Chains extension

A hydroxyl-terminated prepolymer was synthesized by Hiltunen et al. with 1,4-butanediol isocyanate was then used as a chain extender to Increase the molecular weight. According to their studies, the polyaddltlon reaction was Influenced by the structure and functionality of the monomers, the characteristics of the hydroxyl group, polymerization temperature, polymerization time and -OH/-NCO ratio [12]. 

Gu et al. reported that a high-molecular-weight biodegradable lactic acid polymer was successfully synthesized in two steps. First, the lactic acid monomer was oligomerized to low-molecular-weight hydroxyl-terminated prepolymer the molecular weight was then increased by chain extension using 1,6-hexamethylene diisocyanate as the chain extender. The results showed that the obtained polymer had an M of 27,500 g-mol and an Af of 116,900 g mol after 40 min of chain extension at 180°C [13]. 

Ren et al. studied the effect of different diisocyanates as chain-extenders on chain extending of lactic acid oligomers. The results showed that the diisocyanate could be used as chain extenders to get high-molecular-weight chain-linked LA polymers. 1,6-hexamethylene diisocyanate [HDI] as chain-extender was better than methylenediphenyl diisocyanate [MDI] and isophorone diisocyanate (IPDI) [14]. 

Furthermore, carboxyl and hydroxyl reactive chain extenders were used together in lactic acid polymerization by Tuominen et al. The addition of highly reactive chain extenders during the final step of polycondensation led to polymers with higher 

In the preparation of a prepolymer, every effort is made to prevent the formation of any unplanned branching such as biuret groups. The prepolymer is essentially linear except when some cross-link sites have been introduced by using a multifunctional isocyanate or triol. 

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Coarse-grained models have a longstanding history in polymer science. Long-chain molecules share many common mesoscopic characteristics which are independent of the atomistic stmcture of the chemical repeat units [4, 5 and 6]. The self-similar stmcture [7, 8, 9 and 10] on large length scales is only characterized by a single length scale, the chain extension R. 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

Chain extension of the aldopentose (—) arabinose by way of the denved cyanohydnn gave a mixture of (+) glucose and (+) mannose... 

Successive repetitions of the steps shown m Figure 26 3 give unbranched acyl groups having 68 10 12 14 and 16 carbon atoms In each case chain extension occurs... 

Also present in the first test tube is a synthetic analog of ATP in which both the 2 and 3 hydroxyl groups have been replaced by hydrogens This compound is called 2 3 dideoxyadenosme triphosphate (ddATP) Similarly ddTTP is added to the second tube ddGTP to the third and ddCTP to the fourth Each tube also contains a primer The primer is a short section of the complementary DNA strand which has been labeled with a radioactive isotope of phosphorus ( P) When the electrophoresis gel is examined at the end of the experiment the positions of the DNAs formed by chain extension of the primer are located by a technique called autoradiography which detects the particles emitted by the P isotope... 

Ketose (Section 25 1) A carbohydrate that contains a ketone carbonyl group in its open chain form Kiliam-Fischer synthesis (Section 25 20) A synthetic method for carbohydrate chain extension The new carbon-carbon bond IS formed by converting an aldose to its cyanohydnn Reduction of the cyano group to an aldehyde function com pletes the synthesis... 

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. 

Stabilizers, pigments, and other additives are milled in spinning solvent, normally along with small amounts of the urethane polymer to improve dispersion stabiUty this dispersion is then blended to the desired concentration with polymer solution after chain extension. Most producers combine prepolymerization, chain extension, and additive addition and blending into a single integrated continuous production line. 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

Relative hydrolysis and condensation rate studies of multifunctional silanes, Si(OR), under acidic and basic catalysis showed that the first (OR) group hydroly2es much more readily than subsequent groups (195). Sdanol—sdanol condensation is much slower than sdanol—alkoxysilane condensation, even if the alkoxysilane is monofunctional, thus suggesting that chain extension is insignificant ia the presence of a cross-linker (196—199). 

