PLLA preparation

The rates of enzymatic hydrolysis (proteinase K) for branched PLLA (prepared from pentaerythritol with four branches and from polyglycerin with 22 branches) were found to be dependent on the average molecular weight of the PLLA segment in the branched molecules, not on the overall molecular weight of the samples [140]. 

Some reports about the effect of the Sn-based catalyst on PLLA thermal degradation have been published. Noda [40] evaluated the activity of mono-, di-, tetraalkyl and aryl tin(IV) compounds as intramolecular transesterification catalysts and found that monobutyltin trichloride was the most active catalyst with some other compounds indicating almost the same depolymerization activity as Sn(Oct)2. Degee et al. [41] also reported that PLLA prepared with Sn(Oct)2 was less stable thermally than that prepared with Al alk-oxides based on TGA data. However, Cam and Mamed [33] found a contrary effect on thermal decomposition onset temperature in the order of Al > Zn > Sn. 

Figure 3.5 Differential scanning calorimetry thermograms of amorphous PLLA prepared by quenching and PDLLA (heating rate 10°C/min) (adapted from Fambri and Migliaresi, 2010).

Table 4.2 Effect of PPhs on the thermal stability of PLLA prepared by bulk ringopening polymerisation promoted by Sn(Oct)2...

Microparticles can be produced by a simple technique that consists of spraying a polymer, e.g., PLLA, solution in dichloromethane (or dimethylsulfoxide), through a nozzle into a reactor filled with supercritical carbon dioxide (Reverchon et al, 2000). This process is known as supercritical antisolvent precipitation (SAS). The experimental parameters have a limited influence on the particle size (1-4 /im). A modified version of the process, known as the SAS-EM process, allows nanoparticles of a controlled size (30-50 nm) to be produced (Chattopadhay et al., 2002). In order to restrict the use of an organic solvent. Pack and co-workers fed the SAS reactor with a solution of PLLA prepared by homogeneous ring-opening polymerisation in supercritical HCFC-22 (Pack et al, 2003a). 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

PLLA-fr-PCL) multiblock copolymers were prepared from the coupling reaction between the bischloroformates of carboxylated PLLA with diol-terminated PCL in the presence of pyridine [140]. LLA was polymerized with SnOCt2 and 1,6-hexanediol followed by the reaction with succinic anhydride to provide the dicarboxylated PLLA. The carboxyl end groups were subsequently transformed to acid chloride groups by the reaction with thionyl chloride (Scheme 65). As expected, the molecular weight distributions were broad for all samples (1.84 < Mw/Mn < 3.17). 

Recently, we have also prepared nanosized polymersomes through self-assembly of star-shaped PEG-b-PLLA block copolymers (eight-arm PEG-b-PLLA) using a film hydration technique [233]. The polymersomes can encapsulate FITC-labeled Dex, as model of a water-soluble macromolecular (bug, into the hydrophilic interior space. The eight-arm PEG-b-PLLA polymersomes showed relatively high stability compared to that of polymersomes of linear PEG-b-PLLA copolymers with the equal volume fraction. Furthermore, we have developed a novel type of polymersome of amphiphilic polyrotaxane (PRX) composed of PLLA-b-PEG-b-PLLA triblock copolymer and a-cyclodextrin (a-CD) [234]. These polymersomes possess unique structures the surface is covered by PRX structures with multiple a-CDs threaded onto the PEG chain. Since the a-CDs are not covalently bound to the PEG chain, they can slide and rotate along the PEG chain, which forms the outer shell of the polymersomes [235,236]. Thus, the polymersomes could be a novel functional biomedical nanomaterial having a dynamic surface. 

This drawback can be at least partially eliminated by blending PLLA with other polymers (26-29). In addition, ABS has been used for blending (30). The blends were prepared laboratory mill equipped with a twin-screw. It turned out that uncompatibilized blends of PLLA and ABS have a morphology with big phase size and a weak interface. The blends exhibit poor mechanical properties with low elongation at break and decreased impact strength. 

Microspheres were prepared from copolymers of DXO and l-LA and from homopolymer blends of DXO with l-LA and d,l-LA [129-131]. PLLA was partially miscible with PDXO and formed semi crystalline and dense microspheres. PDLLA and PDXO were fully miscible and formed homogeneous and amor-... 

To understand the effect of the molecular weight of PLLA on the properties of nanocomposite, Chen et al. prepared different MWCNT-g-PLLA hybrids with varying molecular weights of PLLA, viz., 1000, 3000, 11000, and 15000. For detailed experimental conditions, refer to Chen et al. (64). The extent of PLLA grafting on MWCNTs was examined by both SEM and TEM (Figure 9.3 and Figure 9.4, respectively). TEM studies revealed that the degree... 

Zhang et al. (65) prepared PLLA-MWCNT nanocomposites by solution-mixing and precipitation method. First, 50g MWCNTs were suspended in lmL DMF. This suspension was stirred overnight in a round-bottom flask along with purified PLLA at 110 °C under N2. The resultant mix was poured into excess methanol, filtered, washed with methanol, and dried at 100 °C under vacuum overnight, which yielded a fluffy black solid (226 mg, 90% yield). SEM and TEM (Figure 9.5) were used to visually examine the degree of dispersion and the extent of absorption of PLLA on the surface of... 

According to the reactants, either diblock or triblock copolymers can be obtained. For instance, PEO-fc-PDMS-b-PEO triblock copolymer and PEO-PDMS diblock copolymers were prepared in high yields by hydrosilylation of a telechelic PDMS which exhibits SiH functions (Mn = 1000) with monofunctional allyl-terminated PEO with Mn = 350 and 500 and telechelic diallyl PEO (Mn = 600), respectively [123]. Their dilute solution properties were investigated. Similarly, interesting PS-b-PDMS thermoplastics have been synthesized from a polystyrene fitted at chain end with a vinyl silane function which reacts with a PDMS bearing SiH end-groups [124]. In addition, hydrosilylation has been used to prepare original copolymers from a,co-disilyl-PDMS 25 and either a,oj-diallyl-polysulfone [125] or a,oj-diallyl poly (L-lactide) (PLLA) as follows [126] ... 

Fig. 16.13 The spectroscopy of pyrene, PRODAN and DCM. Effects of probe structure, PLLA concentration and preparation method on the local microenvironment that surrounds pyrene, PRODAN, and DCM in PLLA/P104-based BPs...

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