Ring-chain equilibration

The thermal stability of polymers of types (1) and (2) is also dependent on the nature of the substituents on phosphoms. Polymers with methoxy and ethoxy substituents undergo skeletal changes and degradation above about 100°C, but aryloxy and fluoroalkoxy substituents provide higher thermal stability (4). Most of the P—N- and P—O-substituted polymers either depolymerize via ring-chain equilibration or undergo cross-linking reactions at temperatures much above 150—175°C. 

An important source of experimental and theoretical studies of equilibria in ring formation is represented by the field of so-called macrocyclisation equilibria (Flory, 1969). Interest in this field appears to have been restricted so far to chemists conventionally labelled as polymer chemists. Experimental evidence of cyclic oligomer populations of ring-chain equilibrates such as those obtained in polysiloxanes (Brown and Slusarczuk, 1965) may be delated to the statistical conformation of the corresponding open-chain molecules (Jacobson and Stockmayer, 1950 Flory, 1969). In these studies experimental results are expressed in terms of molar cyclisation equilibrium constants Kx (14) related to the x-meric cyclic species Mx in equilibrium with the... 

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). 

POLYESTER CYCLIC OLIGOMERS VIA RING-CHAIN EQUILIBRATION (DEPOLYMERIZATION)... 

Figure 3.2 compares the level of cyclics on a molar, wt%, or yield basis as a function of reaction concentration. Note that as the reaction concentration dropped, the yield increased to near 100 %, as predicted from theory. However, the amount of cyclic PBT on a molar or wt% in solution basis remained constant at the critical monomer concentration, which is about 0.050 M, regardless of the concentration of polymer in solution. In fact, if one calculates the amount of cyclic present in an equilibrated melt (1-2 %), it is also about 0.05 M cyclic. The same amount of cyclic was generated via the ring-chain equilibration process, regardless of the reaction concentration only the amount of polymer which remains as a by-product... 

The synthesis of cyclic polydimethylsiloxane was first achieved through ring-chain equilibration of siloxane oligomers in the presence of potassium silanolate, as shown in Fig. 51 [163-165]. Cyclics recovered from ring-chain equilibration reactions have been fractionated by preparative GPC, yielding... 

Fig. 51 Synthesis of cyclic polydimethylsiloxane by ring-chain equilibration beginning...

Investigations of the cyclic populations of simple ring-chain equilibrates such as those set up in polysiloxane and polyester systems have at least two major advantages over the study of ring formation in more complex systems, such as kinetically-controlled non-linear polymerisations. 

For convenience, the cyclic populations of ring-chain equilibrates are expressed in terms of the molar cyclization equilibrium constants Kx for the individual x-meric ring molecules M, thus... 

In their classic paper published in 1950, Jacobson and Stockmayer (21) presented a theory giving a precise expression for the concentrations of macrocyclics Af, in ring-chain equilibrates. The molar cyclization equilibrium constants Kx were given by... 

Wrist s results for the ring-chain equilibrate carried out in toluene solution, as described by Brown and Slusarczuk 18) are in broad agreement with the fmdings of these authors. The gradient in the plot of log against log x over the range... 

An understanding of the thermodynamics of ring-chain equilibration reactions is important in Inorganic Chemistry where such reactions abound (sse, for example. Allcock (65), Gee (66), Gimblett (67), and Moedritzer (65)). Two such reactions are found (i) in polyphosphazene chemistry (69) where a polymerisation-depolymerisation equilibrium can be established (70) and (ii) in silicon-sulphur chemistry, where the analogue of the poly(dimethylsiloxane) system has been studied (71) and shown to contain cyclics such as the dimer ( (013)2 SiS)2 the trimer ((CH3)2SiS)3, as well as small amounts of chain molecules of limited chain length. 

The remarkable ease with which poly(dimethylsiloxane) enters into ring-chain equilibration reactions prompted the investigations described in this section, as well as those outlined in Section IIL(C). Both investigations were concerned with the question as to whether it is possible to study the statistical conformations of chain polymers by the equilibrium cyclic concentration method, if the polymer itself cannot undei o ring-chain equilibration reactions. The polymers chosen for study were n-paraffinic and polystyrene chains. In each case, dimethylsiloxane linkages were incorporated into the molecules and ring[Pg.56]

Despite the numerous polyesters described in the literature, it is only recently that the cyclic populations of ring-chain equilibrates of linear aliphatic polyesters have been characterised. This characterisation was based on the well-known investigations of Carothers 121, 122) and his co-workers and the studies of Billmeyer 123-125) and his group. In particular, Billmeyer s wodc established tetraisopropyltitanate as a catalyst for producing equilibrium between ring and chain molecules in polyester systems. This is in contrast to the catalyst, p-toluene sulphonic acid, used by Jacobson, Beckmann and Stockmayer 127) in what was effectively the first attempt to study experimentally and theoretically the change in the total cyclic population of... 

Finally, it is noted that some studies of ring-chain equilibration reactions in solid PET samples have been carried out (i 7,130). Heating PET chip containing about 2.2% w/w cyclics up to the nonamer resulted in a decrease in cyclic content to about 0.7% w/w, and an increase in molecular weight by a factor of about two. Similar decreases in the concentrations of cyclic oligomers have been found to result when melt equilibrated samples of nylon. 6 are heated in the >lid state, and these will be discussed in detail in the next section. 

The ring-chain equilibrates of nylon-6 at 525 K were prepared starting from 6-capraminohexanoic acid as a catalyst to an effective water... 

Wichterle and his co-workers (167-169) established that ring-chain equilibration reactions can be carried out in nylon-6 at temperatures well below the melting point of the polymer [see Section III.(E)2 for similar reactions in poly(ethylene tereph-thalate)]. Such ring-chain equilibration reactions have been carried out recently (155) by heating samples of nylon-6 or e-caprolactam at 459 K in three clearly defined experiments. The individual cyclic oligomer concentrations were determined for each solid sample of nylon-6. The results obtained are plotted in Fig. 18 and 19, where they are compared with cyclic concentrations in a polymer prepared by rii -chain equilibration in the melt at 525 K (see Fig. 17). 

Fig. 18. Concentrations of cyclics (in mol kg ) in samples of nylonrb prepared (i) by heating a melt polymerised sample in the solid state at 459 K for 32 days (a) and (ii) by heating a cyclic-free polymer in the solid state at 459 K for 34 days(V). These values are compared with cyclic concentrations in a polymer prepared by ring-chain equilibration in the melt at 525 K (o)...

Eq. (6) with oji = 2x. A rotational isomeric state model for polyphosphate chains (2 76,187) was used to compute values of < rj >o required by Eq. (6). Comparison of the experimental values with the theoretical values, as well as with the values for the corresponding cyclic siloxanes (R(CH3)SiO)x [see Section III.(A)5] stronj y supports the contention that polyphos diate chains in sodium phosphate melts adopt random-coil conformations. Further evidence for the structure of sodium phosphate melts would come from precise data for macrocyclic concentrations in quenched sodium phosphate ring-chain equilibrates, if these became available. 

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