A halogenated polyisoolefin copolymer is composed of at least one isoolefin monomer, at least one multiolefin monomer, a halogen content of 0.05-2.5 mol%, and a halogenated oligomer content of less than 65% of total oligomer content in the halogenated polyisoolefin copolymer. The copolymer has low halogen content and significantly reduced levels of halogenated oligomers. The copolymer is especially useful in the pharmaceutical and food industries, for example as pharmaceutical and food product seals and closures.