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Single d(GpG)/cis-diammineplatinum(II) adduct-induced inhibition of DNA polymerization.

A 44 nucleotide DNA template containing a single site-specifically placed cisplatin adduct (cis-[Pt(NH3)2[d(GpG)-N7(1),-N7(2)]]) was annealed with a primer, positioning its 3'-end four bases before the adduct in the template strand. DNA polymerization in the presence of all four nucleotides revealed that both HIV-1 reverse transcriptase (RT) and T7 DNA polymerase strongly paused at one nucleotide preceding the first platinated guanine and at the positions opposite the two platinated guanines. Analysis of single nucleotide incorporation at each pause site showed that polymerization occurs with biphasic kinetics. A small percentage of DNA was bound productively, providing a small amplitude (1-3%) of a fast phase of polymerization, whereas most of the bound DNA (1-34%) was positioned at the pause site in a nonproductive manner and therefore elongated slowly (0.04-0.06 s-1). DNA substrates annealed to the cisplatin-modified template bind to HIV-1 RT with an affinity (10-20 nM) similar to that of unmodified substrates (6-9 nM). The cisplatin-DNA cross-link moderately weakened DNA binding to T7 DNA polymerase (12-115 nM) but significantly slowed the rate of incorporation of the next nucleotide (2-7 s-1 ), with larger effects closer to the cisplatin-DNA adduct. The crystal structure of the same cisplatin-DNA adduct [Takahara, P. M., Frederick, C. A., and Lippard, S. J. (1996) J. Am. Chem. Soc. 118, 12309-12321] reveals not only the bent DNA duplex but also the propeller twisted base pairs near the cisplatin-DNA adduct. The twisted base pairs may cause misalignment of the cisplatin-modified DNA at the binding cleft of T7 DNA polymerase and significantly slow the rate of the protein conformational change preceding polymerization, leading to the slight accumulation of intermediates within five base pairs of the adduct. The ground-state binding of the next correct nucleotide to the enzyme.DNA complex was weakened by the adduct with T7 DNA polymerase but unchanged with HIV-1 RT at sites other than the three strong pause sites. Nucleotide binding to both enzymes at the three strong pause sites was significantly weaker and less selective.

Bacteriophage T7↗

Mechanism of the polymerization reaction initiated and catalyzed by the polyhydroxybutyrate synthase of Ralstonia eutropha.

Polyhydroxybutyrate (PHB) synthases (polymerases) catalyze the polymerization of the coenzyme A thioester of 3-hydroxybutyrate to PHB. The Ralstonia eutropha PHB synthase purified from recombinant E. coli cells exists in aqueous solution in both monomeric (single subunit) and homodimeric (two subunits) forms in equilibrium. Several lines of evidence suggest that the homodimer is the active form of the synthase. The initial mechanistic model for the polymerization reaction proposed that two different thiol groups form the catalytic site. The cysteine at 319 has been shown to provide one thiol group that is involved in the covalent catalysis, but a second thiol group on the same protein molecule has not yet been identified. It is suggested that cysteines at 319 from each of the two molecules of a homodimer synthase provide two identical thiol groups to jointly form a single catalytic site. To verify this model using the strategy of in vitro reconstitution, heterodimers composed of the wild-type subunit and of the C(319) mutated subunit were constructed and the activities at various ratios of the wild-type subunit to the mutated subunit were measured. The experimental results indicate that the homodimer is the active form of the enzyme, that the heterodimer containing the mutated subunit has no activity, and that a single cysteine is not sufficient for catalysis. Two identical thiol groups from C(319) residues on each subunit of the homodimer are required to form the catalytic site for the initiation and propagation reactions. We further demonstrate that a dimer synthase that has initiated the polymerization reaction (primed synthase) is significantly more stable against dissociation than the unprimed (unreacted) dimer synthase. These two properties explain the nature of lag phenomenon during the in vitro polymerization reaction catalyzed by this enzyme

Catalysis↗

Polymerization of propyl malolactonate in the presence of Candida rugosa lipase.

