Polymers, cyclic

There have been a few recent reports on the synthesis of macrocyclic PiPAAms aimed at exploring the effect of topological constraints on the solution properties of PiPAAm. The polymers were obtained by click intramolecular coupling of linear heterofunctional a-azido-co-alkynyl-PiPAAm samples synthesized via RAFT 

Pressly et al. developed an alternative approach using Cu nanoparticles in combination with microwave irradiation to significantly improve the efficiency of the click-based cyclization process [52] this enabled the production of substantially larger quantities of almost pure cyclic polymers based on polystyrene and polyethylene oxide, in much shorter durations. 

Rather than form rings by linking the ends of the chains, it is conceivable that rings could be formed by linking the chain end to any point along the polymer 

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In addition to the above, cyclic polymers, e.g. (RjSiOln, and also three-dimensional polymers can be formed. The exact nature of the polymer (its structure, and whether it is liquid or solid at room temperatures) will depend upon the substituted chloroalkyl-(or aryl-)silicane, or mixture of substituted silicanes, used and upon the experimental conditions. 

J. A. Semiyen, ia J. A. Semiyen, ed.. Cyclic Polymers, Elsevier, AppHed Science PubHsher, London, 1986. 

If an excess of pentachloride is employed, linear polymers of the type (I) will be produced but if the pentachloride is added slowly to a suspension of ammonium chloride then cyclic polymers such as the trimer (II) and tetramer (III) will be produced. 

Heating of the cyclic polymer at 250°C will also lead to the production of the linear polymer, which is rubbery and stable to 350°C. On standing, however, the material hydrolyses and after a few day loses its elastic properties and becomes hard and covered with drops of hydrochloric acid solution. 

The original cyclooligomerization noted above was conducted using alkyl aluminum, zinc and magnesium compounds ". Both open-chained and cyclic polymers were obtained in addition to dioxane. The principal macrocyclic component of the mixture was apparently 12-crown-4 (EO-4) which was said to be valuable as a high boiling neutral solvent... . " The reaction is shown below in Eq. (1.2). 

Applications. Many applications have been proposed for polyphosphazenes, particularly the non-cyclic polymers of high molecular weight, but those with the most desirable properties are extremely expensive and costs will have to drop considerably before they gain widespread use (cf. silicones, p. 365). The cheapest compounds are the chloro series... 

Anionic end-to-end cyclizations have been carried out in several laboratories 66 67,8 ) jjie cydization yields range from 20 to 60%, and the molecular weights accessible reach up to 300,000. Above this limit, separation between polycondensate and cycles becomes increasingly difficult. The presence of some linear contaminant in the cyclic polymer cannot be disregarded. 

Data from f.a.b.-m.s., and also f.d.-m.s., revealed the existence of naturally occurring, large cyclic polysaccharides. The first indication that a molecule may be cyclic comes from its precise molecular-weight determination. Cyclic molecules are 18 mass units less than their linear counterparts. Loss of water may, of course, occur in a number of ways, for example, by dehydration or lactonization, and conclusive evidence for the presence of a cyclic molecule can only be obtained from f.a.b.-m.s. of suitable derivatives, such as the permethyl derivative. Cyclic and dehydrated linear polymers are distinguishable after permethylation, as the cyclic polymer will incorporate one methyl group less than the linear molecule. 

Id. Cyclic Condensation Polymers.—The foregoing discussion has proceeded under the assumption that the only products of bifunctional condensation are open chain polymer molecules—an assumption which obviously will not be exactly valid since cyclic polymers must always occur to some extent. The nature of the error introduced by this assumption will be examined in the course of the following discussion of cyclic polymer components. 

Fig. 55.—Weight fraction distribution of cyclic polymers for a type ii polymer with B Mo/c = 0.01 (g./cc.) as calculated from Eq. (16) for p =0.95 and 1.00 (solid curves) only even integral values of x apply. The chain distribution for p =0.95 is shown for comparison by the broken curve calculated from Eq. (3 ), p. 330.

Catalysts with an unsymmetrical NHC ligand featuring a vinylic side chain have the unique ability to metathesise their own ligand to form a metaUacycle as shown in Scheme 3.7 [119], Ring opening metathesis will then incorporate the monomers, e.g. cyclooctene, into that cycle until a cyclic polymer is cleaved by another intramolecular metathesis step. The catalyst is recovered and can restart this endless route to cyclic polymers [121]. 

Scheme 3.7 REMP Intramolecular metathesis of pre-catalyst 75 to form catalyst 76 incorporation of monomers, release of a cyclic polymer and catalyst recovery...

Edwards CJC, Stepto RFT (1986) In Semiyen JA (ed) Cyclic polymers. Elsevier, London... 

FIGURE 17.20 LCCC separation of low molar mass polyamide 6.6 samples. Top trace amine rich polyamide 6.6 (Mw = lOkg/mol), middle trace acid rich polyamide 6.6 (Mw = 6 kg/mol), lower trace cyclic polymer of polyamide 6.6 (Mw = 700) (Mengerink et al., 2002 with permission from Elsevier). 

Crown ethers are cyclic polymers of ethylene glycol such as 18-crown-6 ... 

Linear Silicones Hydrolysis of dichlorosilanes, give rise to linear silicones. However, a small amount of cyclic polymer is also obtained. 

The formation of the linear cyclic polymers is dependent upon the reaction conditions. Hydrolysis with water alone gives rise to 50-80 per cent linear polydimethyl siloxane a, w-diols and 50-20 per cent polydimethyl-cyclosiloxanes. 

Cyclic polymers form nearly 50 per cent of the product while the tetramer, octamethylcyclotera siloxane (I) constitutes the main cyclic compound. Now the polymerisation of the tetramer is done by heating at 150-200°C with a trace of sodium hydroxide and a very small amount of non-functional material so as to control the Molecular weight. The products obtained is a highly viscous gum having no elastic properties. 

When one considers how cyclic polymers could be formed from propagating species (VII) one is faced with exactly the same problem as with the carboxonium ion theory either there must be a 100% efficient end-to-end ring-closure (which in any case would be confined to polymers with an initial hydroxyl group) or there must be back-biting and the formation of linear fragments, as illustrated by Jaacks [17a]. 

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