in RAFT copolymerization allowed to cir-
cumvent the poor reactivity of BMDO and to perform the
polymerization at 60 ꢀC. S21-Dodecyl-S0-(a,a0-dimethyl-a00-
acetic acid) trithiocarbonate was also used as a chain trans-
fer agent and the polymerization was performed during 10 h
to produce well-controlled copolymers (Ð < 1.3) incorporat-
ing less than 14 mol % of BMDO units.
Analytical Techniques
1H- and 13C-NMR spectra of sections B.3–B.5 were obtained,
respectively, on a Bruker Advance DRX 300 MHz spectrome-
ter at 300 MHz (1H) and on a Bruker DRX-600 spectrometer
at 150.9 MHz (13C). The chemical shifts were referenced to
the solvent peak for CDCl3 (1H-NMR: d 5 7.26 ppm and 13C-
NMR: d 5 77.16 ppm).
The individual conversions of VAc and BMDO were evaluated
by 1H-NMR spectroscopy analyses performed in toluene-d8
at room temperature (Bruker DRX 300). 1,3,5-Trioxane was
used as an internal reference (isolated resonance at 4.7 ppm
in toluene-d8; Supporting Information). At regular time inter-
vals during the copolymerization reaction, the individual
With a special interest in producing well-defined biodegrad-
able poly(vinyl acetate) (PVAc) or potentially poly(vinyl alco-
hol), the copolymerization of cyclic ketene acetals with vinyl
acetate is of particular interest.19,20 To the best of our
knowledge, there is no example of controlled radical copoly-
merization of cyclic ketene monomer with VAc. In such con-
text, this article depicts our first investigations into the
copolymerization of BMDO with VAc by RAFT.
FIGURE 1 1H-NMR (toluene-d8, 300 MHz) spectrum of reaction
SCHEME 2 Radical ring-opening polymerization of 5,6-benzo-2-
methylene-1,3-dioxepane (BMDO).
mixture at time zero. Enlargement between 3.7 and 4.9 ppm.
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molar conversions of the monomers (xVAc for VAc and xBMDO
for BMDO) were determined by relative integration of the
vinyl and vinylidene protons of VAc and BMDO, respectively
(Fig. 1: peak VAc (1) at 4.25 ppm and peak BMDO (2) at
3.85 ppm) and the protons of 1,3,5-trioxane. The resonance
of the trioxane protons overlapping those of VAc, their inte-
gral was therefore calculated as the difference between the
total integral in the range 4.65–4.75 ppm (trioxane 1 VAc
(2)) and the integral of VAc (1) at 4.25 ppm (Fig. 1).
The individual conversion of the monomers was calculated
as follows:
ðHVAcð1Þ=Htrioxane Þ 2ðH
=Htrioxane
Þ
t 3100
ðHVAcð1Þ=HtrioxVaAnceðÞ1Þ
t0
x
VAc 5
(1)
t0
ðHBMDO =Htrioxane Þ 2ðHBMDO =Htrioxane
Þ
t0
x
BMDO 5
t 3100 (2)
SCHEME 3 Synthetic routes for the preparation of BMDO.
ðHBMDO =Htrioxane
Þ
t0
The total weight conversion %wtconv. of the monomers was
calculated as follows:
Synthesis of 1,2-Benzenedimethanol
In a pre-dried three-neck flask fitted with a cooler and a drop-
ping funnel, 400 mL of dry THF and 20 g (0.53 mol) of LiAlH4
were placed under argon. A solution of 95.2 g (0.49 mol) of
dimethyl orthophthalate in 120 mL of dry THF was then added
dropwise under stirring. The mixture was then refluxed at 75–
80 ꢀC during 5 h. After returning to room temperature, the
reaction content was gently put in a 1 L flask filled with 120
mL of water; the gray mixture went deep orange. Then, 30 mL
of 30 % HCl solution was added to obtain a clear and limpid
solution with a white precipitate. The product was then
extracted three times with 400 mL of diethyl ether. The
organic phase was washed with water and dried over Na2SO4.
Finally, the product was concentrated and recrystallized in
%wt conv :5xVAc3%wt VAc 1xBMDO 3%wt BMDO
(3)
where %wtVAc and %wtBMDO are the initial mass fraction of
VAc and BMDO, respectively.
