In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many forms of polymerization and different systems exist to categorize them. In chemical compounds, polymerization can occur via a variety of reaction mechanisms that vary in complexity due to the functional groups present in the reactants and their inherent steric effects. In more straightforward polymerizations, alkenes form polymers through simple radical reactions. Alkanes can be polymerized, but only with the help of strong acids; as alkenes can polymerize in somewhat straightforward radical reactions, they form useful compounds such as polyethylene and polyvinyl chloride, which are produced in high tonnages each year due to their usefulness in manufacturing processes of commercial products, such as piping and packaging. In general, polymers such as PVC are referred to as "homopolymers," as they consist of repeated long chains or structures of the same monomer unit, whereas polymers that consist of more than one monomer unit are referred to as copolymers.
Other monomer units, such as formaldehyde hydrates or simple aldehydes, are able to polymerize themselves at quite low temperatures to form trimers. Such small polymers are referred to as oligomers; because formaldehyde is an exceptionally reactive electrophile it allows nucleophilic addition of hemiacetal intermediates, which are in general short-lived and unstable "mid-stage" compounds that react with other molecules present to form more stable polymeric compounds. Polymerization, not sufficiently moderated and proceeds at a fast rate can be hazardous; this phenomenon can cause fires and explosions. There are two main classes of polymerization reaction mechanisms: chain-growth. In step-growth polymerization, each step may involve the combination of two polymer molecules of any lengths to form a longer polymer molecule; the average molar mass increases and long chains are formed only late in the reaction. Step-growth polymers are formed by independent reaction steps between functional groups of monomer units containing heteroatoms such as nitrogen or oxygen.
Most step-growth polymers are classified as condensation polymers, since a small molecule such as water is lost when the polymer chain is lengthened. For example, polyester chains grow by reaction of alcohol and carboxylic acid groups to form ester links with loss of water. However, there are exceptions. Step-growth polymers increase in molecular weight at a slow rate at lower conversions and reach moderately high molecular weights only at high conversion. In chain-growth polymerization, the only chain-extension reaction step is the addition of a monomer to a growing chain with an active center such as a free radical or ion. Once the growth of a chain is initiated by formation of an active center, chain propagation is rapid by addition of a sequence of monomers. Long chains are formed from the beginning of the reaction. Chain-growth polymerization involves the linking together of molecules incorporating double or triple carbon-carbon bonds; these unsaturated monomers have extra internal bonds that are able to break and link up with other monomers to form a repeating chain, whose backbone contains only carbon atoms.
Chain-growth polymerization is involved in the manufacture of polymers such as polyethylene and polyvinyl chloride. A special case of chain-growth polymerization leads to living polymerization. In the radical polymerization of ethylene, its π bond is broken, the two electrons rearrange to create a new propagating center like the one that attacked it; the form this propagating center takes depends on the specific type of addition mechanism. There are several mechanisms; the free radical mechanism is one of the first methods to be used. Free radicals are reactive atoms or molecules that have unpaired electrons. Taking the polymerization of ethylene as an example, the free radical mechanism can be divided into three stages: chain initiation, chain propagation, chain termination. Free radical addition polymerization of ethylene must take place at high temperatures and pressures 300 °C and 2000 atm. While most other free radical polymerizations do not require such extreme temperatures and pressures, they do tend to lack control.
One effect of this lack of control is a high degree of branching. As termination occurs randomly, when two chains collide, it is impossible to control the length of individual chains. A newer method of polymerization similar to free radical, but allowing more control involves the Ziegler–Natta catalyst with respect to polymer branching. Other forms of chain growth polymerization include cationic addition polymerization and anionic addition polymerization. While not used to a large extent in industry yet due to stringent reaction conditions such as lack of water and oxygen, these methods provide ways to polymerize some monomers th
The National Amalgamated Union of Enginemen, Mechanics and Electrical Workers was a trade union in the United Kingdom. It represented stationary engine drivers and cranemen in a wide variety of industries, as well as less skilled workers in the electrical industry and miscellaneous workers; the union was founded in 1895 as the National Amalgamated Union of Enginemen, Hammer Drivers and Boiler Firemen of Great Britain, before changing its name to the National Amalgamated Enginemen, Hammer and Electric Tram Drivers and Boiler Firemen in 1901. It was affiliated to the Federation of General Workers; the union was the largest of its day catering for stationary engine drivers and cranemen, but it long faced competition for members from two older unions: the Amalgamated Protective Union of Engine Drivers, Crane Drivers and Boiler Attendants, based in London, the Amalgamated Society of Enginemen, Cranemen and Firemen, based in North East England. Both of these had merged into larger general unions by 1920, leaving the National Amalgamated as the only union dedicated to workers in these trades.
Some small unions existed, such as the National Amalgamated Enginemen's and Firemen's Association, based in Manchester, which joined the National Amalgamated Union in 1908, the Northern Union Enginemen's Association, which joined in 1921. The union affiliated to the Transport and General Workers' Union in 1926, it remained a distinct section of the TGWU for many years, its general secretary serving as national secretary of the TGWU Power Group. In 1945, its members who worked in collieries transferred to the new Power Group of the National Union of Mineworkers. 1895: G. H. Copley 1926: George Parker 1928: Samuel Hall 1938: Isaac Hayward 1946: William Tudor TGWU amalgamations
Ghostly Encounters is a Canadian paranormal documentary television series that premiered on July 16, 2005 to January 19, 2011. On Viva/W Network; the program aired on A&E's The Biography Channel, airs on Discovery's Destination America, both in the United States. The show was created by executive producer Phyllis Platt, is produced by Brian Dennis, is hosted by Lawrence Chau; the program uses a combination of interviews and dramatic recreations, examining the events that led its subjects to accept or reject occurrences as supernatural, how the experience has helped or hindered them. Ghostly Encounters won a Gemini Award for best original score in 2007; each episode tells stories from two individuals who believe they have had paranormal experiences. When warranted, the show will break from this format and instead use the full episode for one story; each episode is created using a direct-to-camera interview with the storyteller, dramatic re-enactments, host segments and commentary. An abandoned ballroom on the top floor of the King Edward Hotel in Toronto provides the location for the portion of the program featuring host Lawrence Chau.
The episodes begin with a tease describing the two stories. The tease is followed by the opening credits. After the credits, the first story is introduced using a clip of the subject's interview; the first host segment follows and introduces the theme of the episode, as well as the first story, to the viewers. The first subject's story is told using a combination of his or her interview and dramatic re-enactment footage. B-roll and stock footage are used in telling the story. At the end of the first story, a brief host segment wraps up the first subject and introduces the second subject of the episode; the second story ends again with a wrap-up from the host. At the same time, the host wraps up the show and includes final comments on the stories and theme of the episode based on the commentary from the experts; the show ends with a final comment from each subject reflecting on the lesson their encounter has given them, followed by the closing credits. List of ghost films Official website Official website by Viva Ghostly Encounters on IMDb Ghostly Encounters at The Biography Channel Ghostly Encounters on Facebook