Polyethers and Copolymer

PEG in PEG/PLA blends is used to enhance the hydro-philicity of PLA and to increase its degradation rate and the rate of drug release [207]. Moreover, PEG/PLA blends showed greater strength with lower deformation than PLA. In these blends, PEG acts as a plasticizer for PLA (see also Section 16.4) [210]. PEG/PLA blends can be fabricated by either solution or melt blending [208]. Younes and Cohn [211] reported that solution blends containing more than 20wt% of either compound can crystallize, and the 

The degradation of PLA/PEG blends was reported higher than that of neat PL A. When PEG content was equal or lower than 30wt%, the weight loss occurred mainly due to the enzymatic degradation of PLA. Above 30 wt% of PEG, the weight loss mainly occurred due to the dissolution of PEG [208]. Correspondingly, Cai et al. [207] stated that PEG content decreased from the blends as a consequence of hydrolysis (buffer solution at pH 7.4, 37°C). 

An in vitro degradation study of 50/50 PEO/PLLA found that the blend degraded over a period of 14 days in KH2PO4-NaOH buffer solution due to diffusion of water and dissolution of PEO, while neat PLLA did not exhibit degradation under these conditions. The phase separation of the blend was more obvious when the blends were buffer immersed than when they were not immersed into the buffer solution. An in vivo study conducted in rats showed that the blend could be used for bone formation [219]. 

3 Poly(ethylene oxide)/Poly(propylene oxide)/ Poly(ethylene oxide) Triblock Copolymer Pluronic is a trade name for poloxamers, which are nonionic triblock copolymers of PEO chains on the two ends of the molecules with a poly(propylene) oxide (PPO) chain in the middle. The PEO chain is hydrophilic, while the PPO chain is hydrophobic. The polymer properties can be tailored depending on the content ratio of the constituents. Due to the combined hydro-phobic and hydrophilic properties in the structure (amphiphilic), it can be used as a stabilizer and a surfactant. It also can enhance miscibility and water absorption of polymers, and it has low toxicity itself. Pluronic has been used in drug delivery devices for controlled release purposes [220]. 

In a study of controlled release, BSA, aprotein, was used as a model drug. The absorption of BSA in the blend decreased compared to that in neat PLA [225]. With the presence of longer PEO chains in Pluronic, a considerable decrease in BSA absorption was obtained [226]. For controlled release study, incorporation of more hydrophobic (PPO) than hydrophilic (PEO) portions in the blend enabled reduction in the initial burst of BSA, a model drug, due to lower water absorption. This led to slower release of the BSA [221]. 

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Thermoplastic polyester rubbers are also block copolymers of polyethers and polyesters. The polyester groups are capable of crystallisation and the crystal structures act like cross-links. These materials have good hydrocarbon resistance. Similar thermoplastic polyamide rubbers are also now available. 

Poly(tetramethylene oxide) polyols (see Scheme 4.4) are a special class of polyethers syndiesized via acid-catalyzed ring-opening polymerization of tetrahy-drofuran. Although less susceptible to side reactions, the synthesis of these C4 ethers is less flexible in terms of product composition and structure. Thus, because of diis syndietic route, only two-functional glycols are available and copolymers are not readily available. Molecular weights of commercial C4 glycols range up to about 3000 g/m. 

Poly(ether ester) (PEE) copolymers were consisted of soft segments of polyethers and hard crystalline segments of polyesters. Depending on the polyether/polyester ratio, PEE copolymers exhibit a wide range of mechanical behavior combined with solvent resistance, thermal stability, and ease of melt process ability. 

Horlbeck, G., Siesler, H. W., Tittle, B., and Trafara, G., Characterization of polyether and polyester homo- and copolymers prepared by ring-opening poljnnerization with a new catalytic system. Macromolecules. 10. 284-287, 1977. 

The nature of the hard domains differs for the various block copolymers. The amorphous polystyrene blocks in the ABA block copolymers are hard because the glass transition temperature (100°C) is considerably above ambient temperature, i.e., the polystyrene blocks are in the glassy state. However, there is some controversy about the nature of the hard domains in the various multiblock copolymers. The polyurethane blocks in the polyester-polyurethane and polyether-polyurethane copolymers have a glass transition temperature above ambient temperature but also derive their hard behavior from hydrogen-bonding and low levels of crystallinity. The aromatic polyester (usually terephthalate) blocks in the polyether-polyester multiblock copolymer appear to derive their hardness entirely from crystallinity. 

The solubility of a solute in scC02 is extremely dependent on its structure, with three features of paramount importance. As expected, compounds of low polarity are more soluble than very polar compounds or salts. However, solubility also increases greatly with increasing vapour pressure of a substrate. To account for the contribution of volatility and solvation to the solubility process, Kurt Zosel coined the term Destraktion (from Latin destillare and extrahere) in his pioneering work on natural product extraction with SCFs [5], Finally, some specific functional groups like perfluoroalkyl and polysiloxane substituents, or polyether/polycarbonate copolymers... 

