Keren Zhang

Nucleobase-functionalized acrylic ABA triblock copolymers and supramolecular blends

Reversible addition-fragmentation chain transfer (RAFT) polymerization afforded the unprecedented synthesis of well-defined acrylic ABA triblock copolymers with nucleobase-functionalized external blocks and a central poly(n-butyl acrylate) (PnBA) block. Size exclusion chromatography (SEC) confirmed the molecular weight and molecular weight distribution of the central block. 1H NMR spectroscopy revealed the successful chain extension of the PnBA macro-chain transfer agent (CTA) using adenine or thymine-functionalized acrylic monomers. The acrylic monomer with a flexible spacer to the pendant nucleobases promoted intermolecular recognition of nucleobases and long range segmental motion of polymer main chains. The external block glass transition temperatures (Tgs) of thymine (T) and adenine (A) functionalized blocks were 52 °C and 76 °C, respectively. Thermomechanical and morphological analysis revealed the effect of processing conditions on self-assembly and microphase-separated morphology of nucleobase-functionalized ABA copolymers. Thymine and adenine-functionalized ABA triblocks formed a thermodynamically stable, hydrogen-bonded complex upon blending. The supramolecular blend exhibited a cylindrical microphase-separated morphology with an extended plateau window compared to the individual block copolymers. The complementary hydrogen bonding between adenine and thymine formed a thermally labile, physically crosslinked, network that exhibited enhanced mechanical performance with melt processability. Thus, these ABA nucleobase-functionalized block copolymers demonstrate potential as thermoplastic elastomers for hot melt adhesives and coatings.

Non-isocyanate poly(amide-hydroxyurethane)s from sustainable resources

A two-step synthesis of epoxidation and carbonation afforded a hetero-functional AB monomer with cyclic carbonate and methyl ester (CC-ME) using plant oil-based methyl 9-decenoate and CO2. A unprecedented one-pot synthetic platform of CC-ME with 1,12-diaminododecane and poly(tetramethylene oxide) (PTMO)-based polyether diamine allowed synthesis of both nonsegmented poly(amide-hydroxyurethane) (PA12HU) and segmented PA12HU-PTMOs with varying polyether contents. 1H NMR spectroscopy confirmed complete conversion of cyclic carbonates and methyl esters to hydroxyurethanes and amides, respectively. Thermal analysis revealed distinctive thermal stability and transitions of PA12HU and PA12HU-PTMOs compared to their precursors and model oligomers. PA12HU and PA12HU-PTMOs were melt compression molded into semicrystalline, free-standing films, except for PA12HU-PTMO100 with 100% polyether diamine. PA12HU-PTMO100 was a viscous liquid with a glass transition temperature (Tg) of −64 °C and zero-shear melt viscosity of 449 Pa s. PA12HU formed a semicrystalline, rigid film with Tg of 11 °C. Polyether incorporation afforded creasable PA12HU-PTMO films with broad glass transitions near −50 °C. Thermal and thermomechanical analysis revealed significant phase-mixing of the hard and soft segments from annealed PA12HU-PTMO films. Polyether soft segments mixed with the amorphous hard segments, forming a miscible soft phase; crystallizable hard segments with ordered hydrogen bonding formed a hard phase. Surface morphological analysis of each PA12HU-PTMO film displayed ribbon-like, hard domains with composition-dependent aspect ratios. PA12HU-PTMOs exhibited higher moisture uptake than traditional thermoplastic polyurethane (TPU) due to resultant hydroxyls. Variable temperature FTIR spectroscopy demonstrated that ordered hydrogen bonding in the crystalline domains was disrupted or dissociated as the crystallites melted. Although tensile strength of segmented PA12HU-PTMOs proved lower than traditional polyurethanes due to phase-mixing, these compositions represent the first examples of film-forming, linear isocyanate-free polyurethanes with mechanical integrity and processability.

