Pressure-Induced Polymerization: Addition and Condensation Reactions

Under pressure of 1–100 GPa, unsaturated organic molecules tend to form covalent bond to each other for a negative enthalpy change, which often produces polymeric materials with extended carbon skeleton. The polymerization reactions typically happen in crystal, which promotes the topochemical process. This review summarized the topochemical polymerization processes of several alkynes, aromatics, and alkynylphenyl compounds, including the critical crystal structures before the reaction, bonding process, and the structure of the products. Secondly, this review also summarized the condensation reaction identified in the polymerization process, including the elimination of small molecules such as NH3, etc.

Understand the Specific Regio- and Enantioselectivity of Fluostatin Conjugation in the Post-Biosynthesis

Fluostatins, benzofluorene-containing aromatic polyketides in the atypical angucycline family, conjugate into dimeric and even trimeric compounds in the post-biosynthesis. The formation of the C–C bond involves a non-enzymatic stereospecific coupling reaction. In this work, the unusual regio- and enantioselectivities were rationalized by density functional theory calculations with the M06-2X (SMD, water)/6–311 + G(d,p)//6–31G(d) method. These DFT calculations reproduce the lowest energy C1-(R)-C10′-(S) coupling pathway observed in a nonenzymatic reaction. Bonding of the reactive carbon atoms (C1 and C10′) of the two reactant molecules maximizes the HOMO–LUMO interactions and Fukui function involving the highest occupied molecular orbital (HOMO) of nucleophile p-QM and lowest unoccupied molecular orbital (LUMO) of electrophile FST2− anion. In particular, the significant π–π stacking interactions of the low-energy pre-reaction state are retained in the lowest energy pathway for C–C coupling. The distortion/interaction–activation strain analysis indicates that the transition state (TScp-I) of the lowest energy pathway involves the highest stabilizing interactions and small distortion among all possible C–C coupling reactions. One of the two chiral centers generated in this step is lost upon aromatization of the phenol ring in the final difluostatin products. Thus, the π–π stacking interactions between the fluostatin 6-5-6 aromatic ring system play a critical role in the stereoselectivity of the nonenzymatic fluostatin conjugation.

The Effect of Bond Phase Additive and Sintering Temperature on the Properties of Mullite Bonded Porous SiC Ceramics

Currently, porous SiC ceramics have been a focus of interesting research in the field of porous materials due to their excellent structural properties, high strength, high hardness, and superb mechanical and chemical stabilities even at high temperatures and hostile atmospheres. Porous SiC ceramics have been considered as suitable candidate materials for catalyst supports [1-2], hot gas or molten metal filters [3], high temperature membrane reactors [4], thermal insulating materials [5], gas sensors [6] etc. Porous SiC ceramics are fabricated by various methods including partial sintering [7], carbothermal reduction [8-9], replication or pyrolysis of polymeric sponge [10-12], reaction bonding [13] etc. In all these methods SiC needs to be sintered which requires a very high temperature due to the strong covalent nature of the Si-C bond, selective sintering additives, expensive atmosphere, costly and delicate instrumentation. Processing of porous SiC ceramics at low temperature using a simple technique thus becomes necessary. Bonding of SiC can be done at low temperatures by use of different oxide and non-oxide secondary phases. They include silica, mullite, cordierite, silicon nitride, etc. Various sintering additives are used for the formation of variety of secondary oxide bond phases for formations for porous SiC [14-19] Choice of mullite as a bond for SiC has many advantages. Mullite possesses a high melting point (Tm= 1850°C) and a low oxygen diffusion coefficient (5.6 x 10-14 m2/sec at 50°C). It has a matching thermal expansion coefficient with SiC (CTEmullite= 5.3 ×10-6/K; CTESiC = 4.7 ×10-6/K at RT-1000 °C) and a high strength that can be retained up to a very high temperature. Different sources of aluminum, such as Al2O3, Al, AlN, and Al (OH)3 powders were used for the formation of mullite bonded porous SiC ceramics (MBSC) [20-21]. However, the mullitization temperature of 1550o C is still necessary. In this work, mullite bonded porous SiC ceramics were fabricated by an in situ reaction-bonding process; the mixture of clay and CaCO3 were chosen as sintering additives to lower the mullitization reaction between Al2O3 and oxidation-derived SiO2. The effect amount of alumina, sintering temperature and other sintering aids on material property such as porosity/pore size distribution mechanical and micro structural properties of porous oxide bonded SiC ceramics were studied.