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THE BHP WIRE —
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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
Katherine Johnson’s mathematical calculations helped guide some of America’s most important early space missions while she confronted the racial and gender barriers faced by Black women in twentieth-century America.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
In polymer chemistry, vinyl polymers are a group of polymers derived from substituted vinyl (H2C=CHR) monomers. Their backbone is an extended alkane chain [−CH2−CHR−].[1] In popular usage, "vinyl" refers only to polyvinyl chloride (PVC).
Vinyl polymers are the most common type of plastic. Important examples can be distinguished by the R group in the monomer H2C=CHR:
Vinyl polymers are produced using catalysts. Ziegler–Natta catalysts are used commercially for production of polyethylene and polypropylene. Many are produced using radical initiators which are produced from organic peroxides. Still others (polystyrene) are produced using anionic initiators such as butyl lithium.
An exception from the usual rules, polyvinyl alcohol, (CH2CHOH)n, is produced by hydrolysis of polyvinyl acetate. Vinyl alcohol is not sufficiently stable to undergo polymerization.
Vinyl polymers are subject of several structural variations, which greatly expands the range of polymers and their applications.
With the exception of polyethylene, vinyl polymers can arise from head-to-tail linking of monomers, head-to-head combined with tail-to-tail, or a mixture of those two patterns. Additionally the substituted carbon center in such polymers is stereogenic (a "chiral center"), with the result that the relative absolute configurations of these centers within a polymer can influence the properties of the polymer. This feature is called tacticity. The polymerization conditions and the catalysts affect tacticity.
Another major variation for vinyl polymers arises from the copolymerization of differing vinyl monomers. The simplest example is ethylene-propylene copolymer. The % comonomer is yet another variation.
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In polymer chemistry, vinyl polymers are a group of polymers derived from substituted vinyl (H2C=CHR) monomers. Their backbone is an extended alkane chain [−CH2−CHR−]. In popular usage, "vinyl" refers only to polyvinyl chloride (PVC).
Ethylene-vinyl acetate (EVA), also known as poly(ethylene-vinyl acetate) (PEVA), is an elastomeric copolymer of ethylene and vinyl acetate that produces materials which are "rubber-like" in softness and flexibility. The material has good clarity and gloss, low-temperature toughness, stress-crack resistance, hot-melt adhesive waterproof properties, and resistance to UV radiation. EVA has a distinctive vinegar-like odor and is competitive with rubber and vinyl polymer products in many electrical applications. The types of EVA copolymers differ in the vinyl acetate (VA) content and the way the materials are used. The weight percent of vinyl acetate usually varies from 1 to 40%, with the remainder being ethylene. Many properties of the copolymer are determined by the VA content, although the glass transition temperature is relatively invariant at -25 to -30 °C. EVA copolymer which is based on a low proportion of VA (approximately up to 4%) may be referred to as vinyl acetate modified polyethylene. It is a copolymer and is processed as a thermoplastic material – just like low-density polyethylene. It has some of the properties of a low-density polyethylene but increased gloss (useful for film), softness and flexibility. The material is generally considered non-toxic. EVA copolymer which is based on a medium proportion of VA (approximately 4 to 30%) is referred to as thermoplastic ethylene-vinyl acetate copolymer and is a thermoplastic elastomer material. It is not vulcanized but has some of the properties of a rubber or of plasticized polyvinyl chloride particularly at the higher end of the range. Both filled and unfilled EVA materials have good low temperature properties and are tough. The materials with approximately 11% VA are used as hot-melt adhesives. EVA copolymer which is based on a high proportion of VA (greater than 60%) is referred to as ethylene-vinyl acetate rubber.
This list of polymer classes includes categories of polymers distinguished by characteristics such as their origin, chemical composition, molecular structure, or material properties: Biopolymer Inorganic polymer Organic polymer Conductive polymer Copolymer Fluoropolymer Gutta-percha (Polyterpene) Phenolic resin Polyanhydrides Polyketone Polyester Polyolefin (Polyalkene) Rubber Silicone Silicone rubber Superabsorbent polymer Synthetic rubber Vinyl polymer
Tacticity is the orderliness of the succession of configurational repeating units along the main chain of a macromolecule. Tacticity can be described using pairs of adjacent stereogenic units, known as diads, and longer sequences such as triads, tetrads, and pentads. In vinyl polymers, an isotactic macromolecule predominantly has the same stereochemical relationship between successive stereogenic units, a syndiotactic macromolecule predominantly has the alternative relationship, and an atactic macromolecule has no regular stereochemical sequence. Tacticity can strongly influence the physical properties of a polymer because it affects how regularly its chains can pack and therefore its crystallinity. Isotactic and syndiotactic polymers can form semicrystalline materials, whereas atactic polymers are generally amorphous. Consequently, different tactic forms of the same polymer can differ in properties including mechanical strength, melting point, solubility, solvent resistance, barrier properties, and optical clarity. Polymers with controlled tacticity are produced by stereocontrolled polymerization, in which stereoselectivity can arise from control by the growing polymer chain end or by the stereochemical environment of the catalyst. Tacticity is most commonly quantified using NMR spectroscopy, particularly proton or carbon-13 NMR, by measuring the relative abundances of diads, triads, or longer stereochemical sequences. Other methods sensitive to tacticity are X-ray powder diffraction, vibrational spectroscopy, and secondary ion mass spectrometry.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Brown v. Board of Education in 1954.
