Polyfunctional coupling agents

The linear diblocks are then coupled by a polyfunctional coupling agent such as epoxidised linseed oil to give a star-shaped polymer. As already mentioned, commercial materials of this type have a tetramodal distribution. 

The radial block copolymers are also prepared by anionic polymerization. Firstly, a styrene-butadiene diblock which is active at the butadiene end is formed. A polyfunctional coupling agent such as silicon tetrachloride is then added to produce the radial structure. The process may be represented as follows ... 

In order to overcome this limitation, reactive modification has been introduced polyfunctional coupling agents (or chain structure modifiers) with suitable electrophilic functionality have been shown to react with nucleophilic end groups (hydroxyl and/or carboxyl) present in polyesters, within relatively short times. Through the reaction, the structure of the linear polyesters is modified with a resulting improved rheology suitable for low density foaming PET is well known to respond to such modifications [1,2]. 

The serious drop in strength of epoxy bonded carbon steel structural joints exposed to humid environments for extended periods of time is greatly reduced by surface treating the bond surfaces with polyfunctional mercaptoester coupling agents. 

Polyfunctional mercaptoester chelators have been synthesized and evaluated as potential coupling agents for the adhesion of epoxy resins to steel. The adhesive systems were evaluated by subjecting torsional joints to shear failure after accelerated immersion testing in distilled water. 

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

Coupling of GTP living polymers with halide-terminating agents to form star polymers has been achieved [Hertler, 1996 Webster and Sogah, 1989]. Star polymers are also synthesized by using polyfunctional initiators or by copolymerization with dimethacrylate monomers. 

Radial polymers may also be produced by taking advantage of the fact that butyl lithium-initiated polymerizations are termination-free and that chain ends remain active. These may be coupled with polyfunctional agents such as divinyl benzene or methyl trichlorsilane to give the radial or star-shaped polymers. Being more compact the polymers have lower melt and solution viscosities than linear polymers of equivalent molecular weight. 

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