The molar mass of the polymers was determined by size
exclusion chromatography (SEC) measurements, using either
THF or DMF as eluent (SEC-THF and SEC-DMF, respectively).
For SEC-THF, samples were analyzed at 40 ꢀC at a flow rate
of 1 mL min21 using toluene as a flow rate marker. The sam-
ples were analyzed at a concentration of 5 mg mL21 after fil-
tration through
a 0.45 lm pore-size membrane. The
1
pentane, giving 56 g (yield 85 %) of white crystals. H-NMR
separation was carried out on three Polymer Laboratories
columns (33 PLgel 5 mm Mixed C [300 3 7.5 mm] and a
guard column [PL gel 5 mm]). The setup was equipped with
a refractive index (RI) detector (Waters 410 Differential
Refractometer). The average molar masses (number-average
molar mass (Mn) and weight-average molar mass (Mw)) and
the dispersity (Ð 5 Mw/Mn) were derived from the RI signal
by a calibration curve based on polystyrene (PS) standards
(from Polymer Laboratories).
(300 MHz, CDCl3, d): 7.32 (s, 4H, Ar), 4.68 (s, 4H, ACH2OH),
3.37 (br. s., 2H, ACH2OH); 13C-NMR (300 MHz, CDCl3, d):
139.4, 129.7, 128.5 (6C, Ar), 63.8 (2C, ACH2OH).
Synthesis of 5,6-Benzo-2(chloromethyl)21,3-dioxepane
In a 200 mL large diameter neck round bottom flask, 50 g
(0.37 mol) of 1,2-benzenedimethanol, 48 g (0.37 mol) of chlor-
oacetaldehyde dimethylacetal and 80 mg of p-toluenesulfonic
acid were heated at 110 ꢀC for 3 h until the mixture came dark
orange. After cooling at room temperature, the solidified prod-
uct was dissolved in 400 mL of chloroform to be washed with
100 mL of NaHCO3 solution and 100 mL of water. The crude
product was then dried over MgSO4, concentrated and recrys-
tallized in a mixture of pentane with chloroform to give 61.5 g
(yield 83 %) of white crystals.1H-NMR (300 MHz, CDCl3, d):
7.24–7.20 (m, 4H, Ar), 5.10 (t, 1H, (ACH2O)2CH–CH2Cl), 4.94
For SEC-DMF, the analyses were performed using an EcoSEC
semi-micro SEC system from Tosoh equipped with a dual
flow refractive index detector and a UV detector. The sam-
ples were analyzed in DMF (with LiBr at 0.01 mol L21) at
50 C using a flow rate of 1 mL min21. Toluene was used as
ꢀ
a flow rate marker. All polymers were injected at a concen-
tration of 2 mg mL21, after filtration through a 0.45 lm
pore-size membrane. Separation was performed with a guard
column and two PSS GRAM columns (7 mm, 300 3 7.5 mm).
Mn, Mw, and Ð 5 Mw/Mn were derived from the RI signal by
a calibration curve based on PS standards.
(s, 4H, C6H4A(CH2O)2A), 3.61 (d, 2H, (–CH2O)2CH–CH2Cl); 13
NMR (75.5 MHz, CDCl3, d): 138.62, 127.92, 127.66 (6C, Ar),
106.35 (1C, (ACH2O)2CHA CH2Cl), 72.15 (2C, C6H4–(CH2O)2–
), 44.11 (1C (ACH2O)2CHA CH2Cl).
C
Synthesis of BMDO
Synthesis of BMDO
The overall route is depicted in Scheme 3 and the synthetic
steps are detailed below.
A 20 g (0.1 mol) of 5,6-benzo-2(chloromethyl)21,3-dioxe-
pane, 13 g (0.11 mol) of t-BuOK, and 200 mL of dry THF
were refluxed at 90 ꢀC for 4 h under argon to give a light
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brown turbid mixture. THF was then evaporated and
replaced by 400 mL of diethyl ether before filtration. The
resulting organic layer was concentrated and distillated
under reduce pressure to give 13.2 g (81%) of white needle
shaped crystals.
