Polycondensation acid-catalyzed

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

The main polymerization method is by hydrolytic polymerization or a combination of ring opening as in (3.11) and hydrolytic polymerization as in (3.12).5,7 9 11 28 The reaction of a carboxylic group with an amino group can be noncatalyzed and acid catalyzed. This is illustrated in the reaction scheme shown in Fig. 3.13. The kinetics of the hydrolytic polyamidation-type reaction has die form shown in (3.13). In aqueous solutions, die polycondensation can be described by second-order kinetics.29 Equation (3.13) can also be expressed as (3.14) in which B is die temperature-independent equilibrium constant and AHa the endialpy change of die reaction5 6 812 28 29 ... 

The transesterification and glycolysis reactions proceed via the Aac2 mechanism described above in Section 2.1. The reactions are acid catalyzed as demonstrated by Chegolya el al. [27], who added TPA to the polycondensation of PET and observed a significant increase of the apparent reaction rate. The industrial polycondensation process is accelerated by the use of metal catalysts, with these being mainly antimony compounds. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

Methyl sulfoxide has been used, chiefly by Micheel and his coworkers, as an effective solvent for the acid-catalyzed polycondensation of sugars.93-101 In view of the good yields of material of high molecular weight, the methods developed offered considerable promise. Unfortunately, it was subsequently found 4 that, in acidic media, methyl sulfoxide undergoes a... 

Acid-catalyzed Polycondensation of Sugars in Methyl Sulfoxide... 

Chlorinated aromatic amines polycondensation is catalyzed by hydrochloric acid that is produced by the reaction [15]. 

The above acid-catalyzed polycondensations were carried out at more than 100 °C, whereas Takasu et al. reported room temperature polyesterification with scandium trifluoromethanesulfonate [Sc(OTf)3] or scandium trifluo-romethanesulfoimide [Sc(NTf2)3] [27,28]. Thus, the direct polycondensation of methylsuccinic acid and 1,4-butanediol proceeded in bulk under reduced pressure (0.3-30 mmHg) using 1.4 mol % of Sc(OTf)3 at 35 °C for 96 h to afford poly(butylene methylsuccinate) with Mn of 12400 (Scheme 5). When HfCl4-(THF)2 was used in this room temperature polymerization instead of Sc(OTf)3, only low molecular weight polyester (Mn = 1100) was afforded. The scandium catalysts did not promote transesterification ethanol selectively reacted with acetic acid even in the presence of equimolar methyl acetate. 

Abstract This paper proposes new ways of preparation of hybrid silicones, i.e. an alternated multiblock seqnence of silicone and alkyl spacers, via a polycondensation process catalyzed by the tris(pentaflnorophenyl)borane, a water-tolerant Lewis acid, between methoxy and hydrogeno fnnctionalized silanes and siloxanes at room temperature and in the open air. The protocol was first developed with model molecules which led to polydimethylsiloxane (PDMS) chains, in order to seize the best experimental conditions. Several factors were studied such as the contents of each reactants, the nature of the solvent or the rate of addition. The best conditions were then adapted to the synthesis of hybrid silicones, condensing alkylated oligo-carbosiloxanes with methoxy or hydrogeno chain-ends and complementary small molecules. A systematic limitation in final molar masses of hybrid silicones was observed and explained by the formation of macrocycles, which cannot redistribnte or condense further while formed. 

In other words, of the initial charge of isopropyl chloride, 34% of the carbon is converted to propane under these conditions. A comparable sample of 1-chloropropane under identical reaction conditions, of excess alkyl halide Lewis acid, was converted after five minutes to the extent of 18% to propane. After 15 minutes the relative amount of "propane" had decreased as a result of further acid catalyzed polycondensation reactions. Similarly, isopropyl chloride reacts in HBr-AlBr3 at room temperature to give a gas product which is entirely propane after 2 min. In this system we again begin to see a buildup of heavier hydrocarbon species with time. The fact that the reaction proceeds so rapidly here can probably be attributed to a homogeneous hydrocarbon/acid liquid phase. This last result and additional experiments are summarized in Table IV. 

Silanol polycondensation is catalyzed by strong acids or bases, as well as by species as mild as amines or carboxylic acids combined with their quaternary ammonium salts (67). With strong acids or bases, the process is highly reversible, and the equilibrium can be represented as follows ... 

Polyester, a condensation polymer, can be produced by chainwise, acid-catalyzed ring openings of cyclic ester (lactone) without expulsion of small molecules, and also by stepwise polycondensation of ohydroxycarboxylic acid. 

Another belt-shaped molecule is cucurbituril (53) which was presumably synthesized as early as 1905 by Behrend et al. [42]. The structural formula of 53 was resolved in 1981 by Freeman. Mock, and Shih [43] only when modern spectroscopic methods were available. Cucurbituril (53) is easily obtained from urea, glyoxal, and formaldehyde in an acid-catalyzed condensation reaction with glycoluril (52) as an intermediate. The initially formed polycondensation product, insoluble in all common solvents, is treated with hot concentrated sulfuric acid. After dilution with water and subsequent boiling a solid is obtained in a yield of 40-70%. The H-NMR spectrum shows only three signals of equal intensity and the infrared spectrum suggests retention of the glycoluril moieties. The compound is not suitable for usual mass spectrometry due to its low volatility. However, an X-ray crystal structure of the calcium complex obtained from sulfuric acid solution was undertaken. 

The acid-catalyzed polycondensation of certain mercaptoalcohols represents a method for the preparation of polyalkylene sulfides [87-90]. 

TABLE X Acid-Catalyzed Polycondensation of Mercaptoethanol and Mercaptopropanol ... 

Thus, steel pipes previously coated with bitumen or coal tar pitch can be wrapped with a bonded glass fiber mat based on this type of resin. In this context, 2 is mixed with water in the presence of an emulsifying agent and an acid catalyst and the ensuing emulsion impregnates the mat. Then, the resulting composite is heated in order to remove the water and induce the acid-catalyzed polycondensation of the matrix. 

In situ polymerization is an acid catalyzed polycondensation process pH value cannot only affect the polycondensation rate, but also the surface activity of the amino resin prepolymer. Low pH process promotes the formation of methylene bridges affecting the amino resin solubility and therefore obtains a rough surface. However, for MF microcapsule making, at pH values below 3.0, the fraction of protonated melamine resin prepolymer is increased, and the MF prepolymer becomes more hydrophilic and consequently loses its encapsulation capacity. ... 

Condensation of an ethylene dihalide with an alkali metal sulfide was studied in the latter part of the nineteenth century. The acid catalyzed polycondensation of certain mercapto alcohols represents a second method of synthesis. 

HMF is a suitable precursor for the synthesis of bifunctional furan monomers as summarized in Scheme 4. All these monomers can be used to prepare polycondensates by step-growth reactions with the other corresponding bifunc-tional monomers derived either from petrochemical precursors or from renewable resources. The polycondensates obtained, such as polyesters, polyamides, and polyurethanes, etc. have been characterized [107-113]. Scheme 5 shows another approach for the synthesis of bifunctional monomers through acid-catalyzed condensation of the corresponding mono-functional furan derivatives with an aldehyde... 

This crosslinking agent suppresses the acid-catalyzed self-polycondensation [61]. DiphenyU-odonium 9,10-dimethoxyanthracene-2-sulfonate is used as a photoacid generator. 

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