Chain end-capping

By deactivating the active weak link. For example, commercial polyacetal (polyformaldehyde) resins have their chain ends capped by a stable grouping. (This will, however, be of little use where the initiation of chain degradation is not at the terminal group.)... 

Because of the absence of chain limiter, the catalyst itself may initially act as the chain limiter (Fig. 8.22). The catalyst reacts with the olefinic regions of the polymer backbone and causes chain scission to occur, forming two new chains. The reactive carbene which is produced then moves from chain to chain, forming two new chains with each scission until the most probable molecular weight distribution is reached (Mw/Mn = 2), producing linear chains end capped with [Ru] catalyst residues. 

A general strategy developed for the synthesis of supramolecular block copolymers involves the preparation of macromolecular chains end-capped with a 2,2 6/,2//-terpyridine ligand which can be selectively complexed with RUCI3. Under these conditions only the mono-complex between the ter-pyridine group and Ru(III) is formed. Subsequent reaction with another 2,2 6/,2"-terpyridine terminated polymer under reductive conditions for the transformation of Ru(III) to Ru(II) leads to the formation of supramolecular block copolymers. Using this methodology the copolymer with PEO and PS blocks was prepared (Scheme 42) [ 107]. 

For a review that summarize the chemistry of complexes containing unsaturated carbon chains end-capped by various metal-ligand combinations, see Bruce MI, Low PJ (2004) Adv Organomet Chem 50 179 44 Jiao H, Gladysz JA (2001) New J Chem 25 551-562 Marrone A, Colletti C, Re N (2004) Organometallics 23 4952-4963 Rigaut S, Maury O, Touchard D, Dixneuf PH (2001) Chem Commun 373-374... 

Figure 2. Typical hydrodynamic radius distributions (/(RjO) of individual triblock PMMA-Z>-PS-Z>-PMMA copolymer chain end-capped with oxalyl chloride in a solvent mixture of methyl acetate and acetonitrile (10/1, v/v) at 45 °C and the aggregates formed via the self-assembly of the triblock copolymer chains at 29 °C, where the triblock copolymer concentration is 1 x 10 4 g / mL.[35]...

Figure 6. SEC profiles of PI-6-PS-6-PI triblock copolymer chains end-capped with butyl bromide group (SI44) before and after the coupling reaction in n-hexane, with the self-assembly as well as in THF without the self-assembly.

ILG. The chemistry of carbon chains end-capped by binuclear metal ligand fragments has been reviewed.319,632 Extensive spectroscopic and electrochemical studies of Ru2(ap)4j 2(p-(C=C)m (m= 1-4, 6 X-ray for m = 2, 4) have been reported.633... 

For a review that summarize the chemistry of complexes containing unsaturated carbon chains end-capped by various metal-ligand combinations, see Bruce MI, Low PJ (2004) Adv Organomet Chem 50 179-444... 

The changes in viscosity by changing the chain ends (capping with another monomer) are reversible and reproducible. 

Initiators Typical initiators include butanol (monofiinctional), ethylene or propylene glycol (difiinctional) and trimethylol propane (trifimctional). Both mono-and difiinctional initiators give products with linear chains but the monofiinctional-based products have half of the chain ends capped by an alkyl group. Tri- and poly-fimctional initiators give products with branched chains. 

The compound (Me3Si)2S3N4 may also be synthesized from Me3SiNSNSiMe3 and SCI2 in a 2 1 molar ratio. The reaction of Me3SiNSNSiMc3 with aryl sulfenyl chlorides produces neutral and cationic S—N chains end capped by aryl groups . 

The polymers were characterised by FT-IR, NMR, DSC, TGA and size exclusion chromatography. The methyl and benzyl polymers were stable up to 300 °C whereas the allyl ones were stable up to 150 "C. The degree of branching in the hyper branched polyureas was lower than in the unsubstituted hyperbranched polyurea, because the lower reactivity of N-substituted amines, and their solubilities were improved. Surface chain-end capping of these polymers enhanced the decomposition temperature. 

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