Si trova su / Altri legami
© ConspectusProteins contain a level of complexity – secondary and tertiary structures – that polymer chemists aim to imitate. The bottom–up synthesis of protein–mimicking polymers mastering sequence variability and dispersity remains challenging. Incorporating polymers with predefined secondary structures, such as helices and Ï€–Ï€stacking sheets, into block copolymers circumvents the issue of designing and predicting one facet of their 3D architecture. Block copolymers with well–defined secondary–structure elements formed by covalent chain extension or supramolecular self–assembly may be considered for localized tertiary structures.In this Account, we describe a strategy toward block copolymers composed of units bearing well–defined secondary structures mixed in a "plug–and–play"manner that approaches a modicum of the versatility seen in nature. Our early efforts focused on the concept of single–chain collapse to achieve folded secondary structures through either hydrogen bonding or quadrupole attractive forces. These cases, however, required high dilution. Therefore, we turned to the ring–opening metathesis polymerization (ROMP) of [2.2]paracyclophane–1,9–dienes (pCpd), which forms conjugated, fluorescent poly(p–phenylenevinylene)s (PPVs) evocative of β–sheets. Helical building blocks arise from polymers such as poly(isocyanide)s (PICs) or poly(methacrylamide)s (PMAcs) containing bulky, chiral side groups while the coil motif can be represented by any flexible chain; we frequently chose poly(styrene) (PS) or poly(norbornene) (PNB). We installed moieties for supramolecular assembly at the chain ends of our "sheets"to combine them with complementary helical or coil–shaped polymeric building blocks.Assembling hierarchical materials tantamount to the complexity of proteins requires directional interactions with high specificity, covalent chain extension, or a combination of both chemistries. Our design is based on functionalized reversible addition–fragmentation chain–transfer (RAFT) agents that allowed for the introduction of recognition motifs at the terminus of building blocks and chain–terminating agents (CTAs) that enabled the macroinitiation of helical polymers from the chain end of ROMP–generated sheets and/or coils. To achieve triblock copolymers with a heterotelechelic helix, we relied on supramolecular assembly with helix and coil–shaped building blocks. Our most diverse structures to date comprised a middle block of PPV sheets, parallel or antiparallel, and supramolecularly or covalently linked, respectively, end–functionalized with molecular recognition units (MRUs) for orthogonal supramolecular assembly. We explored PPV sheets with multiple folds achieved by chain extension using alternating pCpd and phenyl–pentafluorophenyl β–hairpin turns. Using single–molecule polarization spectroscopy, we showed that folding occurs preferentially in multistranded over double–stranded PPV sheets. Our strategy toward protein–mimicking and foldable polymers demonstrates an efficient route toward higher ordered, well–characterized materials by taking advantage of polymers that naturally manifest secondary structures. Our studies demonstrate the retention of distinct architectures after complex assembly, a paradigm that we believe may extend to other polymeric folding systems.