Chain Extension Test
In order to investigate the living character of the synthesized
polymers, a chain extension experiment was performed. A
sample of the VB10 copolymer (260 mg, Mn 5 3080 g mol21
,
Ð 5 1.27) was employed as a RAFT agent (macrocontrolling
agent) in the polymerization of vinyl acetate. In particular,
260 mg of VB10 were dissolved in 2 mL of toluene in pres-
ence of AIBN ([VB10]/[AIBN] 5 3), trioxane and vinyl acetate
(3 mol L21). The polymerization reaction was performed at
80 ꢀC. After 24 h, the reaction was stopped and the mono-
1H-NMR (400 MHz, CDCl3, d) 3.72 (s, 2H, (ACH2O)2C@CH2),
5.07 (s, 4H, C6H4CH2OA), 7.08–7.11 (m, 2H, Ar), 7.23–7.26
(m, 2H, Ar). 13C (75.5 MHz, CDCl3, d) 69.67 (1C,
(ACH2O)2C@CH2), 72.23 (2C, C6H4CH2OA), 126.35, 127.59,
135.99 (6C, Ar), 164.41 (1C, (ACH2O)2C@CH2)
1
mer conversion was evaluated by H-NMR analysis. The reac-
tion mixture was dissolved in THF and precipitated in
hexane. The products were dried under vacuum at room
temperature and the molar mass of the obtained polymer
was determined by SEC-THF.
Synthesis of MEA
For the synthesis of methyl (ethoxycarbonothioyl)sulfanyl
acetate (MEA), methyl bromoacetate (7.6 g) dissolved in
ethanol (30 mL) was added dropwise to a dispersion of O-
ethyl xanthic acid potassium salt (9.2 g) in ethanol (40 mL)
over an ice bath. The mixture was stirred for 24 h at room
temperature. The solvent was evaporated and dichlorome-
thane was added. The resulting white precipitate of KBr was
washed twice with water. The organic phase was dried over
MgSO4 and concentrated by evaporation under reduced pres-
sure. The obtained yellow product (yield 95 %) was ana-
lyzed by NMR spectroscopy in CDCl3. 1H-NMR (300 MHz,
CDCl3, d): 1.41 (t, 3H, AOCH2CH3), 3.76 (s, 3H, CH3OCOA),
3.92 (s, 2H, AOCOCH2S), 4.64 (q, 2H, AOCH2CH3).
Hydrolysis of P(VAc-co-BMDO)
About 50 mg of copolymer (VB10) were dissolved in a mix-
ture of 5 mL of KOH in methanol (0.1 M) in a round-
bottomed flask. The resulting mixture was stirred at 60 ꢀC
under reflux. After 2.5 h, the solvent was evaporated under
vacuum and the remaining solid was dissolved in DMF and
1
analyzed by SEC-DMF and H-NMR in DMSO-d6.
RESULTS AND DISCUSSION
Preliminary Comments
Copolymerization of VAc and BMDO
As mentioned in the “Introduction” section, due to its low
reactivity, high polymerization temperatures are often
required to (co)polymerize BMDO. In addition, performing
reactions in bulk is most of the time preferred provided that
the mixture of monomers is homogeneous. In our case, due
to the insolubility of BMDO into vinyl acetate, it was not pos-
sible to perform the reaction in bulk. Several experiments
were then first performed in solution using different solvents
such as toluene and DMF, in order to identify optimal condi-
tions for future RAFT copolymerizations of BMDO and VAc.
In addition, several parameters such as the temperature and
the concentration of radical initiator were also adjusted in
absence of MEA in order to observe a sufficient consumption
of the two monomers before investigating the RAFT
copolymerization.
The homopolymerization of VAc and all the copolymerization
reactions were performed under argon in Schlenk tubes
under RAFT reaction conditions. In a typical copolymeriza-
tion reaction, 1.86 g (21.6 mmol) of VAc, 0.39 mg (0.22
mmol) of BMDO, 39 mg of MEA, 108 mg (1.2 mmol) of triox-
ane, 12 mg (0.073 mmol) of AIBN, and 6.0 mL of toluene
were placed in a Schlenk tube under argon and stirred for
10 min. The mixture was divided into several tubes and
bubbled with argon for 5 min. The flasks were then placed
in a preheated oil bath at 80 ꢀC for 24 h. Each tube was
taken and placed into the ice at different times during the
polymerization in order to evaluate the conversion of the
monomers and the molar mass of the formed polymer. The
reaction mixture was dissolved in THF and precipitated
twice and washed with hexane. The copolymers were dried
under vacuum at room temperature until constant weight.
Different copolymers were prepared by changing the molar
ratio of the two monomers in the feed.
