Poly thermal rearrangement

Other organosilicon polymer precursors for ceramics have either been prepared or improved by means of transition metal complex-catalyzed chemistry. For instance, the Nicalon silicon carbide-based ceramic fibers are fabricated from a polycarbosilane that is produced by thermal rearrangement of poly(dimethylsilylene) [18]. The CH3(H)SiCH2 group is the major constituent of this polycarbosilane. 

Choi Jl, Jung CH, Han SH, Park HB, Lee YM. Thermally rearranged (TR) poly(benzoxazole-co-pyrrolone) membranes tuned for high gas permeability and selectivity. J Membr Sci 2010 349(l-2) 358-368. 

The thermal rearrangement of a-hydroxyl-PI membranes improves the gas permselectivity properties in comparison to a neat PI. By intro-dueing segments within the polymer that do not undergo thermal rearrangement, the gas separation properties of the thermally rearranged membrane can be modified. PI copolymers based on 4,4 -hexafluoroisopropylidene diphthalic anhydride and diamines 3,3 -dihydroxy-4,4 -diamino-biphenyI with 2,3,5,6-tetramethyI-l,4-phenyIenediamine or 9,9 -bis(4-aminophenyl)fluorene, thermally rearranged into poly(benzoxazole-co-imide), were tested [107]. 

Scholes CA, Ribeiro CP, Kentish SE, Freeman BD. Thermal rearranged poly(benzoxazole-co-imide) membranes for C02 separation. J Membr Sci 2014 450 72-80. 

Figure 5.39 Thermally rearranged poly(ether-benzoxazole) [69],...

Han et al. investigated the gas separation behavior of the PBOs (Figure 5.57) prepared from thermal rearrangement of the fluorinated o-HPAs [80]. The thermal rearrangement occurred at a comparatively low temperature (350 °C) than the precursor poly-imides. The cavity sizes and distribution of FFV elements were tuned to obtain a higher combination of permeability (Phj = 206 Barrer) and selectivity by changing the precursor HPA structure and thermal treatment. The reduction of CO2 solubility for PBO in comparison to the precursor HPAs improved the H2/CO2 selectivity (a = 6.2 at 210 °C, in which Ph2 > 200 Barrer) and moved the membrane performance to polymeric upper bound (Robeson upper bound). 

M. Calle, C.M. Doherty, A.J. Hill, Y.M. Lee, Cross-linked thermally rearranged poly (benzoxazole-co-imide) membranes for gas separation. Macromolecules 46 (20) (2013) 8179-8189. 

Unsaturated polymers, particularly of dienic monomers, undergo a number of interesting thermal rearrangements under non-pyrolytic conditions. Golub has studied these reactions in some detail and has recently reviewed the subject. In the past two years accounts of thermal rearrangements of l,2-poly(hexa-l,4-diene)s and l,2-poly(/ra 5-penta-l,3-diene) have appeared. The former polymers have a predominantly 1,8-diene structure and cyclize mainly by a [2 + 2] or type II mechanism accompanied by a small amount of a type III reaction (Scheme 24). The latter is more important in the trans-, A- than in the cir-1,4-isomer. The first unambiguous example of a type III reaction was provided by the polymer of penta-1,3-diene. Scheme 25 shows a macromolecular... 

Carbosilanes, with their Si—CH2—Si skeletons, are formally analogous to silicones, which have Si—0—Si skeletons (p 135). However, whereas silicones have found wide application, the less usefffi bulk properties of carbosilane polymers and their less readily controlled methods of synthesis have prevented their finding comparable use. Apart from the pyrolytic route already described, they can be prepared directly from silicon and di- or poly-chloro alkanes, by thermal rearrangement of disilane derivatives (Si—Si—C -> Si—C—Si), and by dehalogenation of chloro-methylsilanes, e.g. 

Figure 4.12 Mechanism of thermal rearrangement (a) hydroxyl-containing poly-imide and (b) thermally rearranged polybenzoxazole (TR-PBO).

Copolymerization is a method to control the gas transport performances of copolymer membranes as well as to confirm the effect of thermal rearrangement. Poly(benzoxazole-co-imide) membranes were obtained from the thermal rearrangement of poly(hydroxyl imide-c -imide)." Size and distribution of free volume cavities created during thermal conversion could be easily controlled by varying HPI composition in the copolymer. CO2 permeability of copolymer TR... 

Poly(substituted acetylene)s such as PTMSP and PMP, amorphous fluoro-polymers like Teflon AF and Hyflon AD, polymers with intrinsic microporosity, and thermally rearranged (TR) polymers are the candidate polymers for highly permeable glassy polymer membranes. The high free volume in glassy polymers contributes to enhanced diffusion and permeation of small gas molecules. The gas permeation performances of these highly permeable polymers even surpass upper bounds for CO2/N2, CO2/CH4 and H2/CO2 separations. 

Regarding this proposal, it should be noted that while 1,1-eliminations on Si-Si-C units to generate silylenes are well known thermal processes (54) the photochemical variant seems not to have been described. The rearrangement of silylsilylenes (4) to disilenes is known to be rapid (55), and silyl radical addition at the least hindered site would produce the observed persistent radical. Preliminary evidence for the operation of 1,1-photoelimination processes in the polysilane high polymers has been obtained, in that the exhaustive irradiation at 248 nm of poly(cyclohexylmethylsilane) (PCHMS) produces —10-15% volatile products which contain trialkylsilyl terminal groups. For example, the following products were produced and identified by GC— MS (R=cyclohexyl,R = methyl) H(RR Si)2H (49%), H(RR Si)3H (19%), R2R SiH (2%), R 2RSiRR SiH (5%) and R2R SiRR SiH (7%). 

Zinc complexes are important as additives for rubber polymers. Dithiocarbamate complexes are most commonly used here, but bis(8-hydroxyquinolinato)zinc inhibits the thermal decomposition of poly[(trifluoroethoxy)(octafluoropentoxy)phosphazene]. The zinc is thought to complex residual P—OH groups in the polymer chain, which would otherwise lead to rearrangement and chain scission.126... 

The polymers of rubber plastics have unsaturated hydrocarbon chain structure, since they are polymerized from alkadienes. The general formula of poly(l,3-butadiene) or butadiene rubber (BR) and polyisoprene or natural rubber (NR) is drawn in Scheme 12.5, where X is hydrogen in BR and methyl group in synthetic polyisoprene or NR. The free radical mechanism of thermal decomposition starts by homolytic scission of the alkyl C-C bonds. Two primary macroradicals (4 and 5) are formed for which the rearrangement... 

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