Ureido cytosine and cytosine-containing acrylic copolymers

Regioselective Michael addition afforded a novel N1-substituted cytosine acrylate monomer for the synthesis of acrylic random copolymers with cytosine pendant groups. Quantitative post-functionalization converted cytosine to ureido-cytosine (UCy) with an increased self-association strength due to quadruple hydrogen bond formation. Thermogravimetric analysis (TGA) revealed a lower onset temperature of weight loss (∼200 °C) for UCy-containing copolymers, however, they proved to be more thermally stable at ≤130 °C than the cytosine-containing precursors during isothermal rheological experiments. The incorporation of UCy into random copolymers resulted in higher Tgs, enhanced mechanical performance, and better microphase-separation than the cytosine-containing precursors. Both dynamic mechanical analysis and rheological analysis revealed a plateau regime for each UCy-containing copolymer as well as a tan delta transition that corresponded to hydrogen bond dissociation. In contrast, the viscoelastic behavior of cytosine-containing random copolymers resembled entangled, non-associating polymers with increasing Tg as the cytosine content increased. A solution-cast UCy-containing copolymer film exhibited a more well-defined surface morphology with nano-fibrillar hard domains compared to the cytosine control. Variable temperature FTIR spectroscopy verified the presence of hydrogen bonding, and thermogravimetric sorption analysis (TGA-SA) compared the water uptake of UCy and cytosine-containing copolymers. UCy-containing random copolymers showed various advantages for applications as adhesives and thermoplastic elastomers compared to the cytosine copolymers, including superior cohesive strength, higher thermal stability, wider service temperature window, and lower moisture uptake. Free radical copolymerization of a quadruple hydrogen bond containing acrylic monomers provides a versatile avenue to supramolecular polymers with a tunable composition and improved scalability compared to earlier telechelic oligomers. This report describes the first synthesis of an acrylic monomer family and complementary evidence for tunable association in random copolymers.

Styrenic DABCO salt-containing monomers for the synthesis of novel charged polymers

A facile, two-step synthesis afforded styrenic DABCO salt monomers bearing two cyclic quaternary ammonium cations. Free radical polymerization of the DABCO salt monomers yielded homopolymers and copolymers with n-butyl acrylate. DABCO salt-containing ionomers exhibited superior thermomechanical properties with extended plateau regimes compared to ionomers bearing singly-charged pendant groups.

Acetyl-protected cytosine and guanine containing acrylics as supramolecular adhesives

ABSTRACT Hydrogen bonding among nucleobase pairs serves as an efficient noncovalent interaction for designing supramolecular polymers with desired properties for pressure sensitive adhesives. Michael addition yielded acetyl-protected cytosine/guanine containing acrylic monomers with flexible spacers between the hydrogen bonding units and the acrylic backbone. Free radical polymerization of nucleobase-containing monomers afforded acetyl-protected cytosine/guanine homopolymers and random copolymers with n-butyl acrylate. Nucleobase incorporation significantly affected thermal, thermomechanical, rheological, morphological properties, and adhesive performance of polyacrylates. Guanine/cytosine-containing copolymers each exhibited a single glass transition (Tg) that increased with increasing nucleobase content. Self-association of acetyl cytosine and acetyl guanine units converted low Tg polyacrylates to physically crosslinked networks with mechanical integrity. Solution casting acetyl guanine-containing copolymers with 8 mol% or higher guanine content yielded free-standing films with microphase-separated morphologies. Acetyl cytosine-containing copolymers with 15 mol% or more cytosine formed free-standing films with less microphase-separation compared to the guanine copolymers. 1H NMR titration experiments established a 1:1 binding stoichiometry between acetyl cytosine and acetyl guanine monomers in CDCl3, similar to guanine-cytosine association. However, the acetyl protecting group hindered the formation of triple hydrogen bonding, resulting in double hydrogen bonding between acetyl cytosine and acetyl guanine with an intermediate binding strength comparable to their self-associations. Acetyl guanine-containing copolymers with 3 mol% acetyl guanine exhibited higher peel strength on stainless steel and higher extended service frequency range compared to cytosine-containing copolymers and various pressure sensitive adhesive controls.