Chain-stopper

After polymeri2ation is carried out by blending mono- and difunctional chlorosilanes ia excess water, the siloxanes are separated from the water and neutraH2ed. Ratio of the mono-chain stopper to di-chain extender controls the length of the polymer. Once an equiHbrium mixture of chain lengths is catalyticaHy formed, volatile light ends are removed and the desired product results. 

Diols, such as ethylene glycol, are usually not added during the alcoholysis step because their monoesters have only one remaining hydroxyl group, and would function as chain stoppers, thus severely limiting their utihty in the stmcture design of the resin molecules. 

Rubber Chemicals. Sodium nitrite is an important raw material in the manufacture of mbber processing chemicals. Accelerators, retarders, antioxidants (qv), and antiozonants (qv) are the types of compounds made using sodium nitrite. Accelerators, eg, thiuram [137-26-8J, greatly increase the rate of vulcaniza tion and lead to marked improvement in mbber quaUty. Retarders, on the other hand (eg, /V-nitrosodiphenylamine [156-10-5]) delay the onset of vulcanization but do not inhibit the subsequent process rate. Antioxidants and antiozonants, sometimes referred to as antidegradants, serve to slow the rate of oxidation by acting as chain stoppers, transfer agents, and peroxide decomposers. A commonly used antioxidant is A/,AT-disubstituted Nphenylenediamine which can employ sodium nitrite in its manufacture (see Rubber chemicals). 

Chain stoppers are unifunctional molecules of the form AX or BX, where the X moiety is nonreactive. They are used to stop the polymerization at a desired point or to stabilize the polymer chains by endcapping. 

Free-radical chain stoppers—These chemicals interact with chain propagating R02 radicals to form inactive products. 

The use of PbEt4 as an anti-knock agent in petrol depends in part on the ability of the ethyl radicals, generated on its thermal decomposition, to combine with radicals produced in the over-rapid combustion of petroleum hydrocarbons chain reactions which are building up to explosion (knocking) are thus terminated short of this. The complete details of how PbEt4 operates are not known, but there is some evidence that minute Pb02 particles derived from it can also act as chain-stoppers . 

A different approach, presently appfied by the Perstorp company in the production of hyperbranched afiphatic polyesters from 2,2-bis(methylol)propionic acid [8], utihzes a B starter molecule with AB -groups condensed in consecutive steps see Fig. 1 in the middle of the diagram. In principle, the function of the B component can also be regarded as a chain stopper when all building blocks are polycondensed in one step. This also leads to a predictable and stable molecular weight at total conversion due to the excess of B-groups in the system. 

Fig. 1. Top general approach to hyperbranched polycondensates from AB2 monomers Middle modified approach using starters/chain stoppers Bottom new approach to hyperbranched polycondensates by reacting Aa monomers with a molar excess of bB2 monomers...

The key to a controlled molecular weight build-up, which leads to the control of product properties such as glass transition temperature and melt viscosity, is the use of a molar excess of diisopropanolamine as a chain stopper. Thus, as a first step in the synthesis process, the cyclic anhydride is dosed slowly to an excess of amine to accommodate the exothermic reaction and prevent unwanted side reactions such as double acylation of diisopropanolamine. HPLC analysis has shown that the reaction mixture after the exothermic reaction is quite complex. Although the main component is the expected acid-diol, unreacted amine and amine salts are still present and small oligomers already formed. In the absence of any catalyst, a further increase of reaction temperature to 140-180°C leads to a rapid polycondensation. The expected amount of water is distilled (under vacuum, if required) from the hot polymer melt in approximately 2-6 h depending on the anhydride used. At the end of the synthesis the concentration of carboxylic acid groups value reaches the desired low level. 

Another method of achieving the desired molecular weight is by addition of a small amount of a monofunctional monomer, a monomer with only one functional group. Acetic acid or lauric acid, for example, are often used to achieve molecular weight stabilization of polyamides. The monofunctional monomer, often referred to as a chain stopper, controls and limits the polymerization of bifunctional monomers because the growing polymer yields chain ends devoid of functional groups and therefore incapable of further reaction. Thus, the use of benzoic acid in the polyamide synthesis yields a polyamide (XI) with phenyl end groups that are unreactive toward polymerization. 

In glycoproteins, L-fucose and sialic acid are often mutually exclusive at some sites of substitution, and both sugars occur at nonreducing termini.118 In fact, they have been assumed to act as chain-stoppers" in the biosynthetic process, controlling the extent of chain elongation. Nevertheless, from the jelly coat of the eggs of the sea ur-... 

Belgian workers needed to prepare the benzylidenethiophthalide 1 to test its potential use as a chain-stopper for a particular polymerisation process. 2-Thiophthalide 2 was therefore condensed with benzaldehyde in THF at 0°C using potassium t-butoxide as base. Work-up consisted simply of addition of water, acidification with HC1 and extraction with chloroform. None of the expected product 1 was obtained instead, the stilbene acid 3 was formed in 52% yield. 

Scheme 17 ATMET polymerization of high-oleic sunflower oil in the presence of methyl acrylate as chain stopper [115]...

Cross-linked PUs with shape memory properties were prepared by Galia, Meier et al. using linear polyols synthesized by ADMET [140]. In this work, ADMET of a 10-undecenoic acid-derived a,co-diene containing a hydroxyl group was performed in the presence of 0.1 mol% of C4. 10-Undecenol was used as chain stopper, and the mixture of oligomers and diols (from 10-undecenol SM) obtained was cross-linked with MDI. The PUs obtained displayed outstanding values of strain fixity and recovery. 

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