COMMUNICATION
Latent reactive groups unveiled through equilibrium dynamics and
exemplified in crosslinking during film formation from aqueous polymer
colloids{
David J. Berrisford,*a Peter A. Lovell,*b Nadia R. Sulimanab and Andrew Whiting*c
Received (in Cambridge, UK) 25th August 2005, Accepted 28th September 2005
First published as an Advance Article on the web 20th October 2005
DOI: 10.1039/b512073j
The concept of using equilibrium dynamics to provide for both
protection and unveiling of latent functional groups at
appropriate times in aqueous polymer colloid coatings designed
for crosslinking only during film formation is introduced; the
new functional monomer, 4-hydroxyethylsulfonylstyrene
Fig. 1 Representation of the crosslinking principle with spheres
(HESS), readily undergoes emulsion copolymerization with
representing functional groups on polymer chains. A dynamic equilibrium
acrylates to form stable latexes, followed by crosslinking by loss
unveils a reactive group from a dormant group as water evaporates during
of water during film formation.
film formation, resulting in reaction of mutually-reactive functional groups
to give a crosslink (bold line).
Release of volatile organic compounds (VOCs) into the environ-
ment is a major global issue. A significant contribution to VOC
release is made by the use of solvent-borne coatings, but the move
towards materials that liberate no VOCs is difficult to achieve
without loss of coating performance. One issue is the need to
combine stability during storage with a robust crosslinking
methodology that operates only when needed.1 The crosslinking
reaction must not liberate VOCs and the level of coating
performance must at least match that from traditional technolo-
gies. A large number of different chemistries have been investigated
for crosslinking of water-borne coatings, but no current system
satisfies all the requirements. In parallel with growth of this need,
there has been increased understanding of how to engineer
molecular systems, particularly macromolecules, which respond to
their environment and, in doing so, trigger a chemical process.2 A
fundamental requirement for crosslinkable aqueous polymer
colloid (e.g. latex) coatings is that they must be unreactive during
storage, but reactive to crosslinking during film formation. This led
us to consider a previously unexploited, general principle of using
loss of water from the latex during film formation to trigger
crosslinking. Thus, an ideal scenario is an equilibrium process
involving water that unveils a reactive group which is then able to
crosslink (Fig. 1). In principle, a dynamic equilibrium2 is
established between a functional group which is unreactive to
crosslinking and a derivative that readily crosslinks by reaction
with other functional groups present. Excess water in the latex will
quench any reactive intermediate generated during storage,
preventing premature crosslinking and so providing a built-in
protective function. Loss of water during film formation, however,
triggers re-establishment of the equilibrium, favoring the reactive
form and leading to irreversible crosslinking.
To demonstrate the feasibility of this approach, we report the
synthesis, polymerization and crosslinking chemistry for mono-
mers incorporating a hydroxyethylsulfone 1 (Scheme 1), a
prototypical latent reactive functional group.3 Such hydroxyethyl-
sulfones are in equilibrium with vinylsulfones 2 through loss of
water; the equilibrium normally lying well to the left. Vinylsulfones
2 are potent Michael acceptors and readily react with heteroatomic
and carbon nucleophiles (including water); such chemistry is
exploited in reactive dye technology4 and solid phase synthesis.5 In
the context of crosslinking, incorporation of nucleophilic groups
for reaction with 2 can be accomplished through copolymerization
with an appropriate second functional monomer. Conjugate
addition affords 3, which represents a crosslink for reactions
between functional groups pendent to polymer chains. The process
of converting 1 into 3 is rendered irreversible by loss of water.
Furthermore, this conversion increases the local hydrophobicity,
further excluding water and disfavoring hydrolysis of 3. In
contrast, when an excess of water is present, for example during
latex storage, any vinylsulfone 2 that is generated should
recombine with water to regenerate 1, hence preventing premature
crosslinking.
aSchool of Chemistry, Faraday Building, University of Manchester,
Sackville Street, PO Box 88, Manchester, UK M60 1QD.
E-mail: david.berrisford@manchester.ac.uk
bMaterials Science Centre, School of Materials, University of
Manchester, Grosvenor Street, Manchester, UK M1 7HS.
E-mail: peter.lovell@manchester.ac.uk
In order to investigate this principle, we examined a series of
functional monomers containing acrylate, methacrylate and
cDepartment of Chemistry, Durham University, Science Laboratories,
South Road, Durham, UK DH1 3LE.
E-mail: andy.whiting@durham.ac.uk
{ Electronic supplementary information (ESI) available: synthetic meth-
ods and characterization data for compounds 4, 5, 6 and 8, the emulsion
polymerization formulation and procedure, and the film formation, gel
fraction measurement, and tensile testing procedures. See DOI: 10.1039/
b512073j
Scheme 1 Equilibration of 2-hydroxyethylsulfones with vinylsulfones
and the crosslinking reaction (exemplified with R3OH).
5904 | Chem. Commun., 2005, 5904–5906
This journal is ß The Royal Society of Chemistry 2005