Free-Radical Ring-Opening Polymerization of Macrocyclic Aryl Ether Thioether Ketone Oligomers

Solid-Phase Thermal Polymerization of Macrocyclic S-Aryl Thioester Trimer with 5-t-Butylisophtaloyl Skeleton Using Crown Ether Complexes

Thermal polymerizations of macrocyclic S-aryl thioester trimer with a 5- t-butylisophtaloyl skeleton ( m-CTE-3) were investigated. The polymerization of m-CTE-3 without catalysts occurred at over 260°C in the solid state and heating at 280°C for 10 min produced a polymer with a Mw of 370 000. Quaternary onium salts or crown ether complexes enhanced the solid-phase thermal polymerization of m-CTE-3. In the case of the polymerization using 18-crown-6 ether (18-C-6)/potassium halide complexes, the polymerization occurred at around 200°C. It was proved that 18-C-6/KBr was the most effective catalyst to produce high molecular weight polymers with Mw as much as 850 000. It was also suggested that the solid-phase polymerization proceeded in a chain-growth reaction mode.

Macrocycles 32. Syntheses of Cyclic Poly(Ether Ketone)s of Bisphenol-A

Two classes of poly(ether ketone)s were prepared from bisphenol-A, namely by polycondensation with 4, 4′-difluorobenzophenone or with 2,6-difluorobenzophenone and 4′-tert.butyl-2,6-difluorobenzophenone. Two different synthetic methods were compared. First, polycondensations of the free bisphenol-A in DMSO or sulfolane with azeotropic distillation of water. Second, polycondensations of bistrimethylsilyl bisphenol-A in N-methylpyrrolidone. The second approach gave higher yields and higher molecular weights ( Mnvalues up to 85000 Da and Mwvalues up to 190000 Da). The matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectra revealed that the fraction of cyclic oligomers and polymers systematically increased with higher molecular weights. A few polycondensations of silylated 4-tert.butylcatechol with 4, 4′-difluorobenzophenone confirmed the trends observed for silylated bisphenol-A. Under optimum conditions cyclic poly(ether ketone)s were detectable in the MALDI-TOF mass spectra up to molecular weights of 18 000 Da.

Dynamic covalent chemistry

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

Semicrystalline Polymers via Ring-Opening Polymerization: Preparation and Polymerization of Alkylene Phthalate Cyclic Oligomers

Preparation of cyclic oligomeric alkylene phthalates via pseudo-high dilution condensation of alkylene diols with iso- and terephthaloyl chlorides and conversion to high molecular weight polyesters via ring-opening polymerization is described. Sterically unhindered amines such as quinuclidine or 1,4-diazabicyclo[2.2.2]octane (DABCO) catalyze the condensation significantly faster than other tertiary amines and are useful for carrying out this conversion in high yield, in the first direct reaction of diol and diacid chloride to form cyclic polyesters. The mixtures of oligomeric cyclics melt at 150-200 degrees C, providing liquids of low viscosity. Ring-opening polymerization using tin or titanate catalysts affords high molecular weight polymers within minutes. Complete polymerization of PBT oligomeric cyclics can be achieved at 180-200 degreesC, significantly below the polymer's melting point of 225 degreesC, and with molecular weights as high as 445 x 10(3). Polymers formed via such a process are more crystalline than conventionally prepared polyesters.