The
classical division of polymeric materials into thermoplastics
and thermosets based on covalent network structure often implies that
these categories are distinct and irreconcilable. Yet, the past two
decades have seen extensive development of materials that bridge this
gap through incorporation of dynamic crosslinks, enabling them to
behave as both robust networks and moldable plastics. Although their
potential utility is significant, the growth of covalent adaptable
networks (CANs) has obscured the line between “thermoplastic”
and “thermoset” and erected a conceptual barrier to
the growing number of new researchers entering this discipline. This
Perspective aims to both outline the fundamental theory of CANs and
provide a critical assessment of their current status. We emphasize
throughout that the unique properties of CANs emerge from the network
chemistry, and particularly highlight the role that the crosslink
exchange mechanism (i.e., dissociative exchange or associative exchange)
plays in the resultant material properties under processing conditions.
Predominant focus will be on thermally induced dynamic behavior, as
the majority of presently employed exchange chemistries rely on thermal
stimulus, and it is simple to apply to bulk materials. Lastly, this
Perspective aims to identify current issues and address possible solutions
for better fundamental understanding within this field.
The translation of
small molecule chemistries into efficient methodologies for polymer
functionalization spans several decades, enabling critical advances
in soft matter materials synthesis with tailored and adaptive property
profiles. The present Perspective exploresbased on selected
examples50 years of innovation in polymer functionalization
chemistries. These span a diverse set of chemistries based on activated
esters, thiol–ene/yne processes, nucleophilic systems based
on isocyanates, reactions driven by the formation of imines and oximes,
ring-opening processes, cycloadditions, andin a recent renaissancemulticomponent
reactions. In addition, a wide variety of chain types and architectures
have been modified based on the above chemistries, often with exquisite
chemical control, highlighted by key examples. We conclude our journey
through polymer functionalization with thein our viewmost
critically required advances that have the potential to move from
“science fiction” to “science fact”.
Cross-linked networks feature exceptional chemical and mechanical resilience but consequently lack recyclability. Vitrimers have emerged as a class of materials that feature the robustness of thermosets and the recyclability of thermoplastics without compromising network integrity. Most examples of vitrimers have involved new polymers with exchangeable bonds within their backbones. In pursuit of a more universal, commercially viable route, we propose a method utilizing commercially available and inexpensive reagents to prepare vitrimers from vinyl monomer-derived prepolymers that contain cross-linkable βketoester functional groups. Controlled radical copolymerization of methyl methracrylate and (2-acetoacetoxy)ethyl methacrylate afforded linear prepolymers that were converted into vitrimers in a single step by treatment with a trifunctional amine. These materials displayed the characteristic features and reprocessability of vitrimers over as many as six (re)processing cycles. Critically, the networks prepared through this process largely retain the chemical and thermal properties of their linear counterparts, suggesting this method holds significant utility as a user-friendly and commercially relevant approach to the rational design of vitrimers with diverse properties.
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