1
Effect of head-to-head addition in vinyl acetate
controlled radical polymerization: why is Co(acac)
2
-
mediated polymerization so much better?
Aurélie N. Morin,
†
,
‡
Christophe Detrembleur,
¶
Christine Jérôme,
¶
Pascal De Tullio,
£
Rinaldo
Poli,
*,†,‡,
§
Antoine Debuigne,*
, ¶
†
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 Route de Narbonne, BP 44099, F-
31077 Toulouse Cedex 4, France CNRS.
‡
Université de Toulouse, UPS, INPT, F-31077 Toulouse Cedex 4, France.
§
Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France.
¶
Center for Education and Research on Macromolecules (CERM), Chemistry Department,
University of Liège (ULg), Sart-Tilman, B6a, 4000 Liège, Belgium.
£
Drug Reserch Center (CIRM), Laboratory of Medicinal Chemistry , University of Liège (ULg),
1 avenue de l'Hopital, 4000 Sart-Tilman.
KEYWORDS: vinyl acetate; controlled radical polymerization; DFT calculations; ATRP;
OMRP; CMRP; ITP; RAFT/MADIX; TERP; head-to-head addition.
2
ABSTRACT. The controlled polymerization of vinyl acetate has been recently achieved by
several techniques, but PVAc with targeted Mn and low dispersity up to very high monomer
conversions and high degrees of polymerization was only obtained with Co(acac)
2
as controlling
agent in the so-called CMRP, a type of organometallic mediated radical polymerization (OMRP).
Other techniques (including ATRP, ITP, TERP and RAFT/MADIX) have shown a more or less
pronounced slowdown in the polymerization kinetics, which was attributed to the higher strength
of the C-X bond between the radical PVAc chain and the trapping agent (X) in the dormant
species and to a consequent slower reactivation after a less frequent head-to-head monomer
addition. The reason for the CMRP exception is clarified by the present contribution. First, a
detailed investigation by
1
H,
13
C and multiplicity-edited HSQC and DEPT-135 NMR of the
PVAc obtained by CMRP, in comparison with a regular polymer made by free radical
polymerization under the same conditions, has revealed that Co(acac)
2
does not significantly
alter the fraction of head-to-head sequences in the polymer backbone and that there is no
accumulation of Co(acac)
2
-capped chains with a head-to-head ω end. Hence, both dormant
chains (following the head-to-head and the head-to-tail monomer additions) must be reactivated
at similar rates. A DFT study shows that this is possible because the dormant chains are
stabilized not only by the C-Co σ bond but also by formation of a chelate ring through
coordination of the ω monomer carbonyl group. The head-to-head dormant chain contains an
inherently stronger C-Co bond but forms a weaker 6-membered chelate ring, whereas the weaker
C-Co bond in the head-to-tail dormant chain is compensated by a stronger 5-membered chelate
ring. Combination of the two effects leads to similar activation enthalpies, as verified by DFT
calculations using a variety of local, gradient-corrected, hybrid and “ad hoc” functionals
(BPW91, B3PW91, BPW91*, M06 and M06L). While the BDE(C-X) of model H-VAc-X
3
molecules [X = Cl, I, MeTe, EtOC(S)S and Co(acac)
2
] are functional dependent, the BDE
difference between head-to-head and head-to-tail dormant chain models is almost functional
insensitive, with values of 5-9 kcal/mol for the ATRP, ITP and TERP models, 3-6 for the
RAFT/MADIX model, and around zero for CMRP.
4
Introduction
Poly(vinyl acetate) (PVAc) is a large scale market polymer (> 1 million tons/yr) with multiple
applications as an adhesive emulsions, as a protective coating and especially as a raw material to
make other polymers like poly(vinyl alcohol) and poly(vinyl acetate phthalate) that have many
important application of their own.
1
It can only be produced by the radical route, although
copolymers with limited incorporation of vinyl acetate monomer have recently been accessed by
coordination polymerization.
2
The implementation of controlled radical polymerization (CRP) techniques has revolutionized
polymer chemistry because structurally organized macromolecular architectures with controlled
dimensions, low molecular weight dispersity and precisely located functionalities have become
accessible. This opens the way to a multitude of new smart functional materials for advanced
applications.
3
The possibility to control the polymerization of VAc has attracted a great deal of
attention because of the large variety of conceivable new materials that could potentially be
obtained by incorporating PVAc blocks, particularly amphiphilic structures resulting from
hydrolysis to poly(vinyl alcohol). Satisfactory control, however, has not been achieved for the
majority of the techniques so far applied to the CRP of VAc.
5
Table 1. Reported systems for the CRP of VAc and characteristics of the resulting polymer.
Entry
Method
Controlling system
Initiator
X
n
a
Đ
a
remarks
Ref.
1
ATRP (?)
Fe(OAc)
2
/PMDETA
b
CCl
4
~170
~2
Telomerization by transfer to CCl4
4
,
5
2
ATRP
[CpFe(CO)
2
]
2
/M(OiPr)
n
(CH
3
)
2
C(CO
2
Et)I
~120
~2
Slowdown, max. conv. = 60%
6
3
ATRP
CuCl/PMDETA
b
CH
3
CH(CO
2
Me)Br
7.5
1.6
Stopped at 14% conversion
5
4
ATRP
CuCl or CuBr/terpy
c
(CH
3
)
2
C(CO
2
Et)Br
~120
1.7
Linear first-order plot (~75% conv.)
7
5
RAFT
Ph
2
NC(S)SCH(CO
2
Et)
2
AIBN
57
1.56
Đ increases with conv. (up to 73%)
8
6
RAFT
EtOC(S)SCH
2
CN
AIBN
~127
1.31
Linear first-order plot (77% conv.)
9
7
RAFT
EtOC(S)SCH
2
CO
2
Me
AIBN
~580
1.18
Đ at 25% conv. (max. conv ~56%)
10
8
ITP
CH
3
CH(CO
2
Et)I
CPD
d
~400
1.45
Slowdown, max. conv. = 57%
11
9
TERP
(CH
3
)
2
C(CO
2
Et)TeMe
AIBN
34
1.28
Slowdown at low conversions
12
10
OMRP
R
0
-(CH
2
CHOAc)
<4
-Co(acac)
2
e
~1630
1.27
Linear first-order plot (52% conv.)
13
11
OMRP
CpCr(nacnac)(CH
2
tBu)
f
175
1.46
Slowdown, max conv. = 14%
14
12
OMRP
(TMP)Co
g
AIBN
~660
1.27
Max conv. = 10%
15
a
Highest reported number average degree of polymerization (X
n
) and dispersity (Đ = M
w
/M
n
) at maximum conversion.
b
PMDETA
= pentamethyldiethylenetriamine.
c
terpy = 2,2';6',2"-terpyridine.
d
CPD = α-cumyl peroxyneodecanoate.
e
R
0
= initiating fragment of
V70 = 2,2'-Azobis(4-methoxy-2.4-dimethyl valeronitrile).
f
nacnac = ArNC(Me)CHC(Me)NAr (Ar = 2,6-C
6
H
3
Me
2
).
g
TMP =
tetramesitylporphyrin.
