2. The strategies to incorporate AIE
molecules into polymers by
RDRP.
By exploiting the versatility of RDRP, AIE molecules can be facilely
incorporated into the final polymer product through multiple ways to
ultimately synthesize AIE-active polymers; block copolymer containing
separate blocks of non-AIE monomers and AIE monomers (Scheme
1A ), surface-grafted block copolymer (Scheme 1B ), AIE monomers
as crosslinkers for two block copolymers (Scheme 1C ), AIE
moieties as pendent groups in block copolymer (Scheme 1D ),
hyperbranched with random distribution of AIE monomers linked by non-AIE
monomer backbone (Scheme 1E ), four-arm star polymer with AIE
moiety as the central core (Scheme 1F ), end-functionalized AIE
moiety chain extended with non-AIE monomers (Scheme 1G ), direct
linkage of AIE monomers (Scheme 1H ), and unusual AIE behavior
of non-AIE monomers after surface-grafted direct linkage (Scheme
1I ).
Many different combinations of monomers, initiators, linkers, and
catalysts type and amount were discovered and experimented to synthesize
AIE polymers since the advent of AIE discovery. A few of the more
interesting reaction types as examples are listed in Table 1that incorporate AIE characteristics into the final polymeric product.
For the reaction types categorized as “RAFT” above, AIBN is typically
used as the standard initiator and includes one type of RAFT agent as
the chain transfer agent (CTA). Most of the polymerization reactions
listed are considered to be multi-component reactions (MCRs) where more
than one type of monomer is involved. For example, Entry 1 –
Entry 4 (Table 1 ) involves three different monomers and is
considered a random copolymer reaction, where Entry 4 shows a
special case of one of the monomer block (PEG) coupled to the RAFT agent
to form a macro-CTA. Entry 5 and Entry 6(Table 1 ) are considered a four-component and two-component
reaction respectively. Entry 1 – Entry 6 (Table 1 )
are classified as block copolymers containing AIE monomers
(Scheme 1A ). A two-component surface-initiated reaction with
the RAFT agent anchored onto a solid surface (Entry 7 ,Table 1 ) allows for rapid functionalization through
surface-grafted block copolymers of non-AIE and AIE monomers
(Scheme 1B ). Further expansion of this concept enabled the
successful synthesis of a hyperbranched polymer variant using a non-AIE
vinyl-functionalized diamide as a cross-linking backbone (Entry
8, Table 1 ) to incorporate the AIE monomers as pendent groups randomly
distributed across the entire polymer molecule (Scheme 1E ).
Functionalization of symmetrical AIE monomers with vinyl bonds imparts
cross-linking capabilities in a two-component reaction (Entry 9and Entry 10, Table 1 ) to crosslink two block copolymers made
up of the other non-AIE monomer (Scheme 1C ). Similar to a
hyperbranched polymer mentioned before, the incorporation of an AIE
monomer as a pendent group in an MCR (Entry 11, Table 1 ) also
endows the block copolymer with AIE properties (Scheme 1D ).
For all reaction types categorized as “ATRP” above, most of the
examples uses AIE molecule-functionalized macro-initiator to produce the
final polymeric product possessing AIE characteristics. For example,
ATRP initiator core-functionalized with AIE moiety (Entry 12and Entry 13, Table 1 ) generates four-arm star polymer after
polymerizing with non-AIE monomers endows it with unique AIE
fluorescence properties (Scheme 1F ). Another popular choice for
incorporating AIE groups into polymers is through end-functionalized AIE
moieties (Entry 14 – 19, Table 1 ) with subsequent chain
extension using non-AIE monomers (Scheme 1G ). Polymerizing
directly vinyl-functionalized AIE monomers (Entry 20, Table 1 )
provides ease in handling the reaction via direct linkages of the AIE
monomers (Scheme 1H ). Surface-initiated ATRP of AIE-inactive
acrylonitrile monomers (Entry 21, Table 1 ) yielded unusual AIE
fluorescence behavior in the final product without presence of any
phenyl groups or aromatic rings (Scheme 1I ).
For all reaction types categorized as “Others” above, although Cu(0)
polymerizations bear similarities to ATRP, they use different oxidation
states of Cu and are considered different to ATRP. Similar toEntry 1 , AIE monomers are attached to the polymeric product
through ionic bonding with N -containing non-AIE monomers as
backbone (Entry 22, Table 1 ) to form block copolymers with the
AIE moieties as pendent groups (Scheme 1D ). Similar toEntry 20 , unique designs of AIE monomers are polymerized
separately (Entry 23, Table 1 ) through direct linkages
(Scheme 1G ). Similar to Entry 1 – 6 , complex AIE
active monomers functionalized with vinyl bonds (Entry 24, Table
1 ) allows for direct block copolymer synthesis of the AIE monomers
separately (Scheme 1A ). Similar to Entry 14 – 19 , AIE
end-functionalized polymers (Entry 25 and Entry 26,
Table 1 ) are produced through polymerizing with non-AIE monomers
(Scheme 1G ).