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Mechanism of Cyclocopolymerization
Published in George B. Butler, Cyclopolymerization and Cyclocopolymerization, 2020
Tetrahydronaphthoquinone and dimethyl tetrahydronaphthoquinone formed donor-acceptor complexes with divinyl ether.8-21 Since participation of such complexes has been considered in the cyclocopolymerization of 1,4-dienes with monoolefins such as divinyl ether-maleic anhydride and divinyl ether-fumaronitrile systems, radical copolymerization of tetrahydronaphthoquinone and dimethyl tetrahydronaphthoquinone with divinyl ether was studied. It was found that these copolymers have constant 1:1 composition regardless of the comonomer ratio. The terpolymerization of divinyl ethertetrahydronaphthoquinone-dimethyltetrahydronaphthoquinone confirmed the 1:1 donor-acceptor composition in the polymer. Spectroscopic data suggested a cyclized repeating unit in which the copolymer main chain consisted of only divinyl ether units. There was a marked difference between these copolymers and the typical cyclocopolymers, such as those of divinyl ether-maleic anhydride and divinyl ether-fumaronitrile, in which the copolymer main chains consisted of divinyl ether and the comonomer alternately. The overall composition was 1:2. These results were interpreted in terms of the steric effect of the bulky acceptor monomers and the electronic interactions between the comonomers. Competition between an acceptor monomer and the charge-transfer complex toward the cyclized divinyl ether radical in the propagation step appears to favor the charge-transfer complex.
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Published in Joseph C. Salamone, Polymeric Materials Encyclopedia, 2020
The morphology of the PTFE polymerization particle is tailored to the production of a wide variety of new properties or processing characteristics. Radial gradients of composition and/or morphology during the growth of spherical dispersion particles can be generated by changes that occur in the course of a polymerization. The incorporation of a comonomer at a specific stage of a recipe is an important means to reduce creep, increase clarity, modify particle hardness, etc. Changes in the polymerization temperature or pressure, initiator levels, surfactant levels, chain transfer agents, agitation rate, etc., are used to achieve a similar effect. In particular, the fine powder resins utilize tailored morphology to create specific processing and end-use properties. Manufacturers have developed a number of proprietary processes to create a wide range of fine powder grades.13,14
Biodegradable Polymers as Drug Carrier Systems
Published in Severian Dumitriu, Valentin Popa, Polymeric Biomaterials, 2020
The polymer characteristics are affected by the comonomer composition, the polymer architecture, and molecular weight. The crystallinity of the polymer, an important factor in polymer biodegradation, varies with the stereo regularity of the polymer. Polyglycolide is a highly crystalline polymer (45%–55% crystallinity) and therefore exhibits a high tensile modulus and very low solubility in organic solvents. The melting point of the polymer is greater than 200°C. Unlike glycolide, lactide is a chiral molecule and exists in two optically active forms: l-lactide and d-lactide. The polymerization of these monomers leads to the formation of semi-crystalline polymers. The racemic dl polymer is less crystalline than the d- or l-lactide homopolymers (Nair and Laurencin 2007).
Performance evaluation of microbial fuel cell using a radiation synthesized low density polyethylene-grafted-poly (glycidyl methacrylate-co-vinyl acetate) as a proton exchange membrane
Published in Environmental Technology, 2022
Hanan M. Abd-Elmabood, Amany I. Raafat, El-Sayed A. Soliman, Amr El-Hag Ali
Solvent plays an important role in determining the grafting efficiency of a grafted sheet. The careful selection of grafting solvent has to be first made before any further optimization of the radiation grafting conditions. The use of good solvent results in homogeneous grafting in the bulk of the polymer substrate as a result of increasing its swelling ability in such solvent which consequently enhances the diffusion of comonomer into the internal layers of the polymer substrate, and thus the interactions between the internal active sites and the comonomer molecules increase, leading to higher and homogeneous grafting yield. However, the use of a poor solvent leads to surface grafting as a result of low comonomer diffusibility, and consequently, the interactions of comonomer molecules are limited only to the active sites generated at the surface of the polymer substrate [26,27]. Among different solvents, the use of acetone as diluent results in the highest grafting yield with a small amount of homopolymer whereas dimethylformamide gives the lowest degree of grafting and a large amount of homopolymer [28,29]. It is well known that the formation of a homopolymer in the grafting medium result in increases it is viscosity which leading to a retardation of the monomer diffusion into the polymer matrix. Therefore, acetone was selected as the proper solvent and effective medium for grafting of GMA-co-VAc onto LDPE.