In this paper, we examine a new block copolymer architecture-cyclic block copolymers. The physical behavior of cyclic polymers, both in solution1 and in the bulk,2-3 has been the subject of continual research. Current issues concern their dynamics4-5 and topological characteristics.6-9 Due to the challenges present in the synthesis of cyclic polymers,10 the morphology of microphase-separated cyclic block copolymer systems has not been investigated previously. Recently, however, well-characterized cyclic polystyrene-poly(2-vinylpyridine) (PS-P2VP) and polystyrene-poly(dimethylsiloxane) (PS-PDMS) block copolymers have been synthesized from their linear triblock precursors.11-13 The morphology of these microphase-separated systems is the topic of the present study.The focus of our study is to probe the effect of loops versus bridges on the morphological characteristics of microphase-separated block copolymer systems. Cyclic A-B diblocks only assume doubly-looped chain conformations in the microphase-separated state, while their linear A-B-A triblock precursors are looped and bridged. We anticipate this difference to be manifest in the relative spacings of their respective microlattices. The microstructural characteristics of these systems are examined through transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) of systems exhibiting varying degrees of quench into the microphase-separated state. These observations are then compared to our theoretical predictions.Cyclic PS-P2VP and PS-PDMS were prepared by end-coupling their linear triblock precursors bis-anionic LiP2VP-PS-P2VPLi and LiPDMS-PS-PDMSLi with l,4-bis(bromomethyl)benzene and Cl2Si(CH3)2, respectively. The details of the synthesis of the precursor triblocks, coupling reactions, and characterization of the block copolymers are given elsewhere.11-13 The molecular characteristics of the cyclic diblock and linear triblock precursors used in this study are shown in
Information storage and processing is carried out at the level of individual macromolecules in biological systems, but there is no reason, in principle, why synthetic copolymers should not be used for the same purpose. Previous work has suggested that monomer sequence information in chain-folding synthetic copolyimides can be recognized by tweezer-type molecules binding to adjacent triplet sequences, and we show here that different tweezer molecules can show different sequence selectivities. This work, based on (1)H NMR spectroscopy in solution and on single-crystal X-ray analysis of tweezer-oligomer complexes in the solid state, provides the first clear-cut demonstration of polyimide chain-folding and adjacent-tweezer binding. It also reveals a new and entirely unexpected mechanism for sequence recognition, which, by analogy with a related process in biomolecular information processing, may be termed 'frameshift-reading'. The ability of one particular tweezer molecule to detect, with exceptionally high sensitivity, long-range sequence information in chain-folding aromatic copolyimides is readily explained by this novel process.
Sequence-specific binding is demonstrated between pyrene-based tweezer molecules and soluble, high molar mass copolyimides. The binding involves complementary pi-pi stacking interactions, polymer chain-folding, and hydrogen bonding and is extremely sensitive to the steric environment around the pyromellitimide binding-site. A detailed picture of the intermolecular interactions involved has been obtained through single-crystal X-ray studies of tweezer complexes with model diimides. Ring-current magnetic shielding of polyimide protons by the pyrene "arms" of the tweezer molecule induces large complexation shifts of the corresponding 1H NMR resonances, enabling specific triplet sequences to be identified by their complexation shifts. Extended comonomer sequences (triplets of triplets in which the monomer residues differ only by the presence or absence of a methyl group) can be "read" by a mechanism which involves multiple binding of tweezer molecules to adjacent diimide residues within the copolymer chain. The adjacent-binding model for sequence recognition has been validated by two conceptually different sets of tweezer binding experiments. One approach compares sequence-recognition events for copolyimides having either restricted or unrestricted triple-triplet sequences, and the other makes use of copolymers containing both strongly binding and completely nonbinding diimide residues. In all cases the nature and relative proportions of triple-triplet sequences predicted by the adjacent-binding model are fully consistent with the observed 1H NMR data.
Pyrene-based molecular tweezers show sequence-specific binding to aromatic polyimides through sterically-controlled donor-acceptor pi-stacking and hydrogen bonding; (1)H NMR spectra of tweezer-complexes with polyimides having different sequence-restrictions show conclusively that the detection of long range sequence-information results from multiple tweezer-binding at adjacent imide residues.
A high-resolution crystal structure is reported for d(TpA)*, the intramolecular thymine–adenine photoadduct that is produced by direct ultraviolet excitation of the dinucleoside monophosphate d(TpA). It confirms the presence of a central 1,3-diazacyclooctatriene ring linking the remnants of the T and A bases, as previously deduced from heteronuclear NMR measurements by Zhao et al. (The structure of d(TpA)*, the major photoproduct of thymidylyl-(3′-5′)-deoxyadenosine. Nucleic Acids Res., 1996, 24, 1554–1560). Within the crystal, the d(TpA)* molecules exist as zwitterions with a protonated amidine fragment of the eight-membered ring neutralizing the charge of the internucleotide phosphate monoanion. The absolute configuration at the original thymine C5 and C6 atoms is determined as 5S,6R. This is consistent with d(TpA)* arising by valence isomerization of a precursor cyclobutane photoproduct with cis–syn stereochemistry that is generated by [2 + 2] photoaddition of the thymine 5,6-double bond across the C6 and C5 positions of adenine. This mode of photoaddition should be favoured by the stacked conformation of adjacent T and A bases in B-form DNA. It is probable that the primary photoreaction is mechanistically analogous to pyrimidine dimerization despite having a much lower quantum yield.
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