To gain better insight into mechanistic features of enzyme-catalyzed malolactonate polymerization, reactions with propyl malolactonate were analyzed while varying enzyme concentration, reaction media composition, and reaction temperature. Monomer conversion and product molecular weights were characterized by (1)H NMR and MALDI-TOF MS, respectively. A high extent of thermal polymerization of propyl malolactonate was observed, while the polymer chain length in all reactions was controlled by the elimination of alpha-hydrogen from propyl malolactonate with formation of a new initiator and the new chains. The most efficient enzymatic catalysis occurred in toluene (2.11 M monomer) at 60 degrees C. Candida rugosa lipase (10 wt %) accelerated polymerization 25-fold over the rate of thermal polymerization. The maximum poly(propyl malate) number-average molecular weight obtained was 5000 Da at 20 wt % enzyme with a polydispersity of 1.15. These values compare with 1800 Da and 1.5, respectively, in the absence of enzyme.

Candida↗

Permanent, nonleaching antibacterial surfaces. 1. Synthesis by atom transfer radical polymerization.

We have grown an antimicrobial polymer directly on the surfaces of glass and paper using atom transfer radical polymerization (ATRP). The method described here results in potentially permanent nonleaching antibacterial surfaces without the need to chemically graft the antimicrobial material to the substratum. The tertiary amine 2-(dimethylamino)ethyl methacrylate was polymerized directly onto Whatman #1 filter paper or glass slides via atom transfer radical polymerization. Following the polymerization, the tertiary amino groups were quaternized using an alkyl halide to produce a large concentration of quaternary ammonium groups on the polymer-modified surfaces. Incubating the modified materials with either Escherichia coli or Bacillus subtilis demonstrated that the modified surfaces had substantial antimicrobial capacity. The permanence of the antimicrobial activity was demonstrated through repeated use of a modified glass without significant loss of activity. Quaternary amines are believed to cause cell death by disrupting cell membranes allowing release of the intracellular contents. Atomic force microscopic imaging of cells on modified glass surfaces supports this hypothesis.

Anti-Bacterial Agents↗

Modeling of lipase catalyzed ring-opening polymerization of epsilon-caprolactone.

Enzymatic ring-opening polymerization of epsilon-caprolactone by various lipases was investigated in toluene at various temperatures. The determination of molecular weight and structural identification was carried out with gel permeation chromatography and proton NMR, respectively. Among the various lipases employed, an immobilized lipase from Candida antartica B (Novozym 435) showed the highest catalytic activity. The polymerization of epsilon-caprolactone by Novozym 435 showed an optimal temperature of 65 degrees C and an optimum toluene content of 50/50 v/v of toluene and epsilon-caprolactone. As lipases can degrade polyesters, a maximum in the molecular weight with time was obtained due to the competition of ring opening polymerization and degradation by specific chain end scission. The optimum temperature, toluene content, and the variation of molecular weight with time are consistent with earlier observations. A comprehensive model based on continuous distribution kinetics was developed to model these phenomena. The model accounts for simultaneous polymerization, degradation and enzyme deactivation and provides a technique to determine the rate coefficients for these processes. The dependence of these rate coefficients with temperature and monomer concentration is also discussed.

Caproates↗

Synthesis and characterization of polymeric soybean oil-g-methyl methacrylate (and n-butyl methacrylate) graft copolymers: biocompatibility and bacterial adhesion.

Peroxidation, epoxidation, and/or perepoxidation reactions of soybean oil under air at room temperature resulted in cross-linked polymeric soybean oil peroxides on the surface along with the waxy soluble part, sPSB, with a molecular weight of 4690, containing up to 2.3 wt % peroxide. This soluble polymeric oil peroxide, sPSB, initiated the free radical polymerization of either methyl methacrylate (MMA) or n-butyl methacrylate (nBMA) to give PSB-g-PMMA and PSB-g-PnBMA graft copolymers. The polymers obtained were characterized by (1)H NMR, thermogravimetric analysis, differential scanning calorimetry, and gel permeation chromatography techniques. Polymeric oil as a plasticizer lowered the glass transition of the PSB-g-PMMA graft copolymers. PSB-g-PMMA and PSB-g-PnBMA graft copolymer film samples were also used in cell culture studies. Fibroblast and macrophage cells were strongly adhered and spread on the copolymer film surfaces, which is important in tissue engineering. Bacterial adhesion on PSB-g-PMMA graft copolymer was also studied. Both Staphylococcus epidermidis and Escherichia coli adhered on the graft copolymer better than on homo-PMMA. Furthermore, the latter adhered much better than the former.

