The tubular high-pressure LDPE plant will have a capacity of 350,000 tonnes/year, and it will be in Plaquemine, Louisiana.
The plant has the same design as the one at the Sadara complex in Saudi Arabia.
The other plant will produce the company's NORDEL metallocene EPDM. The plant will be in Plaquemine, Loluisiana, and it will produce 200,000 tonnes/year of material.
The remaining plants in DowDuPont's first wave of US investments should start up in 2018.
Those include a 320,000 tonnes/year plant that will produce high-melt specialty index elastomers.
DowDuPont has a second wave of new investments, which it said should come online in 2020.
Among these is a 500,000 tonne/year expansion of its new Light Hydrocarbon 9 (LHC-9) ethane cracker in Freeport, Texas. That cracker currently has a capacity of 1.5m tonne/year.