Cyclic polymer purification

A cyclic polydimethylsiloxane was also prepared by the end-to-end reaction, i.e., a,ft)-dianion-functionalized polymers, which are then cyclized simply by reaction with a difunctional electrophile to give a cyclic polymer. The cyclic polydimethylsiloxane was synthesized from a commercially available o. tw-dihydroxy-polydimethylsiloxane (M = 2.460 g/mol). The Unear precursor was deprotonated using sodium hydride in dilute THF (< 10 2 M) and then end-coupled using a dichlorosilane coupUng agent (Fig. 53). The uncy-clized anionic linear precursors are then removed by a macroporous anion exchange resin. The successful cyclization and purification is monitored by IR and 29Si NMR, GPC, and MALDI-TOF mass [176],... 

An alternative technique for the purification of cyclic polymers was demonstrated by Singla et al, who removed the linear polyfethylene glycol) (PEG) impurities... 

The Huisgen click cycloaddition reaction has been explored for the cyclization of linear poly(styrene) precursors prepared by ATRP. Using an optimized slow, continuous addition technique, nearly quantitative yields of the cyclic polymer can be achieved without the need for fractionation or further purification. The versatility of ATRP provides extremely well defined linear precursors and enables the incorporation of diverse functionality. As a demonstration of this versatility, acetoxystyrene was polymerized, cyclized, and hydrolyzed to yield macrocylic poly(hyroxystyrene). This cyclic polymer is extremely useful as the phenols on each repeat unit can be easily modified to incorporate further functionality, such as the quantitative esterification described. We are currently investigating the attachment of bifurcated... 

Singla, S., Zhao, T., and Beckham, H.W. (2003) Purification of cyclic polymers prepared from linear precursors by inclusion complexation of linear byproducts with cyclodextrins. Macrorrwlecules, 36,6945-6948. 

Cyclic polymers can be synthesised through careful choice of the reaction conditions. The most common strategy is to employ highly dilute reaction conditions in order to favour intramolecular reactions over intermolecular reactions. A S-Bd-S living difunctional polymer obtained under apolar room-temperature conditions was cyclized using the titration method[3] employing a dichlorosilane as linker in slight excess. As a trick for the purification step the excess chlorosilane and monoreacted precursor polymer was reacted with a... 

These thermolysis reactions normally produce polymeric products, free of the cyclic analogs, in essentially quantitative yield and in sufficient purity to give satisfactory elemental analysis upon removal of the sHyl ether byproduct under vacuum. Final purification is generally achieved by precipitation of the polymer into a non-solvent such as hexane. With the exception of poly(diethylphosphazene) (2), which is insoluble in all common solvents (see below), the new polymers are readily soluble in CH CU and CHCU. In addition, the phenyl substituted compounds (3-6) are soluble in THF andvanous aromatic solvents. None of the polymers are water-soluble however, Me2PN]n (1) is soluble in a 50 50 water/THF mixture. 

The biodegradable polymer available in the market today in largest amounts is PEA. PEA is a melt-processible thermoplastic polymer based completely on renewable resources. The manufacture of PEA includes one fermentation step followed by several chemical transformations. The typical annually renewable raw material source is com starch, which is broken down to unrefined dextrose. This sugar is then subjected to a fermentative transformation to lactic acid (LA). Direct polycondensation of LA is possible, but usually LA is first chemically converted to lactide, a cyclic dimer of LA, via a PLA prepolymer. Finally, after purification, lactide is subjected to a ring-opening polymerization to yield PLA [13-17]. 

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [45-53]. Harada et al. [31] treated 6-p-aminoCiO-a-CD (40 mM) with 2M excess 2,4,6-trinitrobenzenesulfonic acid sodium salt (TNBS) as bulky stoppers in aqueous solutions. The resulting precipitate was found to be mainly a cyclic trimer by H NMR and TOF mass spectra. After purification of the crude product, the 2D ROESY spectrum of the cyclic trimer shows cross-peaks between phenyl protons close to an amino group and secondary hydroxyl groups (0(2)H). A trinitrophenyl group is found at the secondary hydroxyl group side. A proposed structure of a cyclic trimer (cyclic daisy chain) is shown in Fig. 3.12. Kaneda et al. [38] reported the preparation of cyclic di[2]rota-xane fashion constructed tail-to-tail by azobenzene derivatives of permethylated a-CDs and showed its computer-generated supramolecular structures (Fig. 3.13). Easton et al. [39] also reported the preparation of cyclic di[2]rotaxane constructed by stilbene-appended a-CDs in tail-to-tail fashion (Fig. 3.14). Kaneda et al. [40]... 

Unlike the high-temperature process where about 8 to 10% cyclics are generated, the equilibrium monomer content of nylon 6 resulting from polymerization at temperature lower than 200 C (i.e., low-temperature polymerization) can be less than 2%. The polymer therefore does not usually require any additional purification. Also, the maximum rate of crystallization of rylon 6 falls within the range of... 

While all of the above-described purification methods have assisted in the isolation of polymer macrocydes, substantial efforts have been made to improve the synthetic methods and to allow the production of macrocydes with narrow polydispersities and increased cyclic purities [18-21]. In this chapter, an attempt will be made to review the general techniques for preparing cycUc polymers, and to highlight the methods which are presently at the forefront of research. 

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