Although not always mentioned in the literature, the use of
dried solvent is necessary when working with BMDO which
turned to be a quite sensitive monomer. For example, disso-
lution of BMDO in DMF, used as received, led to a quantita-
tive degradation of BMDO. In Figure 2(a), a complete
The theoretical number-average molar mass of copolymers
based on VAc and BMDO is calculated as follows:
1
disappearance of vinylidene protons of BMDO in the H-NMR
spectrum was observed together with the appearance of
resonances consistent with a structure shown in Figure 2(a).
This showed the presence of acidic impurities in DMF that
can add onto the double bond of BMDO.9
À
Á
½VAcꢁ03MVAc 1½BMDOꢁ03MBMDO
½MEAꢁ0
% wtconv:
100
Mntheo 5
3
1MMEA
(4)
The use of dried toluene was indeed shown to be appropri-
ate [Fig. 2(b)] and further copolymerizations were thus
conducted in this solvent. In addition, setting the copolymer-
ization temperature at 80 ꢀC, the minimum concentration of
AIBN necessary to observe a sufficient monomer conversion
where MMEA is the molar mass of the RAFT agent, [VAc]0,
[BMDO]0, and [MEA]0 are the initial concentrations of the
two monomers and MEA, respectively, and %wtconv. is the
weight conversion of monomers (eq (3)).
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TABLE 1 Monomer Conversions, Calculated by 1H-NMR Analy-
ses, After 24 h of RAFT Copolymerization of VAc and BMDO in
Toluene at 80 ꢀC
Experiment
Number
BMDO Initial
Molar %
a
xVAc
xBMDO
%wtconv.
V100
VB5
0
53
45
41
27
21
–
53
45
39
26
18
5
44
28
24
15
VB10
VB20
VB30
10
20
30
a
Total weight monomer conversion, calculated as described in the
“Experimental” section.
tion was obtained; however only when dithiocarbonates (i.e.,
xanthates) such as MEA were used as controlling agents.21
Considering the data gained from the preliminary experi-
ments discussed above, the copolymerization reactions were
carried out with AIBN as a radical initiator in the presence
of MEA as a controlling agent in dried toluene at 80 ꢀC
(Scheme 4). First, a pure PVAc was synthesized under these
conditions as a blank experiment (V100 in Tables 1 and 2).
The relatively high value of dispersity (Ð 5 1.38) compared
to the already published results,21,22 may be due to the
transferring ability of toluene used as a solvent and gener-
ally not employed for the polymerization of VAc. Neverthe-
less, the control of the polymerization of VAc achieved under
these conditions allowed to further evaluate the copolymer-
ization with BMDO.
FIGURE 2 Comparison between 1H-NMR spectra of BMDO in
(a) DMF and (b) dried toluene.
within a reasonable time frame (>50 % in less than 24 h)
was around 1 mol % of the total monomer concentration.
Temperatures higher than 80 ꢀC did not really improve the
final conversion obtained.
Four different copolymers were prepared by varying the ini-
tial molar composition in VAc and BMDO (5, 10, 20, and 30
% of BMDO: see experiments noted VB5, VB10, VB20, and
VB30 in Table 1). The monomer conversion and the molar
mass characteristics of the obtained (co)polymers are sum-
marized in Tables 1 and 2, respectively.
RAFT Copolymerization of BMDO and VAc
In order to design a well-defined degradable copolymer
based on BMDO, a low fraction of incorporated BMDO units
is enough. In this particular case, the control of the polymer-
ization can be dictated by the choice of a controlling agent
suitable for the polymerization of the main comonomer. As
we targeted degradable PVAc, the copolymerization of VAc
and BMDO was performed with a low proportion of BMDO
using the RAFT/MADIX (macromolecular design by inter-
change of xanthate) process, well adapted to the homopoly-
merization of VAC. The latter is now well documented and a
very good control over molar mass and molar mass distribu-
Whatever the reaction performed, the total molar monomer
conversion versus time profiles obtained is similar [Fig.
3(a)]. A plateau was reached after 8 h of reaction after
which the conversion did not increase anymore (similar val-
ues obtained after 24 h of reactions). As this trend also
holds for the homopolymerization of VAc, this may be related
to depletion of radicals generated by AIBN at 80 ꢀC (t1/2
ꢀ
around 1 h at 80 C, calculated with t1/2 5 ln2/kd and kd (80
ꢀC) 5 1.69 3 1024 s21).23 An increase of BMDO content in
the starting monomer mixture led to a decrease of the
SCHEME 4 P(VAc-co-BMDO) synthesis via RAFT/MADIX.
