The two samples could have had different average molecular weights, different molecularweight distributions, or different spatial structures (linear, branched, or cross-linked). Two samples of poly(methy1 methacrylate) were listed by the manufacturer to be 100% pure, which was true, yet one had a significantly lower softening temperature than the other. No polymerization occurs in the dental processing of the mouth protector they are processed as thermoplastics. The manufacturer uses a free-radical polymerization method to produce the polymer, which is then molded into sheets. The higher the percentage of vinyl acetate, the softer the copolymer. For this application it is called gutta-percha.Р”Р„ copolymer of ethylene and vinyl acetate has the following structural formula:Р”В¬opolymers containing 18%to 33% vinyl acetate are sold in fully polymerized sheets, which are heated to about 90' C and vacuumor handformed over gypsum dental models to produce athletic mouth protectors. trans-Polyisoprene is rigid it is compounded mainly with zinc oxide (but also with some waxes and zinc silicate) and is used for endodontic points as part of the filling material for root canals. In the vulcanized form it is very flexible and is used in surgical gloves, rubber dams for restorative procedures, and elastics for orthodontic applications. cisPolyisoprene is cross-linked by the process of vulcanization with sulfur or other chemicals such as peroxides.

Notice that for the cis type the CH, and H are on the same (cis) side, whereas for the trans type they are on opposite (trans) sides. Compounds used in the vulcanization of latex surgical gloves interfere in the polymerization of addition silicones, and thus contact should be avoided. In this instance the platinum catalyst attacks the hydrogen in the silanecontaining dimethyl siloxane, and this complex reacts with the vinyl-terminated dimethyl siloxane to form a cross-linked silicone rubber. Hydrosilylation One final example of an addition reaction is with a vinyl-terminated silicone and a silane (-H)-containing siloxane, as shown above.

The oligomer and setting reaction are further discussed in Chapter 12, Impression Materials. The ring is opened by the sulfonium fluoborate catalyst, and polymerization results in a cross-linked rubber. The polyether oligomer has a threemembered ring containing nitrogen shown as R in formula VI above. Therefore agar and alginate impressions are incompatible with this die material. Water interferes with the setting reaction because it reacts with the epoxide. The amine opens the ring and crosslinking results in a rigid polymer.

Portal knights common resin