Water-borne adhesives are preferred because of restrictions on the use of solvents. Low viscosity prepolymers are emulsified in water, followed by chain extension with water-soluble glycols or diamines. As cross-linker PMDI can be used, which has a shelf life of 5 to 6 h in water. Water-borne polyurethane coatings are used for vacuum forming of PVC sheeting to ABS shells in automotive interior door panels, for the lamination of ABS/PVC film to treated polypropylene foam for use in automotive instmment panels, as metal primers for steering wheels, in flexible packaging lamination, as shoe sole adhesive, and as tie coats for polyurethane-coated fabrics. PMDI is also used as a binder for reconstituted wood products and as a foundry core binder. 

Fig. 1. The family of D-aldoses derive from D-glyceraldehyde by chain extension at the carbonyl carbon atom.

Another approach to processible bismaleimide resins via a Michael addition chain extension, is the reaction of bismaleimide, or alow melting mixture of bismaleimides, with aminobenzoic hydrazide to provide a resin that is soluble in various solvents, such as acetone [67-64-1methylene chloride [75-09-2] and dimethylform amide [68-12-2] (33). The idealized chemical stmcture for a 2 1 BMI—aminobenzoic hydrazide resin is as follows ... 

In the presence of the organic siHcate, the heavy-metal salts trigger the chain extension and cross-linking reactions that lead to siHcone mbber and volatile ethanol as a byproduct. Useful metal soaps iaclude stannous octanoate [1912-83-0], ziac octanoate [557-09-5], dibutyltin dilaurate [77-58-7], and dibutyltin diacetate [1067-33-0]. The reactivity of the different salts varies considerably. Stannous octanoate effects a cure ia 0.5—2 min ziac octanoate may require 24—96 h the dibutyltin dilaurate, 10—20 min. Heat and moisture accelerate the curing rate, but to a lesser degree than ia the case of the polysulfide mbbers. 

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 addition to linear chain extension, excess diisocyanate leads to cross-linking iato a network because the diisocyanate groups can also react with the hydrogen atoms of the —NH— groups ia the chains. Furthermore, the weU-known polyurethane foam mbber can be made by a dding water to the mixture because the isocyanate groups react vigorously with water to Hberate carbon dioxide gas as follows ... 

The amine groups thus formed can also react vigorously with the isocyanate groups to continue the chain extension and cross-linking reactions. Hence, ia the systems there are simultaneous foaming, polymerization, and cross-linking reactions, which produce foam elastomers (or plastics). 

The Hquid polymer is then compounded with metal oxides or peroxides, as weU as fillers (carbon black) and can undergo cold vulcanization, ie, chain extension and cross-linking iato a soHd matrix. It is largely used as a sealant and gasket material for wiadows, automobile wiadshields, etc. 

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. 

It will be seen that reactions (7.5b) and (7.5c) if they occur more than once per hydrocarbon molecule (radical) will lead to chain extension and, if on average more than twice, to cross-linking. 

By introducing branch points into the polymer chains, for example by incorporating about 2% of 1,2,3,-trichloropropane into the polymerisation recipe, chain extension may proceed in more than two directions and this leads to the formation of networks by chemical cross-links. However, with these structures interchange reactions occur at elevated temperatures and these cause stress relief of stressed parts and in turn a high compression set. 

These polymers are liquids which may usefully be cast or used for impregnation and caulking compounds. In addition they may be vulcanised by a variety of agents, ostensibly by a chain lengthening process. It should, however, be noted that these polymers normally contain small quantities of tri-chloropropane in the original monomer mix so that the three-dimensional chain extension will lead to cross-linking. 

Such reactions allow chain extension and/or cross-linking to occur without the elimination of small molecules such as water, i.e. they react by a rearrangement polymerisation type of reaction. In consequence these materials exhibit a lower curing shrinkage than many other types of thermosetting plastics. 

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