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TABLE 2 Experimental Conditions and Molar Mass Characteris-
tics of the Copolymers Obtained by RAFT Copolymerization of
BMDO and VAc Performed in Toluene at 80 ꢀC with AIBN as an
Initiator.
Copolymer
Composition
(molar ratio)
a
Experiment
Number
Mn theo
(g mol21
Mn
)
(g mol21
)
Ð
VAc
BMDO
V100
VB5
4290
4650
4220
3140
2380
3370
3020
2590
1900
1300
1.38
1.26
1.20
1.21
1.25
100
95
–
5
FIGURE 4 SEC-THF chromatograms of samples withdrawn
from the reaction medium during the MEA-mediated RAFT
copolymerization of VAc and BMDO (VB5: VAc/BMDO 95/5:
mol/mol) performed at 80 ꢀC in toluene.
VB10
VB20
VB30
90
10
18
24
82
76
a
Determined by SEC-THF based on a polystyrene calibration for sam-
ples isolated after 24
h of polymerization and after precipitation in
the RAFT polymerization. The molar mass values obtained
(Table 2) were difficult to compare to the theoretical ones
considering the use of a calibration based on PS standards.
Nevertheless, Mn increased with conversion and the Ð values
remained lower than 1.30 (Fig. 6). This was also confirmed
for the other copolymerization reactions (Table 2).
n-hexane.
polymerization rate, the plateau value reached being
decreased accordingly with the lack of radicals observed
after 8 h (down to ca. 20 % for VB30).
Radical depletion suspected above was confirmed in the case
of VB10 for which AIBN was further added once the conver-
sion reached the plateau value of 36 % after 24 h of reaction
[Fig. 3(a)]. The copolymerization did resume and the conver-
sion determined after additional 24 h increased to 48 %. In
addition, the SEC-THF chromatogram of the copolymer
obtained after this AIBN addition was compared to the chro-
matogram of the starting copolymer (Fig. 7). The shift of the
distribution toward higher molar mass values was a good
indication (but not an evidence, see below) that a majority
of chains were living.
Samples were withdrawn at different times and analyzed by
SEC-THF. Figure 4 shows the evolution of the corresponding
chromatograms in the case of VB5. The shift of the chroma-
tograms towards higher molar mass values suggests a con-
trolled nature of the copolymerization. Nevertheless,
polymer peaks partially overlapping those related to the
residual monomers, particularly when conversion was low,
the molar mass and dispersity values were difficult to esti-
mate (Supporting Information Fig. S1). Therefore, for each
sample, the polymer was isolated by precipitation in n-hex-
ane, filtered, dried, and then analyzed by SEC-THF (Fig. 5).
The final copolymers were additionally analyzed by 1H-NMR
spectroscopy. An example of such analyses is shown in Fig-
ure 8 in the case of VB10. The characteristic resonances of
BMDO units (methylene at 2.6 ppm) and VAc units (4.9 ppm,
The molar mass distribution of the precipitated copolymers
shows a slight tailing toward the low molar mass values,
which could be due to the formation of low molar mass spe-
cies resulting from chain transfer reactions to toluene during
FIGURE 3 Evolution of total molar monomer conversions ver-
sus time for the MEA-mediated RAFT copolymerizations of VAc
and BMDO performed at 80 ꢀC in toluene using AIBN as an ini-
tiator. Molar ratio VAc/BMDO 5 95/5 (VB5); 90/10 (VB10); 80/20
(VB20); 70/30 (VB30).
FIGURE 5 SEC-THF chromatograms of the precipitated copoly-
mers for VB5 experiment.
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FIGURE 6 Molar mass and Ð evolution versus weight conver-
sion obtained by SEC-THF during the MEA mediated RAFT
copolymerization of VAc and BMDO (VB5: 95/5, mol/mol) per-
formed at 80 ꢀC in toluene.
FIGURE
8
1H-NMR analysis of P(VAc-co-BMDO) (VB10) in
CDCl3.
