12-15 Synthesis of Polymers acn6e6S PROPAGATION STEPS RiHH+"CH—→51HcH rized as cation ae8ooy6o CH:=CHCI -(CH.CH) The average molecular weiaht of polyethene is almost 1 millio ic polymerizations require initiation by base keenentne,-,HG0aoknoamasonoa,suseato ame2reaec b23twneeoteThg32egsenronanC 1111 12-15 Synthesis of Polymers Polymerization reactions can be categorized as cationic, radical, anionic, and metal catalyzed. Acid-catalyzed cationic polymerizations have already been covered. Initiators include H2SO4, HF, and BF3. Radical polymerizations lead to commercially useful materials. The polymerization of ethene in the presence of an organic peroxide at high pressures and temperatures proceeds by a radical polymerization process. Polyethene (polyethylene) polymerized in this way is actually a branched polymer. Branching occurs as a result of hydrogen abstraction along the growing chain by another radical center. The average molecular weight of polyethene is almost 1 million. Polychloroethene (PVC or polyvinylchloride) is a polymer of chloroethene (vinyl chloride). The peroxide initiator and the intermediate radical chains add only to the unsubstituted end of the monomer (producing the most stable radical) which results in a very regular head-to-tail structure of molecular weight over 1.5 million. Pure PVC is fairly hard and brittle. It can be softened by the addition of carboxylic acid esters (plasticizers) for use in elastic materials such as vinyl leather, plastic covers, and garden hoses. Polypropenenitrile (polyacrylonitrile) can be prepared from propenenitrile (acrylonitrile) using hydrogen peroxide with FeSO4 as a catalyst. Polypropenenitrile, -(CH2CHCN)n-, also known as Orlon, is used to make fibers. Anionic polymerizations require initiation by bases. Anionic polymerizations are initiated by strong bases such as alkyllithiums, amides, alkoxides, and hydroxide. The adhesive properties of “Super Glue” result from the hydroxide initiated polymerization of 2-cyanopropenoate. The electron withdrawing natures of the carbonyl and nitrile groups create a partially positive carbon center at which the hydroxide can initially attack. The negative charge on the resulting anion is then resonance stabilized by both the carbonyl and nitrile groups