Extension reactions, chain

Stabilizers and pigments are normally slurried with macroglycol and added to the polymeric glycol charge, prior to diisocyanate addition. Therefore, care must be taken to avoid additives that react significantly with diisocyanates or diamines under processing conditions. Also, stabilizers should be chosen that have no adverse catalytic effect on the prepolymer or chain-extension reactions. 

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

In recent years, proprietary catalysts for advancement have been incorporated in precataly2ed Hquid resins. Thus only the addition of bisphenol A is needed to produce soHd epoxy resins. Use of the catalysts is claimed to provide resins free from branching which can occur in conventional fusion processes (10). Additionally, use of the catalysts results in rapid chain-extension reactions because of the high amount of heat generated in the processing. 

In recent years there has been some substitution of TDI by MDI derivatives. One-shot polyether processes became feasible with the advent of sufficiently powerful catalysts. For many years tertiary amines had been used with both polyesters and the newer polyethers. Examples included alkyl morpholines and triethylamine. Catalysts such as triethylenediamine ( Dabco ) and 4-dimethyla-minopyridine were rather more powerful but not satisfactory on their own. In the late 1950s organo-tin catalysts such as dibutyl tin dilaurate and stannous octoate were found to be powerful catalysts for the chain extension reactions. It was found that by use of varying combinations of a tin catayst with a tertiary amine... 

Bromodifluoromethylphosphonium bromide has also been used m chain extension reactions to form dienes [42] (equation 42) or in bisdifluoromethylena-tion of diones [43] (equation 43) or halofluoromethanes [44], which may themselves be used in the preparation of new phosphomum salts (Table 16)... 

Deoxy-3-fluoro-D-glucose (25%) and -mannose (40%) were prepared from 2-deoxy-2-fluoro-D-arabinose by a chain-extension reaction (cyanohydrin synthesis). Likewise, 4-deoxy-4-fluoro-D-glucose ° (27%) and -mannose (45%) were prepared from 3-deoxy-3-fluoro-D-arabinose. ... 

The two reactor feeds were controlled to give copolymers with the desired densities, and a physical blend and a diblock OBC were produced. DEZ was added to the first reactor to achieve the desired melt index (/2 = 20 dg min1, equivalent to a Mn of -15-20 kg mol1). This material was fed to the second reactor, and production was continued under different conditions. The material collected after the second reactor had a lower melt index (/2 = 3.9 dg min1), indicating a higher molecular weight consistent with the chain extension reaction from the CCTP process. 

These iniferter sites containing an N-H group can be easily transformed into the corresponding thiol which leads to disulfide by oxidative coupling and can form chelation with metal ions (Eq. 47) [171,172]. Poly(St) prepared for polymerization with 44 and 45 was applied to the chain-extension reaction by the S-S bond or chelation bond formations. 

For the synthesis of D-glucuronic acid, methods of oxidation of suitable D-glucose derivatives have been devised during the past two decades these procedures have been comprehensively reviewed by Marsh,6 Mehltretter,7 and Heyns and Paulsen.8 For special purposes, for example, for the preparation of 6-I4C-labelled D-glucuronic acid, chain-extension reactions of 1,2-O-isopropylidene-a-D-xy/o-pen-todialdo-I,4-fiiranose by the cyanohydrin synthesis9 or by ethynyla-tion10 are used, but these frequently yield mixtures of D-glucuronic acid and L-iduronic acid. 

Pyromellitic dianhydride (PMDA) is generally used in PET at concentrations ranging from 0.05 to 2%. Reactive extrusion of PET with PMDA has been reported by Incamato et al. [9], These authors used PMDA to increase the molecular weight of PET industrial scraps sourced from a PET processing plant. They found that concentrations of PMDA between 0.50 and 0.75 % promote chain extension reactions that lead to an increase of MW, a broadening of the MWD and branching phenomena which modify the PET scrap in such a way that makes... 

Figure 14.4(a) shows the structure for a common diepoxide chain extender. Haralabakopoulos et al. [12] have reported that the molecular weight of PET increases via chain extension reactions with commercially available diepoxides. Low concentrations of extender and short reaction times generally favoured chain extension. In addition, purging with nitrogen resulted in chain extended polymers having the highest values of intrinsic viscosity (e.g. 0.82 dL/g). 

The etherification of a,(jJ-bis (hydroxyphenyl)PSU with C1MS was demonstrated to be quantitative (for all procedures A, B and C), based on FTIR and 200 MHz H-NMR analyses (6). A careful analysis of the 200 MHz -H-NMR spectra of a,w-bis(vinylbenzyl)PSU prepared according to procedures A and B shows a signal assigned to aromatic formal protons (-OCH2O-, 6=5.64 ppm). This is due to a chain extension reaction of the q,U)-bis(hydroxyphenyl)PSU with methylene chloride. 

Table I. Infrared Study of the Chain Extension Reaction...

According to O. Bayer, the latter procedure, which is used especially for the preparation of elastomeric polyurethanes, is carried out in two separate stages. First, a carefully dried, relatively low-molecular-weight, aliphatic polyester or polyether with hydroxy end groups is reacted with an excess of diisocyanate. A chain extension reaction occurs in which two to three linear diol molecules are coupled with diisocyanate, so as to yield a linear polymer with some in-chain urethane groups and with isocyanate end groups. 

Methylene chloride, reagent grade, was obtained from Pharmco Products, Inc. and was distilled from phosphorus pentoxide prior to use. Benzene can be substituted for methylene chloride in the chain extension reaction with similar results, however the use of benzene in the reaction has not been independently checked. 

The first step of the chain extension reaction mechanism has been shown to b e etiolate formation, the by-product of which Is ethane gas. The nitrogen line should be attached to a large adapter to provide adequate venting of the gas. 

PRODUCTS OF ZINC CARBENOID-MEDIATED CHAIN EXTENSION REACTIONS... 

As oxidation processes were clarified, it was observed in other chain extension reactions that R02 radicals reacted with oxidation products hydroperoxides, alcohols, and ketones. The high reactivity of hydroperoxides and alcohols strongly influences the mechanism of oxidation processes. Chain rupture results from recombination of R02 radicals. 

A related reaction of more general applicability occurs with the octos-3,7-diulose derivative 27, which was made from the C-3 epimer of dialdose acetal 4 by treatment with (2-oxopropylidene)triphenylphosphorane in a Wittig chain extension reaction, followed by hydrogenation of the resulting enone and oxidation of the alcohol groups at C-3 and C-7. Cyclilzation to give the crystalline tertiary alcohol 28 in 81% yield is promoted by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in refluxing benzene (Scheme 7) [16]. 

For the total synthesis of aminodeoxy sugars from nonsugar precursors, it is logical to turn to amino acids as primary sources of chiral templates. The challenge is to find innovative and stereocontrolled methods of chain extension reactions that lead to higher-carbon aminodeoxy sugars. 

There is considerable evidence in the literature that chain extension reactions can occur between epoxide and hydroxyl groups and that this reaction (4) is enhanced in the presence of the tertiary amines 28.29,32,36,40,42-48,50) jn tjie case Qf TGDDM... 

Hence, the most plausible explanation of our FTIR observations of the simultaneous appearance of hydroxyl, carbonyl and ether groups upon TGDDM epoxide consumption is epoxide isomerization and/or oxidation followed by epoxide-hydroxyl chain extension reactions. 

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