ratio of 6.7% (10.7% conv., Mw = 73.6 kDa, PDI = 4.3), the
polymer obtained from FRP of cleavable MVM has a very
different degradation profile. The high molecular weight polymer
chains were degraded into small chains and the molecular weight
shows a very large reduction after degradation (Fig. 1C). The
degradation studies show, contrary to common understanding,
that a predominantly intramolecular linked knot structure is
formed by RAFT rather than a branched structure.
divinyl benzene (contains ca. 20% molar ratio monovinyl
monomer) studied by Perrier and co-workers resulted in a
product that was recognized as a highly branched structure.24
We reason that this is mainly due to the rigid styrenic monomer
sterically hindering a cyclization looping back to the chain, as
well as the high ratio of RAFT agent to monomer.
In conclusion, this study demonstrates that the kinetic model
can be applied to RAFT polymerization of MVMs, which
accurately predicts that due to the deactivating nature of the
RAFT agent, single cyclized molecules, consisting predominantly
of intramolecular cyclization, are formed. Through RAFT poly-
merizations, we fulfil and explain the production of the 3D single
cyclized structure from the homopolymerization of MVMs. The
understanding of the naturally deactivation enhanced RAFT
process opens a new route to allow a broad range of nanosize 3D
polymeric materials to be designed and synthesized in a facile
manner. More importantly, the results show once again the
highly applicable nature of the kinetics model to the controlled
living polymerizations.
The newly developed kinetic model can now be used to gain a
further understanding of why RAFT polymerization occurs in a
two phase process and how it differs from FRP by suppressing
intermolecular crosslinking. More specifically, the model predicts
which reaction process (propagation, intramolecular or inter-
molecular crosslinking) will occur by taking into account: the
growth boundary, the chain length and the chain concentration.
Fig. S7 in ESIw outlines the models for both RAFT polymeriza-
tion and FRP. For polymerizations with a large kinetics chain
length, such as FRP (Eq. S5, ESIw), the growth boundary (dotted
circle) is very large allowing all three reaction processes to occur.
The propagation and intermolecular crosslinking reactions are
simply higher due to statistical probability2 regardless of chain
length and concentration, thus instantly combining chains and
forming an insoluble gel. The true effect of the kinetic model
becomes apparent when considering RAFT polymerization. In
this case the growth boundary is constricted (smaller dotted
circle) because the growth of the propagating chain depends on
the deactivation. The deactivation in this case is caused naturally
by the propagating chain reacting with either free RAFT agents
(at early stage) or with RAFT end-capped polymer chains
(at later stage) when the RAFT agent is becoming exhausted
(Eq. S6, ESIw). At early stages, the propagating chain only has
the probability to either propagate linearly or perform intra-
molecular cyclization before transfer to the RAFT agent. The
constant of chain transfer (ktr) that is 102 to 103 times higher
than the vinyl propagation (kp) helps to eliminate the inter-
molecular crosslinking since the propagation centre cannot
reach another primary chain. Combining the effects of the small
growth boundary and the short chain length (shaded area)
constraining chain overlap (at the initial reaction stages), the
intermolecular crosslinking reactions are indeed suppressed
during the early phase of RAFT polymerization.
Heath Research Board (HRB) of Ireland and Science Founda-
tion Ireland (SFI)–SFI Principal Investigator programme, DEBRA
Ireland, SFI-Strategic Research Program (07/SRC/B1163) and
National University of Ireland, Galway, are gratefully acknow-
ledged for funding.
Notes and references
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3085–3087 3087