Acrylates↗

Catalytic polymerization of a cyclic ester derived from a "cool" natural precursor.

(-)-Menthide, a seven-membered lactone derived from the natural product (-)-menthol, was polymerized using a structurally defined zinc-alkoxide catalyst to form an aliphatic polyester. The polymer was fully characterized by NMR spectroscopy, size exclusion chromatography, and matrix-assisted laser desorption ionization mass spectrometry. The polymerization reaction occurred in a controlled fashion and polymer samples with M(n) values up to 90 kg mol(-1) were obtained by varying the catalyst loading. Monitoring of the rate of polymerization by in situ FT-IR spectroscopy (ReactIR) revealed a first order dependence on (-)-menthide. The temperature dependence of the observed rate constant between 30 and 90 degrees C was well described by the Arrhenius equation and gave E(a) = 38.4 +/- 0.9 kJ mol(-1). Thermodynamic parameters (deltaH(p) degrees = -16.8 +/- 1.6 kJ mol(-1), deltaS(p) degrees = -27.4 +/- 4.6 J mol(-1) K(-1)) for the polymerization of (-)-menthide were also determined by measuring the equilibrium monomer concentration at different temperatures ranging from 40 to 100 degrees C. The equilibrium monomer concentrations at 25 and 100 degrees C were calculated to be 0.031 +/- 0.018 and 0.120 +/- 0.063 M, respectively.

Catalysis↗

Exploiting the vitamin B12 pathway to enhance oral drug delivery via polymeric micelles.

Vitamin B12 (VB12)-modified dextran-g-polyethyleneoxide cetyl ether (DEX-g-PEO-C16) was synthesized by linking VB12 residues to a DEX-g-PEO-C16 copolymer via a 2,2'-(ethylenedioxy)bis(ethylamine) spacer. The level of VB12 substitution on the DEX-g-PEO-C16 copolymer reached 1.68% (w/w). In aqueous solution, DEX-based copolymers form micelles that can entrap within their hydrophobic core up to 8.5% w/w of cyclosporin A (CsA), a poorly water soluble immunosuppressant. The permeability of Caco-2 cell membranes to CsA incorporated in VB12 modified and unmodified polymeric micelles was monitored in the presence and absence of intrinsic factor (IF). The apical (AP) to basolateral (BL) permeation of CsA through Caco-2 cell monolayers after 24 h of transport was significantly higher (1.8 and 2.3 times in absence and presence of IF, respectively) in the case of CsA loaded in VB12-modified polymeric micelles, compared to CsA in unmodified micelles. The results point to possible improvement in the application of polysaccharide-based polymeric micelles as targeted polymeric drug carriers for the oral delivery of poorly water soluble drugs.

Biological Transport↗

Molecular blending by polymerization of intercalated solvent. Poly(gamma-benzyl-L-glutamate)/benzyl methacrylate as a model system.

The aim of the present research is to obtain blending between a polymer and a (polymerized) solvent on the molecular level. Because of its rigid rod structure, poly(gamma-benzyl-L-glutamate) (PBLG) is chosen as the polymer. Benzyl methacrylate (BzMA) has been chosen as the solvent for two reasons. First, the structure of the solvent is very similar to the structure of the side chain of PBLG, favoring interactions between the two materials. Second, the solvent can be polymerized, because of the presence of a C=C bond. In cast films of PBLG and BzMA separate zones of the polymer and solvent are present. Wide-angle X-ray diffraction and Raman results show that upon heating the cast films homogenization occurs and solvent molecules intercalate between the helices of PBLG. At 150 degrees C a hexagonal packing is obtained. The dimensions of the obtained packing depend on the solvent concentration, which confirms that solvent molecules are indeed present within the crystalline lattice. DSC experiments imply that the observed changes upon heating correspond to thermodynamic processes. On cooling the homogeneous samples, disordering of the hexagonal packing occurs. Polymerization of the homogeneous samples results in a disordering of the hexagonal packing and in a contraction of the unit cell. The latter once more confirms that solvent molecules are indeed present within the crystalline lattice. The applied principle of polymerization of a solvent in a molecular homogeneous system can be favorable for many applications, for which morphology control at the molecular level is required.

Biopolymers↗

Effect of NaBH4 concentration and reaction time on physical properties of glutaraldehyde-polymerized hemoglobin.

