Block copolymer stereo

Another type of stereo complex, i.e., a tightly wound 3/1-helix, is formed when polymers of L-lactic acid (PLLA) and D-lactic acid (PDLA) are mixed in the blend.247-248 Also in solution aggregation of block copolymers containing PLA segments has been found 249-250 however, no studies toward the morphology of the aggregates have been conducted. 

Ethylene-a-olefin copolymers, stereo-block enhanced inter-spherulitic and Lustiger, 1993... 

Fujiwara T, Mukose T, Yamaoka T, Yamane H, Sakurai S, Kimura Y. Novel thermo-responsive formation of a hydrogel by stereo-complexation between PLLA-PEG-PLLA and PDLA-PEG-PDLA block copolymers. Macromol Biosci 2001 1 204-208. 

Kang K, Perron ME, Prud homme RE, Zhang Y, Gaucher G, Leroux JC. 2005. Stereo complex block copolymer micelles core shell nanostructures with enhanced stability. Nano Lett 5 315 319. 

SOLUTION POLYMERIZATION Solution SBR typically made in hydrocarbon solution with alkyl lithium-based inihator. In this stereo-specific catalyst system, in principle, every polymer molecule remains live until a deactivator or some other agent capable of reacting with the anion intervenes. Able to control molecular weight, molecular weight distribution, and branching. Able to make random and block copolymers with designed chain sequence. Able to make copolymer with controlled styrene content. Able to control the butadiene structure of vinyl/ ds/ trans. Higher purity due to no addition of soap. 

Methods of improving thermal resistance are to prepare a stereo complex (sc PLA) or a stereo-block copolymer (sbc PLA). Melting point and heat distortion temperature (HDT) will increase significantly. 

Formation of AC-2 results in a more substantial transformation of bonds than the formation of AC-1. A different spatial structure of AC-2 is formed, which has the flower shape and permits both mono and bidentate coordination of 1,3-dienes, as well as putting less restrictions on their coordination with a monomer for all the positions of substituent at the C = C bond, in relation to a crystal surface. This is the reason for AC-2 stereospecificity and the aforementioned possibility of stereo block copolymer formation during a-polymerisation in the presence of Ziegler-Natta catalytic systems. AC-2 can orient unsaturated C = C bonds of 1,3-dienes to bidentate coordination on the bidentate AC. 

Moreover, stereo-complexation can occur in block copolymer mixtures provided that the copolymers contain l-LA and d-LA sequences of sufficient lengths. PLA multiblock stereo-copolymers can easily form a stereo-complex as mentioned above. Examples also include stereo-complexation between bloek copolymers of L-lactide or o-lactide with s-caprolactone, between L-lactide-rich PLA and D-lactide-rich PLA, between copolymers of L-lactide or D-lactide with glycolide and between block copolymers of L-lactide or D-lactide with PEG. " ... 

Stereo-complex films were reported by Masutani et al7 % combining bifunctional PLLA and PDLA pre-polymers, multi-stereo-block copolymers having different block lengths and sequences are obtained. The resultant copolymers are readily fabricated into transparent films by hot-pressing. The films present excellent thermal stability and thermo-mechanical properties because of the easy formation of stereo-complex crystals. This synthetic method based on tbe dual terminal couplings allows obtaining stereo-block copolymers of PLLA and PDLA showing excellent thermo-mechanical properties and melt processability. 

Stereo-complexation not only occurs in melt and in organic solution, but also in aqueous solution. When a water soluble PLLA/PEG block copolymer is mixed with a similar PDLA/PEG in dilute solution, the strong interactions between PLEA and PDLA blocks lead to the formation of stereo-complex micelles. 

Li et al. reported on stereo-complex hydrogels from totally aqueous PLLA/ PEG and PDLA/PEG solutions. The copolymers are synthesized by ring-opening polymerization of L-lactide or o-lactide in the presence of mono- or dihydro)q l PEG, using zinc lactate as catalyst. Hydrogels are obtained from aqueous solutions containing equimolar amounts of PLLA/ PEG and PDLA/PEG block copolymers due to the stereo-complexation between PLLA and PDLA blocks. 

N.m.r. has been used to probe various aggregation phenomena. Heatley and Begum observed that mieellization of styrene-butadiene-styrene tri-block copolymers in non-solvents for butadiene is accompanied by marked hindrance to butadiene mobility. Bovey et a/. have shown that C Ti s for poly(vinylidene fluoride) and PMMA are the same in a mixed solution as in single solutions. The unusual compatibility of these two polyn rs therefore does not arise from complex formation. Self-association of stereo-r ular PMMA s in dilute solution has been studied. In toluene, isotactic PMMA is 10—17% associated at 27 C whereas a highly syndiotactic sample is 76% associated. 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

Block copolymers may also be produced from just one monomer species. In this case the stereo arrangement in one block is different from another. For example, in one block the units might be arranged isotactically and in the attached block, syndiotactically. 

TLC has been used in the study of many homopolymers polystyrene, poly(methyl methacrylate), poly(ethylene oxide), polyisoprene, poly(vinyl acetate), poly(vinyl chloride) and polybutadiene. Their molecular weight, molecular-weight distributions, microstructure (stereo-regularity, isomerism and the content of polar end groups), isotope composition and branching have been studied. For copolymer characterisation (e.g. purity and compositional inhomogeneity), random copolymers such as styrene-methacrylate, and block copolymers such as styrene-butadiene, styrene-methyl methacrylate and styrene-ethylene oxide have been separated. A good review article on polymers... 

Usually the Zigler-Natta co-ordination initiator system is used to graft oc-olefins onto other polymers to give stereo block/graft copolymers, which contain isotactic/atactic sequences. In the Zigler-Natta co-ordination catalyst [69] system, the diethyl aluminium hydride reacts with pendant groups to form macromolecular trialkyl aluminium. The residual initiator is freed by extraction methods. 

In this notation A and B can represent different stereo configurations, or microstructures of the same mer. Likewise, either A and/or B may be random or tapered copolymer blocks rather than homopolymeric blocks. A tapered A-B block is one that begins with a section of pure A mers and gradually incorporates B mers until at the end it becomes a section of pure B mers. The A-B-C designation represents the incorporation of a third monomer segment and would most probably comprise a three-phase system. An A-B-A block polymer based on styrene and butadiene would mean in this notation a polystyrene block-polybutadiene block-polystyrene block structure, that is, S-B-S. 

Semicrystalline polyolefin blends and method of their preparation were described. The blends were reported to show enhanced inter-spherulitic and interlamellar strength. The first polymer should have higher crystallinity and crystallization temperature than the second. Thus, 50-99.9 wt% PP was blended with ethylene-a-olefin copolymers, either a stereo block polypropylene or an ethylene-propylene copolymer, EPR... 

Yang et al. systemically investigated the preparation, aggregation properties and drug release behaviour of PLA/PEG stereo-complex mi-celles. °° Predetermined amounts of PLLA/PEG or PDLA/PEG copolymer are separately dissolved in distilled water or phosphate buffered saline (PBS). The two solutions with equal molar concentrations are then mixed to yield a micellar solution by self-assembly due to interactions between l-LA and d-LA blocks. ... 

Chen et ah investigated comparatively single micelles and stereo-complex micelles prepared by nanoprecipitation of PLA-PEG diblock copolymers composed of a monometho PEG with = 5000 Da and PLLA or PDLA blocks with DP of 14, 35 and 70. The CMC and micelle sizes of single... 

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