Cyclic fraction

A much better agreement between theory and experiment is found in the closely-related field of macrocyclisation equilibria. Investigations of the cyclic populations in ring-chain equilibrates set up in typical polymeric systems such as polyesters, polyethers, polysiloxanes, and polyamides take a major advantage from the relative ease with which the cyclic fraction can be separated from the linear fraction and analysed for the relative abundance of the individual oligomeric rings. This is conveniently done by means of modern analytical techniques such as gas-liquid and gel-permeation chromatography (Semiyen, 1976). 

The most basic raw petrochemical materials are liquefied petroleum gas, natural gas, gas from cracking operations, liquid distillate (C4 to C6), distillate from special cracking processes, and selected or isomerized cyclic fractions for aromatics. Mixtures are usually separated into their components at the petroleum refineries, then chemically converted into reactive precursors before being converted into salable chemicals within the plant. 

Other examples of successful combinations of liquid chromatography and MALDI-TOF have been reported by Kruger et al. who separated linear and cyclic fractions of polylactides by LC-CC [184]. Just et al. were able to separate cyclic siloxanes from linear silanols and to characterize their chemical composition [185]. The calibration of an SEC system by MALDI-TOF was discussed by Mon-taudo et al. [186]. Poly( dimethyl siloxane) (PDMS) was fractionated by SEC into different molar mass fractions. These fractions were subjected to MALDI-TOF for molar mass determination. The resulting peak maximum molar masses were... 

Cyclic fractions are formed either from monomers by some variant of propagation or, and this is the main source, from the growing chain. Examples of the former could be that already cited [314] or the observation of Weissermel and Nolken [334] that chloromethyloxirane reacts with triethyaluminium at low temperature to give a product which is a mixture of polymer and cyclic dimer... 

Macrocycles are sometimes only formed in the presence of monomers. When all monomer has been consumed by polymerization, depolymerization also stops [333]. The tendency of common heterocycles to the production of giant molecules, and the possibility of generating cyclic fractions from their chains is illustrated in Table 6. 

Cationic polymerization of the unsubstituted oxirane (ethylene oxide) leads to the mixture of relatively low molecular weight (M < 103) linear polymer and up to >90% of cyclic oligomers, predominantly cyclic dimer (1,4-dioxane) 191,102]. The same behavior was observed for polymerization of substituted oxiranes, propylene oxide [103], epichlorohydrin [104], and other oxiranes having one or more substituents in the ring, although the distribution of cyclic fraction varied, depending on the structure of monomer. 

Thus, cationic polymerization of oxiranes is of little synthetic value, if the preparation of linear polymers is attempted. The high tendency for cyclization may be employed, however, for preparation of macrocyclic polyethers (crown ethers). Polymerization of ethylene oxide in the presence of suitable cations (e.g., Na+, K+, Rb +, Cs + ) leads to crown ethers of a given ring size in relatively high yields, due to the template effect [105], Thus, with Rb+ or Cs+ cations, cyclic fraction contained exclusively 18-crown-6. 

In this sequence of reactions, it is the monomer that forms oxonium ion [thus Activated Monomer (AM) mechanism] and the growing chain end is neutral. As shown in the series of papers [107-115], if the conditions are created, when AM mechanism predominates, by keeping the low instantaneous ratio of [monomer]/[HO-] (slow addition of monomer to reaction mixture), back-biting is effectively eliminated. Linear polymers, free of cyclic fraction are obtained under these conditions. The mechanism and kinetics of AM polymerization of oxiranes is discussed in detail in recent monograph [6]. 

In the cationic polymerization of unsubstituted oxetane, formation up to 50% of cyclic tetramer has been observed, in addition to linear polymer for polymerization conducted at 100° C, initiated with BF3 [119]. The content of cyclic fraction decreased, however, with decreasing temperature. 

In the polymerization of 3,3-dimethyloxetane initiated with (CzH5)30 + BF4 at 20° C, up to 20% of cyclic fraction was observed with cyclic tetramer as a main component [120]. 

By introducing still larger (but at the same time electron-withdrawing) chloromethyl substituents (3,3-bis(chloromethyl)oxirane BCMO), the content of cyclic fraction was further considerably reduced. Only —2% of cyclic trimer was isolated from the product of polymerization of BCMO initiated with (C2H5)3A1-H20 system at temperature as high as 180° C [121]. 

