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Peroxide curable fluoroelastomers having copolymerized units of vinylidene fluoride or tetrafluoroethylene major monomer, at least one other fluorinated major monomer, a cure site monomer having the general formula CH2=CH--(CF2)nI, where n is an integer between 2 and 8, and iodine bound at the termi

Peroxide curable fluoroelastomers having copolymerized units of vinylidene fluoride or tetrafluoroethylene major monomer, at least one other fluorinated major monomer, a cure site monomer having the general formula CH2=CH--(CF2)nI, where n is an integer between 2 and 8, and iodine bound at the terminal positions of polymer chains, process well and have excellent tensile properties.

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1. A semibatch polymerization process for the manufacture of a fluoroelastomer comprising: (A) charging a reactor with a quantity of an aqueous solution comprising a surfactant; (B) feeding to said reactor a quantity of an initial major monomer mixture to form a reaction medium, said initial maj

1. A semibatch polymerization process for the manufacture of a fluoroelastomer comprising: (A) charging a reactor with a quantity of an aqueous solution comprising a surfactant; (B) feeding to said reactor a quantity of an initial major monomer mixture to form a reaction medium, said initial major monomer mixture comprising i) from 10 to 70 weight percent, based on total weight of said monomer mixture, of a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) between 75 and 30 weight percent, based on total weight of said monomer mixture, of one or more additional copolymerizable monomers, different from said first monomer, wherein said additional monomer is selected from the group consisting of fluorine-containing olefins, fluorine-containing ethers, and mixtures thereof; (C) polymerizing said monomers in the presence of a free radical initiator to form a fluoroelastomer dispersion while maintaining said reaction medium at a pH between 1 and 7, at a pressure between 0.5 and 10 MPa, and at a temperature between 25° C. and 130° C.; and (D) feeding to said reactor a quantity of an incremental major monomer mixture to maintain constant pressure in said reactor, said incremental major monomer mixture comprising i) from 20 to 70 weight percent, based on total weight of said monomer mixture, of a first monomer, said first monomer selected from the group consisting of vinylidene fluoride and tetrafluoroethylene, and ii) between 80 and 30 weight percent, based on total weight of said monomer mixture, of one or more additional copolymerizable monomers, different from said first monomer, wherein said additional monomer is selected from the group consisting of fluorine-containing olefins, fluorine-containing ethers, and mixtures thereof; wherein, at a time between when 0% and 50% of said quantity of incremental major monomer mixture has been fed to said reactor, addition of a quantity of a chain transfer agent to said reactor is begun, said chain transfer agent having a formula RIx,where R is a perfluoroalkyl group containing 3 to 10 carbon atoms and x is 1 or 2, and wherein at a time between when 10% and 90% of said quantity of incremental major monomer mixture has been fed to said reactor, addition of a quantity of a cure site monomer to said reactor is begun, said cure site monomer having the general formula CH2=CH--(CF2)nI, where n is an integer between 2 and 8. 2. A semibatch polymerization process of claim 1 wherein addition of a quantity of said chain transfer agent to said reactor is begun prior to when 20% of said quantity of incremental major monomer mixture has been fed to said reactor. 3. A semibatch polymerization process of claim 2 wherein addition of a quantity of said chain transfer agent to said reactor is begun prior to when 5% of said quantity of incremental major monomer mixture has been fed to said reactor. 4. A semibatch polymerization process of claim 1 wherein addition of a quantity of said cure site monomer to said reactor is begun after 25% of said quantity of incremental major monomer mixture has been fed to said reactor. 5. A semibatch polymerization process of claim 4 wherein addition of a quantity of said cure site monomer to said reactor is begun after 33% of said quantity of incremental major monomer mixture has been fed to said reactor. 6. A semibatch polymerization process of claim 1 wherein said cure site monomer is 4-iodo-3,3,4,4-tetrafluorobutene-1. 7. A semibatch polymerization process of claim 1 wherein said chain transfer agent is of formula RI2,where R is a perfluoroalkyl group containing 3 to 10 carbon atoms.