Constant rate copolymers

Several parenteral microencapsulated products have been commercialized the cote materials ate polypeptides with hormonal activity. Poly(lactide— glycohde) copolymers ate the sheU materials used. The capsules ate produced by solvent evaporation, polymer-polymer phase separation, or spray-dry encapsulation processes. They release their cote material over a 30 day period in vivo, although not at a constant rate. 

Polydiorganosilane copolymers were synthesized by adding 2.2 moles of a sodium dispersion in a light oil (Aldrich) at a constant rate (320 meq/min) into a toluene solution which contained a total of 1 mole of diorganodichlorsilane monomers. 

Nondegradable polymers are also useful as matrices for ocular implants. This application requires the polymer to be hydrophilic, to minimize local tissue irritation. Need for ocular implants stems from the challenges posed to conventional ocular medicines (i.e., eye drops) such as rapid dilution, tear washout, poor patient compliance, and limited bioavailability. Ocular implants from hydrophilic polymer matrices that provide localized sustained release may overcome the above limitations. The first polymeric sustained release product to reach the market was Ocusert , a pilocarpin sustained release ocular implant developed by Alza. Ocusert has the drug reservoir as a thin disc of pilocarpine-alginate complex sandwiched between two transparent discs of microporous membrane fabricated from ethylene-vinyl acetate copolymer. The microporous membranes permit the tear fluid to penetrate into the drug reservoir compartment to dissolve pilocarpine from the complex. Pilocarpine molecules are then released at a constant rate of 20 or 40 pg/hr for a four- to seven-day management of glaucoma. 

Tests in vivo give best results for copolymers with 30% water content. The specific feature of their performance, as compared to the copolymers of other compositions, is a relatively high and constant rate of heparin elution (Fig. 2). 

The authors of the above-discussed results consider the constant rate of heparin elution to be absolutely necessary for a successful functioning of the implant. Catheters made of these copolymers are much better, in respect of their thromboresistant properties, than the commercially available poly(tetrafluoroethylene) polyethylene, and plasticized polyvinyl chloride catheters (Table 8). The tested catheters were clotted in 9 cases of 81 (11 %), whereas usual silicone rubber catheters were clotted in 5 cases of 8 (63 %)70). 

The rate of dispersion (co)polymerization of PEO macromonomers passes through a maximum at a certain conversion. No constant rate interval was observed and it was attributed to the decreasing monomer concentration. At the beginning of polymerization, the abrupt increase in the rate was attributed to a certain compartmentalization of reaction loci, the diffusion controlled termination, gel effect, and pseudo-bulk kinetics. A dispersion copolymerization of PEO macromonomers in polar media is used to prepare monodisperse polymer particles in micron and submicron range as a result of the very short nucleation period, the high nucleation activity of macromonomer or its graft copolymer formed, and the location of surface active group of stabilizer at the particle surface (chemically bound at the particle surface). Under such conditions a small amount of stabilizer promotes the formation of stable and monodisperse polymer particles. 

After an induction period, the polymerization rate reaches a maximum and then becomes almost constant for over 20 hours. The constant rate of polymerization of the homogeneous system indicates that living polymers are present in this case. Indeed, block copolymers of propylene and ethylene could be obtained with this homogeneous system when ethylene was dissolved in liquid propylene [see also, related experiments with the heterogeneous system (3/)]. 

Samer [137] studied miniemulsion copolymerization in a single CSTR. Two separate feed streams, miniemulsion (or macroemulsion for comparative studies) and initiator were fed at constant rates into the reactor. SLS was used as the surfactant, HD as the costabilizer, and KPS was the initiator. In the miniemulsion configuration (costabilizer included in recipe), the emulsion stream was continuous. Constant volume was provided by an overflow outlet. Salt tracer experiments were used to validate the ideal mixing model assumed for a CSTR. Total monomer conversion was measured via in-hne densitometry, and copolymer composition via offline NMR. 

Fig. 19. The stress-strain curves recorded for the two triblock copolymer samples during cold drawing of films with a constant rate of 1 mm/min. ABA-isolated clean pnBA central block Mn=65,200 pMMA outer blocks, Mn=13,150, overall Mw/Mn=1.34. ABA-sequential clean pnBA central block, Mn=67,500 pMMA-grad-pnBA outer blocks 13mol% nBA and 87 mol% MMA, Mn=10,600, overall Mw/Mn=1.24. Inset small angle X-ray scattering intensities for these samples. Reprinted with permission from [94]. Copyright (2000) John Wiley Sons, Inc.

Fig. 7. Plots of oxygen uptake against time [333] (a) linear, polymers that show no induction period but absorb oxygen at a relatively constant rate (polymethylmethacrylate, polystyrene, polycarbonate) (b) autoretardant, polymers that exhibit no induction period but initially absorb oxygen at a relatively rapid rate, followed by a slower steady rate (polyethylene, polypropylene, nylons) (c) polymers that display autocatalytic behaviour (the modified acrylics, acrylonitrile—butadiene—styrene copolymer) (d) polymers that can be considered a combination of autocatalytic and autoretardant, (c) and (d) can be considered as autocatalytic, since the processes usually become autoretardant in the later stages of oxidation.

In other words, when a homogeneous polyethylene solution was cooled at a constant rate, the point of beginning crystallization was reached. This point was indicated by the first visual turbidity. At all concentrations, homopolymer A had a point of beginning crystallization about 10°C higher than that of copolymer B. This finding seemed reasonable since the crystallization temperature depends on the comonomer content and on the degree of short-chain branching of the polyethylene. From the curve we concluded that the temperature at which styrene diffused into LDPE A was about 10°C above that for diffusion into LDPE B at a comparable rate of diffusion. 

Indirect fluid beds have already proved efficient in drying very heat-sensitive polymers with large constant-rate drying periods, as in drying PVC, polyethylene, acrylonitrile-butadiene-styrene (ABS) copolymers, and polycarbonates (PC). 

Both polymer centrifuge cakes are discharged hot (50°C-60°C), with diluent contents as high as 35 (wb) or 53.2% (db). Between 35 (wb) and 5% (wb), most homopolymers and copolymers exhibit constant-rate drying characteristics i.e., all moisture evaporation is from the particle surface. Drying is rapid, and residence time is heat transfer-dependent. Since the product temperature limit for these... 

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