Lactides, ROP

Fig. 27 Amino bis(phenolate)-supported zirconium and hafnium complexes and Al-fluorous complexes for isoselective lactide ROP...

Fig. 29 Rare-earth complexes with heteroselective rac-lactide ROP...

Scheme 16 Indium salt as initiator for heteroselective lactide ROP [173]...

Fig. 30 Zirconium, zinc, and yttrium complexes for heteroselective lactide ROP...

The major side reactions in lactide ROP are intramolecular and intermolecular transesterification (Fig. 3). Intramolecular transesterification leads to formation of mixtures of linear and cyclic polymers, ultimately reducing the molecular weight of the polymer sample. Intermolecular transesterification, or chain transfer, leads to a broadening of the polymer molecular weight distribution. The extent of these processes can be experimentally verified using mass spectrometry, most commonly using ESI or MALDI-ToF mass spectrometry and by l3C hi) NMR spectroscopy [24, 25]. 

Figure 15 Metal catalysts bearing tripodal nitrogen donor ligands for lactide ROP...

Figure 22 Examples of aluminum salen and salan catalysts for lactide ROP reported by Hormnirun et alJ ...

Group 4 metals have also been used widely in conjunction with salen-type ligands (Figure 25). In 2006 Gregson et reported several chiral and achiral titanium salen alkoxide complexes for the ROP of lactide. All catalysts reported were modestly active and heteroselective (P 0.51-0.57). Several achiral Ti and Zr salan catalysts were reported by Gendler et for melt polymerization of lactide. While no stereoselectivity has been reported for either system, the Zr complexes were more active towards lactide ROP than the Ti analogs. 

Figure 26 Variations of salen-type ligands used in metal-based lactide ROP catalysts...

Fig. 3.10 Thiourea-based organocatalysts. a-c Chemical structure of the bifunctional thiourea-tertiary amine catalyst, bis(3,5-trifluoromethyl)phenyl cyclohexylthiourea (thiourea), and N,N-dimethylcyclohexylamine (Moditied from Dove et til. [42]). d Proposed dual activation pathway of lactide ROP [41] (Adapted with permission from Pratt et al. [41]. Copyright 2013 American Chemictil Society)...

In a first batch of lactide ROPs an appropriate solvent and catalyst were determined, see Table3.3. The polydispersity of poly(lactic acid) synthesized in TCM compared to DCM was significantly lower. The addition of thiourea co-catalyst lead to an improved PDI in case of DCM, while no change was observed in TCM. However, when using poly(4-X-styrene)-OH as macroinitiator only the DBU/thiourea co-catalyst system was found to yield copolymer. Considering Fig. 3.12b, the lactide... 

Fig. 3.17 Proposed lactide ROP mechanism through the dual activation of DBU/thiourea...

In summary, almost 80 distinct ATRP polymerizations of 4-X-styrene monomers and over 215 copolymerization via lactide ROP were conducted. A set of optimized polymerization parameters were successfiilly identified that allowed the highly reproducible synthesis of well-defined diblock copolymers on a gram scale fhat adopt the double-gyroid nanomorphology during microphase separation as will be discussed in the following chapter. To reduce the synthesis cost and make the polymers more environmentally sustainable, the fraction of expensive fluorinated styrene monomer was gradually decreased. 

FIGURE 9.2 Examples of lactide ROP with anionic initiator oiganometallic catalyst/alcohol initiator system for possible chain-end/midchain functionalization. 

Scheme 3.11 The synthesis of ABA triblock copolyesters based on ricinoleic acid and L-lactide. ROP Ring-opening polymerisation [23]...

Scheme 21.17 Reaction pathway for the lactide ROP with Sn(OAc)2 as model.

These prehminary results then motivated the investigation of NHCs as organocatalysts for lactide ROP, and the representative imidazol-2-yhdene IMes... 

Scheme 10.6 Alcohol adducts of NHCs as single-component catalyst/initlator and temperature switch of the lactide ROP by alternating high (90°C) and low (20°C) temperatures.

Scheme 10.17 Star polylactides and a representative dendritic initiator for lactide ROP.

Incorporation of the initiating protic source at the a-chain end of the polymer was confirmed by H NMR and mass spectroscopic analysis. The requirement for a strong acid was evidenced by the inactivity towards lactide ROP of trifluoroacetic acid and HCl-OEt2 under comparable conditions. An activated monomer mechanism was proposed in which the protonation of lactide by the acid activated the monomer towards ring opening by nucleophiUc addition of the initiating/ propagahng probe species (Scheme 14.15). 

---