The US is about 1.5 days away from exhausting its entire blood supply. Hence, there is an urgent need for the development of universal blood substitutes. One such blood substitute is glutaraldehyde-polymerized bovine hemoglobin. Despite the commercial development of glutaraldehyde-polymerized bovine hemoglobin-based blood substitutes, there has been little published work on the effect of reaction conditions on the physical properties of the polymerized hemoglobin dispersion. In this study, the effect of varying the concentration of the quenching agent NaBH(4), glutaraldehyde concentration, and reaction time on the physical properties of PolyHb dispersions was studied by measuring the absolute molecular weight distribution, as well as oxygen-binding properties such as P(50), Hill coefficient, and methemoglobin level (MetHb) of these dispersions. Bovine hemoglobin was polymerized with glutaraldehyde using a parallel synthetic approach. Asymmetric flow field-flow fractionation (AFFF) coupled with multi-angle static light scattering (MASLS) was used to measure the absolute molecular weight distribution of the PolyHb dispersions. In general, high glutaraldehyde concentrations (20-30 times the molar concentration of hemoglobin) adversely affected the oxygen-binding properties of PolyHb dispersions, while NaBH(4) concentrations (up to 300 times the molar concentration of hemoglobin) and reaction times (up to 24 h) did not appear to have any effect on the oxygen-binding properties of PolyHb dispersions.

Animals↗

Surfactant Mediated Cationic and Anionic Suspension Polymerization of PEG-Based Resins in Silicon Oil: Beaded SPOCC 1500 and POEPOP 1500.

A novel surfactant has been synthesized for use in cationic and anionic ring-opening suspension polymerization of PEG-based macromonomers in silicon oil. A polymer of acrylate esters containing pentamethyldisiloxane and PEG was prepared by radical polymerization. The surfactant can stabilize an emulsion of PEG-based macromonomers, initiator, and solvent in silicon oil such that polymer beads are obtained by ring-opening polymerization, initiated either by a Lewis acid (cationic ring opening) or potassium tert-butoxide (anionic ring opening). The average bead size could be controlled by varying the stirring rate and the amount of surfactant and solvent. The surfactant does not interfere with the polymerization and can be removed together with residual silicon oil by a simple washing procedure.

Journal Article↗

Optimization of polymerization conditions of furan with aniline for variable conducting polymers.

A high-throughput multiparameter optimization of chemical oxidative polymerization conditions has been developed for a facile synthesis of furan homopolymers and furan/aniline copolymers using a combinatorial method. The polymerization yield, molecular structure, and properties of the polymers would be optimized against typical polymerization parameters, including oxidant species, medium species, temperature, oxidant/monomer ratio, monomer concentration, dopant concentration, and furan/aniline comonomer ratio. The electrical conductivity, lead ion adsorptivity, chemical resistance, and thermal behavior of the polymers were also elaborated. It is found that only a combination of FeCl(3) and nitromethane as oxidant and medium, respectively, is appropriate for the furan homopolymerization. The homopolymerization yield increases consistently with an increase in the monomer concentration from 0.05 to 0.2 M and the FeCl(3)/furan molar ratio from 0.25 to 1.25. Although the as-prepared polyfuran exhibits very low conductivity, down to 10(-11) S cm(-1), the HCl- and HClO(4)-doped polyfurans possess much higher conductivities of 9.2 x 10(-8) and 2.38 x 10(-5) S cm(-1), respectively. In addition, the conductivity of the furan/aniline copolymer rises steadily with increasing aniline content, although the copolymerization yield shows a minimum at the furan/aniline molar ratio of 60/40, which is evidence of the occurrence of a real copolymerization between the furan and aniline monomers. The difficulty of synthesizing conducting polyfuran could be overcome to some extent by the polymerization in an appropriate condition optimized in this study. Particularly, the difficulty of synthesizing poly(furan-co-aniline) having much higher conductivity than the polyfuran would be largely conquered by chemical oxidative copolymerization of furan with aniline.

Aniline Compounds↗

Preparation of Designer Resins via Living Free Radical Polymerization of Functional Monomers on Solid Support.

