2034 Communications to the Editor
Macromolecules, Vol. 38, No. 6, 2005
In solution, AIBN decomposes to form two cyano
Scheme 1. Reaction Cycle To Produce Chain-End
Functional Polymers via Reversible
Addition-Fragmentation Chain Transfer
Polymerization with Recovery of the
Chain Transfer Agent
isopropyl radicals, which react on the reactive CdS of
the thiocarbonyl-thio moiety present at the chain end
of the polymer. The addition of the cyano isopropyl
radical to the reactive CdS bond of the thiocarbonyl-
thio group leads to the formation of an intermediate
radical. This intermediate has then the possibility to
fragment either back to the original attacking radical
or to free the leaving group (R group). In the presence
of an excess of cyano-isopropyl radicals, the equilibrium
is displaced toward the formation of the R group radical,
which can then combine irreversibly with free cyano
isopropyl radicals present in solution, thus forming dead
polymeric chains, capped by the cyano-isopropyl group.
The ratio of AIBN to polymer chain as well as the
temperature and the length of the reaction are impor-
tant parameters to consider in order to fully remove the
thiocarbonyl-thio end group from the polymeric chains.
To attain complete conversion, the length of the reaction
should follow the half-life time of the radical initiator.
Furthermore, excess of AIBN relative to polymer needs
to be used. Indeed, in our initial attempts, a lower ratio
of AIBN to PMMA (10:1) led to disproportionation
g/mol, PDI ) 1.24). The reproducibility of the polymer-
ization and end group removal reaction confirms that
the CTA is almost quantitatively recovered and with
virtually no loss to its activity. Scheme 1 summarizes
the recovery cycle for the process.
1
7
reactions between polymeric chains. Figure 2 (lower
spectrum) shows the appearance of singlets at 5.7 and
The precipitation of polymers is an easy technique to
separate the chain-end functionalized polymer and
recovered CTA for laboratory scale experiments, but it
is less appropriate for scale-up processes. Alternative
methods of recovering the CTA, more practical for large-
scale processes, include (a) increase of temperature
toward the end of the polymerization to recover the CTA
by distillation or sublimation; (b) following the reaction
with excess AIBN, the recovered CTA and end-func-
tionalized, nonreactive, polymeric chains can be kept in
solution, and a new batch of monomer may be added to
restart polymerization. In the latter technique, a new
batch of polymeric chains is therefore formed in the
presence of the previous batch of product, with similar
molecular weight and polydispersity; the same CTA is
therefore reused to produce a similar material. However,
the time for reaction of the polymer with AIBN when
removing the thiocarbonyl-thio group must be suf-
ficiently long for complete decomposition of the AIBN.
These alternative processes are currently being tested
in our laboratories.
6
.2 ppm, characteristic of vinyl protons of the unsatur-
ated PMMA chains formed through disproportionation
termination reactions. By increasing the concentration
of free radicals in solution (ratio AIBN:PMMA ) 20:1),
we avoided such side reactions. However, dispropor-
tionation may be wanted in order to introduce a certain
amount of double bond end groups on methacrylate
polymeric chains.
To assess the activity of the recovered chain transfer
agent (CTA) and to test the recovery cycle, a similar
reaction as that described above was undertaken, with
a ratio of MMA:MCPDB:AIBN ) 100:1:0.2 in toluene.
The temperature was raised to 80 °C for 24 h (98%
conversion). The polymer was precipitated by dropwise
addition into hexane and characterized by SEC (Mn )
1
6 300 g/mol, PDI ) 1.23). The polymer was dissolved
in toluene, and a second batch of AIBN was added to
cleave the dithiobenzoate moiety from the polymeric
chains. After a further 2.5 h at 80 °C, the solution was
cooled, and the poly(methyl methacrylate) was precipi-
tated by dropwise addition of the solution into hexane.
The polymer was filtered off and characterized by SEC
The process we report here not only is a simple and
effective method to recycle a RAFT or MADIX agent but
also has great potential to introduce specific chain-end
functionalities to both ends of a polymeric chain with
an efficiency of nearly 100%. Indeed, the careful choice
of the initial CTA followed by free radical initiator, from
the breadth of free radical initiators available com-
mercially, to remove the thiocarbonyl-thio group will
allow the production of polymers with specific function-
alities at both ends, with known molecular weight and
low polydispersity. To illustrate further this technique,
we applied this process to a variety of methacrylate,
acrylate, acrylamide, and styrenyl polymers and a choice
of free radical initiators (see Table 1). Among these
results, sample i is noteworthy, as it demonstrates the
one-step synthesis of a telechelic poly(methyl acrylate)
(Mn ) 16 000 g/mol, PDI ) 1.25). The filtrate was
evaporated to dryness, and the pink product was added
to a clean dry ampule together with methyl methacry-
late, AIBN, and toluene using the exact amounts as for
the first reaction. The solution was degassed with N2
for 10 min and then heated to 80 °C for 60 h. The
solution was cooled, and the poly(methyl methacrylate)
was precipitated by dropwise addition of the solution
into hexane. The polymer was filtered off and character-
ized by SEC to reveal a similar molecular weight and
polydispersity as those of the previous product (Mn )
1
6 300 g/mol, PDI ) 1.17). The polymer was dissolved
in toluene, and a second batch of AIBN was added to
cleave the dithiobenzoate moiety from the polymeric
chains. After a further 2.5 h at 80 °C, the solution was
cooled, and the poly(methyl methacrylate) was precipi-
tated by dropwise addition of the solution into hexane.
The polymer was filtered off and characterized by SEC
to reveal a similar molecular weight and polydispersity
as those of the previous cleaved product (Mn ) 16 500
of M ) 32 600 g/mol and PDI ) 1.27, with a carboxylic
n
acid functionality at both chain ends, useful as a
building block for methacrylate resins. Further modi-
fication of the end group functionalized polymers can
be achieved using classical synthetic organic chemistry,
an example of which has been successfully achieved by
tagging sample d with a fluorescent marker. Reduction