Cyclic polymers solvents

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). 

The most import uni cyclic ethers arc (he furan compounds Furfural is manufactured by treating pentosan with sulfuric acid and steam stripping mil the furfural a n is formed The sources of pentosan arc such agricultural products uv comet tbs. oat hulls, and hagavtc Furfural is used to produce furan and tetrahydro furan fTHF) These products are used as organic intermediates, extraction solvent, polymer solvents, and in polyurethane applications... 

The hydrolysis of, for example, dimethyl dichlorosilane to cyclic dimethylsiloxanes and linear diols can be regarded as following a similar pattern, if OHs+Cl" is taken as the analogue of NH4+C1". Increase of acid concentration, or the use of a mutual solvent, increases the proportion of cyclic polymers their proportion is decreased by the use of insufficient, alkaline or scarce water in the foi-m of hydrated salts (see pp. 80 ff. in ref. 65). 

A few years later, by using the same experimental procedures, the same scientists prepared cyclic polybutadienes170 (Scheme 82). THF was the polymerization solvent, which results in a 60% 1,2-content. From the g values was concluded that some of the cyclic polybutadienes (PBd) were contamined by their linear precursors. A high-resolution column set was used in order to separate the linear from the cyclic polymer. In this work the characterization analysis was the most comprehensive presented so far. Recently, these cyclic PSs were analyzed by high-pressure liquid chromatography under critical conditions, which is a method that can separate linear and cyclic macromolecules according to their archi-... 

The thermodynamic parameters 1/) and K introduced above, pertaining to polymer-solvent interactions in dilute solutions, may be determined from thermodynamic studies of dilute solutions of the polymer, e.g., from osmotic pressure or turbidity measurements at different temperatures. These parameters may also be determined, at least in principle, from viscosity measurements on polymer solutions (see Frictional Propcitics of Polymers). The parameter ij, which is a measure of the entropy of mixing, appears to be related to the spatial or geometrical character of the solvent. For those solvents having cyclic structures, which are relatively compact and symmetrical (e.g., benzene, toluene, and cyclohexane), xp has relatively higher values than for the less symmetrical acyclic solvents capable of assuming a number of different configurations. Cyclic solvents are thus more favorable... 

In dilute solutions and in more strongly donor solvents the monomeric species normally predominate but in diethyl ether at concentrations >0.1M association occurs36 and linear or cyclic polymers may be present. The behavior of several compounds is shown in Fig. 7-4, which includes also the halides MgBr2 and Mgl2. 

Equations (19) and (20) are valid in theta solvent. The more compact structure and the lack of chain ends result in different chemical and physical properties of cyclic polymers, including lower translational friction coefficients, higher glass transition temperatures [167], faster crystallization [168], higher refractive index [169], higher density [170], higher critical solution temperature [167], and lower intrinsic viscosity [167, 171, 172]. 

The dynamics of cyclic chains have been studied mainly by computer simulation [175-177]. Cifre and co-workers consider the CST of cyclic polymer solution in QSSF using Brownian dynamics simulation techniques [178]. In Fig. 17a, < Rq > for a ring chain with N = 25 beads suddenly increases as e > sq. They further exploited the dependence of ec on the N and hydrodynamic interactions (Fig. 17b). In both theta solvent and good solvent, ec satisfies... 

The solution viscosity of cyclic polymers is lower than those of their linear analogues. The viscosity of cyclic and linear PS was preferentially studied and compared in cyclohexane, a theta solvent. Quite similar intrinsic viscosity ratios, [>/]c/[>/]i/ were determined by several research groups for this system for instance, values of 0.64-0.70 depending on the molar mass, of 0.67, and of 0.65 are reported. However, at high molar masses (>20 000gmor ), the ratio of intrinsic viscosities was found to approach unity.A similar tendency was reported also for cyclic and linear poly(2-vinylpyridine) and polybutadiene. For cyclic and linear PDMS, the ratio of intrinsic viscosities, [r ]cl[h] i under 0-conditions in bromohex-ane at 18 °C, was found to be equal to 0.66. ... 

Concerning the radii of g)nation of PDMS, determined by means of SANS, a ratio good solvent. All these experimental results are in good agreement with an early theoretical treatment of stmcture-property relationships of cyclic polymers. 

Much experimental work is reported for adsorption on to solid particles this has the benefit that adsorbed amounts can easily be obtained, but the results may sometimes be surprising. An example of this are the results for the adsorption of polydimethyl siloxane on to silica from solutions in hexane and carbon tetrachloride (Patel et al. 1991), the latter being the poorer solvent. In these circumstances a greater adsorbed amount is expected from the solution in the poorer solvent. This was not observed, because carbon tetrachloride also has an affinity for the silica surface and was not displaced by the polymer. A further interesting comparison made in this paper, that bears directly on the discussion above about the relative importance of loops and tails, is the difference in adsorption from a good solvent between cyclic and linear polymers. Figure 5.16 summarises their findings. There is a crossover in adsorbed amount for linear and cyclic polymers at a certain relative molecular mass. 

When this reaction was attempted with X = CH j no polymer was obtained, unless the reaction was carried out in THF or dioxane. Stille (53) and we (54) discovered concurrently and independently that this polymerization Involves Incorporation of the cyclic ether solvent. 

ABA triblock copolymers are ideal precursors for cyclic polymers, because one can increase Ve by using a pair of reactive terminal A blocks rather than a pair of reactive end groups. More importantly, the terminal blocks can be preassociated or preassembled intramolecularly in a block-selective solvent for the B block at high dilution. This leads... 

Bensafi, A., Maschke, U., and Benmouna, M. (2000) Cyclic polymers in good solvents. Polymer International, 49,175-183. 

Other attempts made by Rempp et al. [48, 49] and VoUmert et al. [50] used a similar method but changed the solvent conditions using, for example, a mixture of benzene and tetrahydrofuran (THF). This allowed good coupling for the synthesis of cyclic polymers with different building blocks such as poly(2-vinyl pyridine), polybutadiene, poly(dimethylsiloxane), poly(propylene oxide), poly(ethylene oxide), and a variety of other polymers (Scheme 4). 

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