Merrifield resin is converted to a solid-supported free radical initiator by reacting with the TEMPO-Na. Heating TEMPO-methyl resin with a variety of functionalized styrene and acrylate monomers gives larger resin beads via living free radical polymerization. We have coined the term Rasta resin to describe resin beads prepared in this fashion. The process can be described as a solvent-free suspension polymerization. It is particularly well suited for preparation of resin beads from monomers which contain electrophilic groups that would be destroyed upon suspension polymerization in water. Rasta resins have a novel macromolecular architecture wherein long straight chain polymers bearing reactive functional groups emanate from the phenyl groups of a cross-linked polystyrene core. With judicious choice of co-monomers and polymerization strategy, the solvent affinity, loading capacity, and distance of functionality from the cross-linked core may be controlled giving beads with properties that are tailored to specific uses as synthesis supports and scavenging resins.

Journal Article↗

Exploration of the conformational space of a polymeric material that inhibits human immunodeficiency virus.

Baertschi et al. (Antiviral Chem. Chemother. 1997, 8, 353-362) clarified the nature of a polymeric degradation product formed from the cephalosporin ceftazidime. Interest in the polymeric material arises from its ability to inhibit the RNase H and polymerase activities of HIV-1 reverse transcriptase (RT). To shed light on the structure of the polymeric material like that which forms from degradation of third-generation cephalosporins, we apply molecular modeling and other computational chemistry techniques. Aminothiazole methoxime (2-amino-4-thiazolyl-methoxyimino; ATMO) is the parent structure related to the isolated degradation product of ceftazidime. The MMFF94 force field and Monte Carlo multiple minimum method as implemented in MacroModel are used to generate low-energy conformers. We built up oligomeric models starting from the trimer to the 16-mer and performed distribution analyses on the dihedral angles from the Monte Carlo runs to analyze the three-dimensional shapes of the oligomers. Although the larger oligomers are too long for a complete search of conformational space, the low-energy conformers examined do not show secondary structure or repetitive conformations. Polymeric ATMO material may, therefore, exhibit only random coil conformations. Topological similarity of ATMO structures to other reported RT inhibitors is also examined.

Anti-HIV Agents↗

Synthesis and characterization of monomeric, oligomeric, and polymeric aluminum 8-hydroxyquinolines.

We report the synthesis and characterization of monomeric, oligomeric, and polymeric aluminum 8-hydroxyquinolines. The new structures of aluminum quinolate are contrived for expanding the application of AlQ(3) in the area of solution process by modifying AlQ(3) structure for improving solution processibility and crystallization resistance. Oligomeric aluminum 8-hydroxyquinoline (OALQ) was obtained using methylaluminoxane (MAO) and 8-hydroxyquinoline (8-HQ). Polymeric aluminum 8-hydroxyquinoline (PALQ) consists of 8-HQ and a polymeric Al-O backbone, simply prepared by stoichiometrically reacting 8-hydroxyquinoline, pentaerythritol propoxylate, and triethyl aluminum in the presence of chloroform. The absorption and emission spectra of OALQ and PALQ bear a clear resemblance to those of AlQ(3), and the molecular orbitals of OALQ and PALQ are virtually identical to those of AlQ(3). In the SEM images of AlQ(3) and OALQ, cylindrical rods of >100 microm in length and 5-10 microm in diameter for AlQ(3) and 20-100 microm in length and 1-5 microm in diameter for OALQ were observed, respectively. The size of the cylindrical rods of OALQ decreased compared with that of AlQ(3). As for the image of PALQ, an amorphous phase with bulge spots (ca. 5 microm) was observed. These microscope data correspond well to the X-ray powder pattern results. The chemical shifts (31.1, 57.0 ppm) and peak broadness of (27)Al NMR of AlQ(3) and its DFT calculation results present that mer- and fac-AlQ(3) appear in equilibrium through pentacoordinated intermediates. With the combination of DFT optimization and NMR calculation, models of OALQ and PALQ, hexa-, penta-, and tetracoordinated structures, were proposed, which exist in polymeric Al-O backbone and with inter- and intracoordination of Al-O bonds.

Journal Article↗

A novel consecutive chain transfer reaction to p-methylstyrene and hydrogen during metallocene-mediated olefin polymerization.