It is evident that reactions of unsaturated polymers with bisnitrile oxides lead to cross-linking. Such a procedure has been patented for curing poly(butadiene), butadiene-styrene copolymer, as well as some unsaturated polyethers and polyesters (512-514). Bisnitrile oxides are usually generated in the presence of unsaturated polymers by dehydrochlorination of hydroximoyl chlorides. Cross-linking of ethylene-propylene-diene co-polymers with stable bisnitrile oxides has been studied (515, 516). The rate of the process has been shown to reduce in record with the sequence 2-chloroterephthalonitrile oxide > terephthalonitrile oxide > 2,5-dimethylterephthalonitrile oxide > 2,3,5,6-tetramethylterephthalo-nitrile oxide > anthracene-9,10-dicarbonitrile oxide (515). 

Block copolymers such as styrene-butadiene-styrene (SBS) and its hydrogenated versions (SEBS), along with polyester-polyether block copolymers, can also be used to improve PBT impact. The SEBS and SBS copolymers [47], and especially their functionalized, grafted derivatives [48], show surprisingly good affinity for the polyester. 

The cationic ring-opening polymerization of epichlorohydrin in conjunction with a glycol or water as a modifier produced hydroxyl-terminated epichlorohydrin (HTE) liquid polymers (1-2). Hydroxyl-terminated polyethers of other alkylene oxides (3 4), oxetane and its derivatives (5 6), and copolymers of tetrahydrofuran (7-15) have also been reported. These hydroxyl-terminated polyethers are theoretically difunctional and used as reactive prepolymers. 

Recent work on the synthesis, structure and some properties of macromolecules bearing furan rings is discussed. Two basic sources of monomers are considered, viz. furfural for monomers apt to undergo chain polymerization and hydroxymethylfurfural for monomers suitable for step polymerization.Within the first context, free radical, catiomc and anionic systems are reviewed and the peculiarities arising from the presence of furan moieties in the monomer and/or the polymer examined in detail. As for the second context, the polymers considered are polyesters, polyethers, polyamides and polyurethanes. Finally, the chemical modification of aU these oligomers, polymers and copolymers is envisaged on the basis of the unique reactivity of the furan heterocycle. 

A manifold of dendrimers have been presented in the literature ranging from polyamidoamine, polyfpropylene imine), aromatic polyether and polyester, aliphatic polyether and polyester, polyalkane,polyphenylene, polysilane, and phosphorus dendrimers. Combinations of different backbones as well as architectural modifications have also been presented. For example, the incorporation of chirality in dendrimers, copolymers of linear blocks with dendrimer segments (dendrons), and block copolymers of different dendrons has been described. Numerous applications have been proposed for dendrimers such as biotemplates, liquid membranes, catalysts, or in medical applications. ... 

Polyolefin poiysulfides (Thiokol) are thio analogues of polyethers which are flexible, amorphous, oil-resistant elastomers. The number of sulfur links in the repeating unit, which is called rank, is always greater than two. The solvent resistance, resistance to gaseous permeation, and flexibility of these polyolefin sulfides increase with rank. The actual strength of polymers and copolymers of olefin sulfides varies. Many of these products contain the following repeating unit ... 

It has been noted that C02 behaves very much like a hydrocarbon solvent with regards to its ability to dissolve small molecules consequently, many monomers exhibit a high solubility within C02. On the other hand, most high-molar-mass polymers are scarcely soluble in C02, and the only polymers that show good solubility under relatively mild conditions (T < 373 K, P < 35 MPa) are amorphous fluoropolymers, silicones, and polyether polycarbonate copolymers. 

Polyamide-polyether block copolymers (Pebax , Elf Atochem, Inc., Philadelphia, PA) have been used successfully with polar organics such as phenol and aniline [32-34], The separation factors obtained with these organics are greater than 100, far higher than the separation factors obtained with silicone rubber. The improved selectivity reflects the greater sorption selectivity obtained with the polar organic in the relatively polar polyamide-polyether membrane. On the other hand, toluene separation factors obtained with polyamide-polyether membranes are below those measured with silicone rubber. 

Newer developments are polyester/polyether block copolymers (Hytrel, Amitel), etc. By choosing various levels of block length ratio, a broad spectrum of stiffnesses (or hardnesses) can be obtained, which practically fills the gap between rubbers and thermoplasts. TPE s form a rapidly growing class of materials, which find an increasing number of applications. 

Dielectric relaxation study of two-phase microstructures in segmented copolymers was first attempted by North and his co-workers (55, 57,58,59). Dielectric measurements down to 10 5Hz were made on MDI-based segmented polyether- and polyester-urethanes using a dc transient technique. These materials displayed large, low-frequency... 

Part I of this series explored the structure-property relationships of tetramethylene terephthalate/polyether terephthalate copolymers as a function of variations in the chemical structure, molecular weight, and concentration of the polyether units (10). Of the polyether monomers tested, poly (tetramethylene ether) glycol of molecular weight approximately 1000 was found to provide copolymers having the best overall combination of physical properties and ease of synthesis. 

Polymer Preparation. The polyether-ester copolymers were prepared by titanate-ester-catalyzed, melt transesterification of a mixture of PTME glycol, the dimethyl ester of an aromatic diacid, and a diol present in 50-100% molar excess above the stoichiometric amount required (Figure 1). The reactions were carried out in the presence of no more than 1 wt %, based on final polymer, of an aromatic-amine or hindered-... 

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