Direct polycondensation route

Poly(lactic acid) (PLA) is a biodegradable polymer that has a variety of applications. It has been widely used in the biomedical and pharmaceutical fields for several decades due to its biocompatibility and biodegradability in contact with mammalian bodies. For many years, however, the application of PLA was very limited, due to the high cost of synthesis in the laboratory. For the most part, the direct polycondensation route (see Figure 8.1) was employed to produce PLA from lactic acid. The resultant PLA had a low molecular weight and poor mechanical properties. 

More recently, models were developed to achieve the elimination of the template under milder conditions. The first relies on the preparation of a dialkyl carbonate system by polycondensation/hydrolysis of the corresponding precursor family 26. Thermal treatment of a non-porous xerogel of 26 at 250-350 °C results in the simultaneous elimination of CO2 and hybrid materials with residual hydroxyalkyl and olefinic functions according to Scheme 18. It is interesting to note that this approach also allows the preparation of materials which cannot be prepared by other routes, such as direct polycondensation of the corresponding allyl and hydroxyalkyl precursors. 

Direct polycondensations of aromatic diamines with dicarboxylic acids have generally been described as a poor route to high molecular linear polyamides. Recently, high molecular weight polyamides have been obtained with limited success by a melt polymerization of 4,4 -diaminodiphenylmethane (MDA) with aliphatic dicarboxylic acids14. ... 

In recent years methods have been developed to produce terephthalic acid with satisfactory purity, and direct polycondensation reaction with ethylene glycol is now the preferred route to this polymer. 

Several process routes have been developed or are practised on industrial scale Ring Opening Polymerization (ROP), Direct Polycondensation in high boiling solvents (DP-S) and Direct Polymerization in bulk followed by chain extension with reactive additives. 

Moon, S.I., Lee, C.W., Miyamoto, M. and Kimura, Y. (2000) Melt polycondensation of L-lactic add with Sn(II) catalysts activated by various proton adds A direct manufacturing rout to high molecular... 

The polymers are synthesised via two routes direct polycondensation of lactic and glycolic acids or ring-opening polymerisation of the cyclic lactide and glycolide dimers.The nomenclature for polymers prepared by different routes is full of contradictions, but polymers prepared from lactic acid by polycondensation are strictly referred to by the acid, as in poly(lactic acid), and those prepared by ring-opening polymerisation by the dimer, as in polylactide (PLA). ... 

Direct polycondensation is the cheaper of the two routes, but the polymers produced have lower molecular weights and are more polydisperse than those produced by ring-opening polymerisation. 

While direct polycondensation of LA should be the cheapest route to PLA, the ring-opening polymerization (ROP) of lactide is the method used commercially. Though the ROP of lactide was first studied long back (1932), only low molecular weight polymer was produced until lactide purification techniques were devised by DuPont in 1954. Over the past decades, many researchers have studied the... 

Poly(lactic acid) (PLA) is produced from the monomer of lactic acid (LA). PLA can be produced by two well-known processes — the direct polycondensation (DP) route and the ring-opening polymerization (ROP) route. Although DP is simpler than ROP for the production of PLA, ROP can produce a low-molecular-weight brittle form of PLA. Generally, several substances are involved in the production of PLA, and these relationships have been summarized in Figure 2.1. The lactic acid for the process is obtained from the fermentation of sugar. Lactic acid is converted to lactide and eventually to PLA. It should be noted that there are two different terms, poly(lactic acid) and polylactide , for the polymer of lactic acid. Both terms are used... 

The PLA can be synthesized by various synthesis routes such as direct polycondensation, azeotropicdehydrative condensation, ROP, melt polycondensation (MP), and solid state polymerization (SSP) of low molecular weight (MW) PLA (Lunt, 1998). 

PLA can be obtained in two ways through direct polycondensation of the hydroxy acid or by ROP of cyclic lactide monomer. The different reaction pathways to PLA are depicted in Scheme 6.3. Different nomenclatures of polymers obtained by the different routes are often observed in the literature those obtained from lactic acid by direct polycondensation are referred to as poly (lactic acid), while those obtained from lactide monomer by ROP are referred to as poly (lactide). The general abbreviation used in both cases is PLA. 

In addition to nucleophilic aromatic substitution, there are a number of other synthetic routes to poly(aryl ethers). Friedel-Crafts condensation of arylsulfonyl chlorides and aryl carboxylic acid derivatives with aryl ethers has been employed to prepare polysulfones (2b) and poly(ether ketones) (105,106), respectively. Direct polycondensation of various benzoic acids containing a phenyl ether structure has been carried in 1 10 phosphorous pentoxide/methanesulfonic acid (107). The success of this method is a consequence of the high selectivity of the electrophilic reagent for substitution para to the ether linkage. 

Moon, S, I,. Lee, C. W., Miyamtot, M., Kimma, Y. Melt Polycondensation of L-Lactic acid with Sn(II) Catalysts Activated by Various Proton Acids A Direct Manufacturing Route to High Molecular Wei Poly(L-lactic acid). Journal of Polymer Science, Part A Polymer Chemistry, 2000, 38, 1673-1679. 

As mentioned previously, the main drawbacks of the thermal route to poly-borylborazine are (1) the presence of both direct intercyclic bonds and three-atom bridges between the rings, and (2) a difficulty in controlling the polycondensation rate. One solution we investigated to address these drawbacks is a route based on the room temperature reaction of /i-chloroborazine with trialkylaminoborane.31 32 We used 2-methylamino-4,6-dichloroborazine instead of 2,4,6-trichloroborazine to prepare a two-point polymer (scheme 4), which is theoretically less cross-linked. 

Figure 4. shows the route from the high boiling residue of the direct synthesis to silicon carbo-nitride fibers. Methylchlorodisilanes and trichlorosilanes as additives are mixed in a specific ratio and react with methylamine and a small amount of ammonia to form an aminodisilane/oligosilazane. The subsequent polycondensation reaction of this mixture by heating to 250 °C yields a soluble and melt spinnable polysilazane. In comparision with the polysilane the properties of the polysilazane depend on the ratios of the disilanes/silanes and methylamine/ammonia and also on the reaction conditions. 

Recently, the synthesis of nano-sized HA has been proposed via reverse-micro-emulsion preparation, which is reported to be effective for controlling the hydrolysis and polycondensation of the alkoxides of the constituents. Using this preparation route, the nanoparticles crystallize directly to the desired phase at the relatively low temperature of 1050 °C and maintain surface areas higher than 100 m g after calcination at 1300 °C for 2h [107-109]. 

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