This paper describes the first example of consecutive chain transfer reaction, first to p-methylstyrene (or styrene) and then to hydrogen, during metallocene-catalyzed propylene polymerization by rac-Me(2)Si[2-Me-4-Ph(Ind)](2)ZrCl(2)/MAO complex. The PP molecular weight is inversely proportional to the molar ratio of [p-methylstyrene]/[propylene] and [styrene]/[propylene] with the chain transfer constants of k(tr)/k(p) = 1/6.36 and 1/7.5, respectively. Although hydrogen does not influence the polymer molecular weight, it greatly affects the catalyst activity. Each PP chain formed contains a terminal p-methylstyrene (or styrene) unit. The terminal p-MS unit can be metalated to form a stable polymeric anion for living anionic polymerization to prepare new PP diblock copolymers, such as PP-b-PS, which are very difficult to prepare by other methods. The overall process resembles a transformation reaction from metallocene to living anionic polymerization.

Journal Article↗

Stochastic simulations of polymer growth and isomerization in the polymerization of propylene catalyzed by Pd-based diimine catalysts.

A model is presented that employs a stochastic approach to the simulation of polyolefin chain growth and isomerization. The model is applied to propylene polymerization catalyzed by Pd-based diimine catalysts. The stochastic approach links the microscopic (quantum chemical) approach with modeling of the macroscopic systems. The DFT calculated energies of the elementary reactions and their barriers have been used as input parameters for the simulations. The influence of the catalyst's steric bulk, as well as polymerization temperature and olefin pressure on the polymer branching and its microstructure, is discussed. The results are in good agreement with available experimental data. In the propylene polymerization catalyzed by Pd(II) complexes with methyl backbone- and -Ph-(i)Pr(2) imine substituents a number of branches of 238 branches/1000 C have been obtained. An increase in polymerization temperature leads to a decrease in the number of branches. Change in olefin pressure does not affect the global number of branches, while it strongly affects the polymer microstructure, leading to hyperbranched structures at low pressures. Further, the simulations confirm the experimental interpretation of the mechanistic details for this process: (1) both 1,2- and 2,1-insertion happen with the ratio of ca. 7:3; (2) there are no insertions at the secondary carbons; and (3) most of the 2,1-insertions are followed by a chain straightening isomerization. Thus, for this catalyst the total number of branches is controlled exclusively by the 1,2-/2,1-insertion ratio. For the catalysts with different substituents the branching can be controlled by a 1,2-/2,1-insertion ratio as well as the fraction of the insertions at the secondary carbons. The results of the present studies demonstrate that a stochastic approach can be successfully used to model the polyolefin microstructures and their catalyst, temperature, and pressure dependence. Further, it can also facilitate interpretation of the experimental results, and can be used to draw general conclusions about the influence of the specific elementary reaction barriers on the polymer structures; this can be helpful for a rational design of the catalysts producing a desired microstructure.

Journal Article↗

Crystal engineering for topochemical polymerization of muconic esters using halogen-halogen and CH/pi interactions as weak intermolecular interactions.

We now report the molecular and crystal structure design of muconic ester derivatives on the basis of crystal engineering using halogen-halogen contacts and CH/pi interactions. The solid-state photoreaction pathway of the dibenzyl (Z,Z)-muconates as the 1,3-diene dicarboxylic acid monomers depends on the structure of the ester groups. The substitution of a halogen atom for the aromatic hydrogen of a benzyl group induces topochemical polymerization to produce stereoregular polymers in a crystalline form, whereas the unsubstituted benzyl derivative isomerizes to yield the corresponding E,E isomer under similar conditions. The topochemical polymerization process is directly confirmed by the fact that the single-crystal structures before and after the polymerization are very similar to each other. From the crystal structure analysis for a series of substituted benzyl (Z,Z)- and (E,E)-muconates, it has been revealed that the planar diene moieties are closely packed to form a columnar structure in the crystals. The stacking of the polymerizable monomers is characterized by a stacking distance of 4.9-5.2 A along the columns. This structure is supported by a halogen-halogen interaction between the chlorine or bromine atoms introduced at the p position of the benzyl groups in addition to an aromatic stacking due to the CH/pi interaction between the benzylic methylene hydrogens and aromatic rings. The design of a monomer packing corresponds to the type and position of the introduced halogen atom and also the polymorphs. To make a stacking distance of 5 A using both halogen-halogen and CH/pi interactions as supramolecular synthons is important for the molecular design of muconic ester derivatives appropriate for topochemical polymerization.

Journal Article↗