No absolute concentrations were measured it was however estimated that cyclic tetramer constituted —0.4% mol of the final products. Thus, the overall concentration of the cyclic fraction would correspond to few mol% (this may be still not the equilibrium distribution). 

The preceding discussion shows that in the cationic polymerization of cyclic acetals chain transfer to polymer can not be avoided. If however polymerization is carried out at high initial monomer concentration (preferentially in bulk) the content of cyclic fraction may be limited to a few percent. As the cyclic fraction is composed mainly of medium-size rings, the high molecular weight polymer may be separated from cyclic fraction by fractionation. 

For such mechanism the size of the ring of cyclic polymer formed would be closely related to the thickness of lamellar crystals. The thickness of crystals has been measured by SAXS method and good linear dependence between the molecular weight of cyclic polymer (M -1300-2100) and the long period of the crystal (—7-9 nm) was observed. The quantitative relation between M of cyclic fraction and parameters of crystalline phase can be expressed as follows ... 

The main components of cyclic fraction in thiirane polymerization are cyclic dimer and cyclic tetramer [158],... 

Formation of block copolymers in the sequential polymerization may be affected by chain transfer to polymer. As already discussed, in several systems the intramolecular chain transfer to polymer leads to formation of cyclic fraction. Cyclic macromolecules, being neutral, do not participate in further reaction and constitute the homopolymer fraction in resulting copolymer. Intermolecular chain transfer to polymer may lead to disproportionation, i.e., formation of fraction of macromolecules which do not carry active species ... 

Fig. 2. Gas-liquid chromatogram (g.Lc.) of a cyclic fraction from a ring-chain equilitoate of poly-(ethylmethylsiloxane) containing cyclics (CH3CH2(CH3)SiO) xWifhx = 11-19...

Branched Alkanes. 2,6,10,14-Tetramethyl-hexadecane (phytane) and 2,6,10,14-tetramethyl-pentadecane (pristane) were found in considerable amounts (3). The dominance of phytane over pristane was observed. Other aliphatic isoprenoid alkanes C15 (2,6,10-trimethyl-dodecane, farnesane), C-, (2,6,10-trimethyl-tridecane), and C-g (2,6,10-trimethyl-pentaaecane) were also identified (4). In tne thiourea adduct of the branched-cyclic fraction, a homologous... 

Cyclic Alkanes. In the cyclic fraction the following polycyclic isoprenoid compounds were identified (4) 27 29 steranes> methyl-C2g sterane> C 7 32 Pentacyclic triterpanes of hopane type (except C ft), anathe bicyclic tetraterpane perhydro-8-caro-tane (Table IIJ. 

If the equilibrium concentrations of the individual oligomers are taken together, the equilibrium concentration of the cyclic fraction is obtained. On the basis of the... 

Recently we stressed that the proportion of the cyclic fraction is given by the ratio [M]cril./([M]o — [M]e — [M]crll ), thus it is different for different starting monomer concentrations, as shown in Fig. 3.2 for 1,3-dioxolane polymerization 18). Figure 3.2 is based on the data of Ref.14), (note that only the proportions change, the absolute value of rM]cri, does not depend on [M]o). 

Apparently, this largely forgotten phenomenon was, at least partially, responsible for the long-lasting controversy concerning the proportion of the cyclic fraction in cyclic acetal polymers 16>. 

As described in the previous section, under suitable reaction conditions one can prepare living polyacetals (poly-l,3-dioxolane and poly-1,3-dioxepane) in almost quantitative yields having well defined molecular weights (DPn = ([M]0 — [M]e)/[I]0), and a low content of cyclic fraction (a few percent). 

In contrast to the hydrolysis technology, the methanolysis process allows for the one-step synthesis of organosiloxane oligomers and methyl chloride without formation of hydrochloric acid (64,65). The continuous methanolysis can also yield quantitatively linear silanol-stopped oligomers by recycle of the cyclic fraction into the hydrolysis loop. 

The position of the equilibrium depends on a number of factors, such as concentration of siloxane units and the nature of substituents on the silicon, but is independent of the starting siloxane composition and the polymerization conditions (81,82). For a bulk polymerization of dimethylsiloxane, the equilibrium concentration of cyclic oligomers is approximately 18 wt % (83). The equilibrium mixture of cydosiloxanes is composed of a continuous population to at least D400, but D4, D5, and D6 make over 95 wt % of the total cyclic fraction (84). 

---