Chain Extension Test
–CH–main chain) were observed. The presence of the reso-
nance at 5.1 ppm can be assigned to the –CH– protons of
VAc units connected to BMDO unit in the copolymer and
confirms that copolymerization indeed took place. Further-
more, the typical resonances of end groups originating from
MEA at 4.1 ppm (f, CH3CH2O–), 1.4 ppm (g, CH3CH2O–), and
3.6 ppm (a, CH3OC(5O)–) confirm that the control of the
copolymerization was induced by MEA.
The experiment involving a further addition of AIBN in VB10
discussed above seemed to show the livingness of the copoly-
mer chains. However, as the increase in conversion did not
represent a sufficient increase in molar mass to unambigu-
ously conclude on the livingness of the polymerization, a
proper chain extension experiment was carried out. In that
respect, the copolymer VB10 was used as a macroRAFT agent
for the homopolymerization of VAc (VB10-ext). The extension
reaction was carried out at 80 ꢀC and the employed [VAc]/
[VB10] ratio was such that the expected extension should dou-
ble the molar mass of the starting copolymer (Mn VB10 5 2590
g mol21; Mn theo targeted 5 6000 g mol21). After 24 h of reac-
tion, 55% of VAc was converted. The copolymer was recovered
by precipitation in n-hexane and analyzed by SEC-THF (Fig. 9).
Further characterization of the copolymer by 13C-NMR
showed the absence of resonances between 100 and 120
ppm characteristic of acetal carbons and confirmed that
BMDO was only introduced in the main chain by radical ring
opening reactions (Supporting Information Fig. S2).
Figure 9 shows that the molar mass distribution has been
shifted toward higher molar mass values. However,
a
FIGURE 7 SEC-THF chromatograms of the reaction medium
(without precipitation step) during the MEA-mediated RAFT
copolymerization of VAc and BMDO (VAc/BMDO: 90/10 mol/
mol) performed at 80 ꢀC in toluene: (a) after first 24 h of reac-
tion (VB10), (b) after the addition of AIBN to VB10 reaction
medium and 24 h of additional polymerization.
FIGURE 9 SEC-THF chromatograms showing the evolution of
molar mass for the chain extension experiment using P(VAc-
co-BMDO) (VB10) as a macroRAFT agent for the polymerization
of VAc (VB10-ext).
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shoulder was clearly visible on the low molar mass side cor-
responding to the presence of a certain proportion of dead
chains in the starting VB10. Chain extension of P(VAc-co-
BMDO) was nevertheless possible.
Universitaire de France) for her nomination as senior member.
The authors would like to thank O. Boyron (UMR 5265 C2P2,
LCPP Team, Villeurbanne) for SEC analyses.
Hydrolysis of P(VAc-co-BMDO) Copolymer
REFERENCES AND NOTES
A preliminary study on the hydrolytic degradation of P(VAc-
co-BMDO) (VB10) copolymer was performed under alkali con-
ditions at 60 ꢀC during 2.5 h. Under these conditions, both the
VAc units and the ester links between BMDO and VAc units
should be hydrolyzed. This should result in the formation of
species including short segments of poly(vinyl alcohol).
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CONCLUSIONS
With the view of targeting degradable poly(vinyl acetate) based
macromolecules, the RAFT copolymerization of BMDO and VAc
was successfully carried out using MEA as a controlling agent.
The copolymerization reactions were performed in toluene at
80 ꢀC in the presence of AIBN as an initiator. Monomer mix-
tures with low BMDO contents (lower than 30 mol %) were
used. In all the cases, well-defined P(VAc-co-BMDO) exhibiting
molar masses from 1300 to 3000 g mol21 were obtained with
molar mass dispersities lower than 1.26. The livingness of the
obtained chains was shown by successful chain extension
experiments with VAc, although the presence of dead chains
was also evidenced. The final P(VAc-co-BMDO) was shown to
degrade when treated by a mixture of KOH/MeOH in reflux dur-
ing 2.5 h, confirming that ester groups were successfully intro-
duced in the main chain. This is the first study on RAFT
copolymerization of VAc and BMDO additionally demonstrated
that the manipulation of BMDO and the design of macromolecu-
lar architectures obtained by radical ring opening copolymeriza-
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reaction conditions, particularly to reach higher molar masses.
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ACKNOWLEDGMENTS
GGA is grateful to CNR-DSCTM for the support in the frame of
Short Term Mobility Program. BC is grateful to the IUF (Institut
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