Degree of polycondensation

Oils and gums are nonhighly branched- or straight-chain polymers whose viscosity increases with the degree of polycondensation. 

Cellulose layers are produced from native, fibrous or microcrystalline cellulose (Avicel ). The separation behaviors of these naturally vary, because particle size (fiber length), surface, degree of polycondensation and, hence, swelling behavior are all different. 

In 1959 and 1960, Challa published the first results of quantitative experiments on the poly condensation equilibrium in PET [22, 41, 42], He determined the polycondensation equilibrium constant K at different temperatures and average degrees of polycondensation and found that this parameter depends only slightly on temperature, but increases significantly with increasing degree of polycondensation. The monomer BHET was found not to follow the principle of equal reactivity. 

Later, Fontana [43] performed experiments on transesterification and reinterpreted Challa s results. He concluded that the value of the polycondensation equilibrium constant is close to 0.5, being independent of temperature or degree of polycondensation and that the normal Flory-Schuz distribution does hold in the PET system. In Figure 2.8, the polycondensation equilibrium constant K from different sources [22, 43, 44] is shown as a function of the average degree of polycondensation, Pn. 

Figure 2.8 The polycondensation constant K as a function of the average degree of polycondensation Pn, according to Challa [42], Krumpolc and Malek [44] and Fontana [43]...

Rafler et al. showed in an early work [102] that the diffusion coefficient of EG varies with the overall effective polycondensation rate and they proposed a dependency of the diffusion coefficient on the degree of polycondensation. This dependency is obvious, because the diffusion coefficient is proportional to the reciprocal of the viscosity which increases by four orders of magnitude during polycondensation from approximately 0.001 Pas (for Pn = 3) to 67Pas (for Pn = 100) at 280 °C. In later work, Rafler et al. [103, 104, 106] abandoned the varying diffusion coefficient and instead added a convective mass-transport term to the material balance of EG and water. The additional model parameter for convection in the polymer melt and the constant diffusion coefficient were evaluated by data fitting. 

Figure 2.30 Diffusion coefficient, Deg,pet, in molten PET films as a function of degree of polycondensation and temperature [8]...

The monomers TPA and EG are mixed upstream to the esterification reactor in a jacketed slurry preparation unit equipped with a stirrer for highly viscous fluids (e.g. Intermig ). The typical molar ratio of EG to TPA lies between 1.1 and 1.3. The esterification temperature and the molar ratio of monomers are the main controlling factors for the average degree of polycondensation of the esterification product (prepolymer), as well as for its content of carboxyl end groups and DEG. The latter mainly occurs as randomly distributed units of the polymer molecules. 

Figure 5.7 Degree of polycondensation as a function of retention time and temperature [12b]. From Weger, F., Solid-state postcondensation of polyesters and polyamides, presentation given at the FrankI and Thomas Polymer Seminar, 16 June, 1994, Greenville, SC, USA, and reproduced with permission of EMS Inventa-Fischer, GmbH Co. KG...

Phosphanes with polyether substituents have noticeable water solubility if the degree of polycondensation n is at least 16 up to 32. They are of interest as recoverable, nonionic ligands. Several examples of this class of compound have been synthesized (structures 5-7) (46). However, the synthetic methodol ogy is limited to the reactions between phosphorus compounds and polyether derivatives. 

Quantitative determinations of the degree of polycondensation in systematic studies have demonstrated that kinetic parameters such as the temperature have little effect on the level of polycondensation in xerogels 40 and 73, and the solvent has a more important effect149,151,152. Variations of 5 to 20% are observed like those presented in Table 3. From these data and those shown in Table 4, it is evident that the degree of polycondensation and the textural properties of the xerogels are generally not related for these precursors. A variation of 5% of the degree of polycondensation is measured while the specific surface area is increased 10 fold. 

Temperature of the sol-gel processing. Only in cases of the xerogels 40, 73 and 74 has a systematic study demonstrated how much the texture is temperature-dependent. From data in Table 4, an increase in the temperature of the perforation of the gel leads to a decrease in microporosity, an increase in mesoporosity and an increase in specific surface area155. It also appears that the texture of the material is not directly correlated to the degree of polycondensation (see Table 3)151,152. The same effect was also found for the xerogel P(C6H4SiOi.5)3 (30 m2g-x at 20 °C and 340 m2g 1 at 60 °C)109. 

Another demonstration of the independence of the reactivity of functional groups with chain length was given by determining the degree of polycondensation for polymers derived from a homologous series of acids, each acid reacting with 1,6-hexanediol under identical conditions [32]... 

In several instances, discussed in more detail later in this section, experimental evidence strongly indicates that the principle of equal reactivity of functional groups is not obeyed for all chain lengths, and that the equilibrium constant may itself be a function of the degree of polycondensation. 

High datas of deformation- strengthening properties of given block-copolysulfonarilates are explained that the last ones combine inflexibility of polyarilates as well as elasticity of polysulfones. In this case the elasticity is gained by OASO of different degree of polycondensation, which makes the system in the hole more plastic. 

Substituting a phosphine with a polyether chain may also make the phosphine water-soluble. However, diphosphines of the type 15 (Structures 15-17) are only soluble in water when n > 15 [24]. This type of materials can also be used to prepare thermally responsive catalysts [58]. Other examples related to 15 are the class of compounds 16 (cf. Section 3.2.2.3 n = 12, 16, 110) and 17 (n= 18) [12, 25]. The number n gives the average value of the degree of polycondensation . So far, these polyether-based diphosphines have mainly been used in asymmetric hydrogenation of prochiral substrates such as a-acetamidocinnamic acid where ee values vary from 11 to 91% depending on the reaction medium and ligand used (cf. Section 4.6.3). 

Tl(rV), V(IV), In(III) led to the polychelates (125M), (126M). If the degree of polycondensation is not too high, the yellow polymer ligands are soluble in polar solvents. The intensively coloured polychelates are insoluble. 

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