Solid polycondensation

Moon, S. I., Lee, C. W., Taniguchi, I., Mi5 moto, M., Kimura, Y. Melt/ solid polycondensation of L-lactic acid An alternative route to poly(L-lactic acid) with high molecular weight. Polvm. Commun.. 2001,42, 5059-5062. 

S.l. Moon, I. Taniguchi, M. Miyamoto, Y. Kimura, C.W. Lee, Synthesis and properties of high-molecular-weight poly (L-lactic acid) by melt/solid polycondensation under different reaction conditions. High Perform. Polym. 13 (2) (2001) S189-S196. 

The disadvantage of the PLA prepared by the direct polycondensation is often a limited molecular weight in combination with a low yield. Some progress in increasing the molecular weight of the PLA has recently been achieved, though, by sequential melt/solid polycondensation [5, 31]. 

Moreover, the molecular weight remained around 100 000 Da, being much lower than that of the PLLA obtained by the ring-opening polymerization of Z-lactide. Therefore, they examined the melt/solid polycondensation of lactic acid in which the melt polycondensation of Z,-lactic acid was subjected to solid-state polycondensation below Tm of PLLA [8]. In solid state, the polymerization reaction can be favored over the depolymerization or other side reactions. Particularly, in the process of crystallization of the resultant polymer, both monomer and catalyst can be segregated and concentrated in the noncrystalline part to allow the polymer formation to reach 100% [9]. Figure 3.2 shows the whole process of this melt/solid polycondensation of Z-lactic acid. In this process, a polycondensation with a molecular weight of 20 000 Da is first prepared by... 

Figure 3.2 The whole process of the melt/solid polycondensation of L-lactic acid...

The melt/solid polycondensation of LA has been extensively studied by many researchers. This process usually leads to high molecular weight products, however, is not usually used commercially due to low production rates and mixing problems experienced in the process. 

Difficulties due to side reactions (cyclization) and a broad molecular weight distribution accompanying the polycondensation of active esters led to the application of methods wherein the polymers are built up stepwise. In 1968, Sakakibara et al.31) introduced the solid-phase technique using Merrifield s resin. By stepwise addition of tert-pentyloxycar-bonyl tripeptides, they have synthesized (Pro-Pro-Gly)n with n = 5, 10, 15 and 20. 

The synthesis of high-molar-mass PLA and PGA by two-step polycondensations of lactic and glycolic acids, respectively, has recently been reported.374,375 It involves the formation of a low-molar-mass oligomer followed by a polycondensation step either in the solid state374 or in the melt under vacuum.375 The procedures are detailed in Section 2.4.1.5.2. 

The preparation of PGA given below (Scheme 2.51) is a polycondensation of glycolic acid followed by a solid-state postpolycondensation.374 (Reproduced from ref. 374. Copyright 2000 Elsevier Science Ltd, by permission of the copyright owner.) This method is different from the commonly used ring-opening polymerization of lactide (see Section 2.3.6). 

Polycondensation At room temperature, 0.4% mass of Sn(II) chloride dihydrate (SnCl2-2H20) and 0.4% mass of p-toluenesulfonic acid monohydrate (p-TSA) are introduced into the mixture. The mixture is heated to 180°C under mechanical stirring. The pressure is reduced stepwise to reach 13 mbar, and file reaction is continued for 20 h. The reaction system becomes gradually viscous, and a small amount of L-lactide is formed and refluxed through the reflux condenser. At file end of the reaction, the flask is cooled down, file product is dissolved in chloroform and subsequently precipitated into diethyl ether. The resulting white fibrous solids are filtered and dried under vacuum (average yield 67%). 

Second step solid state post-polycondensation 260-280 °C, 0.7 mbar (4 h)... 

The polycondensation of 13 is conducted in vacuo using a process similar to that used for the polycondensation of 11. After heating to 150°C, the product becomes solid and the mechanical stirring is stopped to yield polymer 14. This product is a colorless waxy solid at 150°C, which becomes a white powder after cooling to TIoom. DSC and TGA give a 7 g 55°C and a ceramic yield of 47% (at 1000°C). 

Aromatic polycarbonates are currently manufactured either by the interfacial polycondensation of the sodium salt of diphenols such as bisphenol A with phosgene (Reaction 1, Scheme 22) or by transesterification of diphenyl carbonate (DPC) with diphenols in the presence of homogeneous catalysts (Reaction 2, Scheme 22). DPC is made by the oxidative carbonylation of dimethyl carbonate. If DPC can be made from cyclic carbonates by transesterification with solid catalysts, then an environmentally friendlier route to polycarbonates using C02 (instead of COCl2/CO) can be established. Transesterifications are catalyzed by a variety of materials K2C03, KOH, Mg-containing smectites, and oxides supported on silica (250). Recently, Ma et al. (251) reported the transesterification of dimethyl oxalate with phenol catalyzed by Sn-TS-1 samples calcined at various temperatures. The activity was related to the weak Lewis acidity of Sn-TS-1 (251). 

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

In the effort to make pure blue-emitting materials Shim and coworkers [146] synthesized a series of PPV-based copolymers containing carbazole (polymers 95 and 96) and fluorene (polymers 97 and 98) units via Wittig polycondensation. The use of trimethylsilyl substituents, instead of alkoxy groups, eliminates the electron donor influence of the latter and leads to chain distortion that bathochromically shifts the emission (Amax = 480 nm for 95 and 495 nm for 97). In addition, a very high PLQY was found for these polymers in the solid state (64 and 81%, respectively). Single-layer PLEDs fabricated with 95 and 97 (ITO/polymer/Al) showed EL efficiencies of 13 and 32 times higher than MEH-PPV, respectively (see also Ref. [147] for synthesis and PLED studies of polymers 99 and 100) (Chart 2.20). 

Esterification is the first step in PET synthesis but also occurs during melt-phase polycondensation, SSP, and extrusion processes due to the significant formation of carboxyl end groups by polymer degradation. As an equilibrium reaction, esterification is always accompanied by the reverse reaction being hydrolysis. In industrial esterification reactors, esterification and transesterification proceed simultaneously, and thus a complex reaction scheme with parallel and serial equilibrium reactions has to be considered. In addition, the esterification process involves three phases, i.e. solid TPA, a homogeneous liquid phase and the gas phase. The respective phase equilibria will be discussed below in Section 3.1. 

Transesterification is the main reaction of PET polycondensation in both the melt phase and the solid state. It is the dominant reaction in the second and subsequent stages of PET production, but also occurs to a significant extent during esterification. As mentioned above, polycondensation is an equilibrium reaction and the reverse reaction is glycolysis. The temperature-dependent equilibrium constant of transesterification has already been discussed in Section 2.1. The polycondensation process in the melt phase involves a gas phase and a homogeneous liquid phase, while the SSP process involves a gas phase and two solid phases. The respective phase equilibria, which have to be considered for process modelling, will be discussed below in Section 3.1. 

---