Andrew "Hydra" Hall, of eHOBBY and Hydra's TF Pulp fame, stopped has posted details on the back story for TakaraTomy's releases of Power Core Combiners as United EX.  Here is the detail he provided via the information in the latest issue of Figure King as well as what will likely be printed on the package of each figure. Credit to The Allspark

"In the 21st century, the war of the Transformers has shifted to space, and seemed for a while that peace had returned to Earth. However, during that time the Autobots had been working together with Earth scientist Professor Chase to develop a new combination system. With it, Autobots would use Power Core Combination to combine themselves with 4 Drones. The Autobot warriors chosen for the process became part of the group known as the Master Class. At the same time, a covert counter-team of Decepticons with the same capability, "Master Chaos," was formed."

The story told on the package backs reflects the atmosphere of the United series, while including characters from earlier Transformers series such as wheelchair-bound genius Chip Chase.

Combatmaster Prime Mode
A Decepticon strategic commander who has been active since the early stages of the war, Combat Master combines with 4 military vehicles. The weapons that line his body provide devastating firepower. In the story, his drones were stolen from the Autobot Chopper Master, who they were originally developed for.

Drones: Morser Drone, Panzer Drone, Kanonier Drone, Spaher Drone

Jetmaster Prime Mode
The Autobot spy Jetmaster combines with 4 jet and helicopter Drones. Jetmaster was the first Autobot to successfully achieve Power Core Combination. The character features details referencing earlier Transformers series, such as that the data for his drones was gathered from the Aerialbots, and that he was born on Planet Z.

Drones: Zone Drone, Feminia Drone, Micro Drone, Tenth Drone

Aerial Warrior Choppermaster
Targetmaster: Firebug

Polar Commander Rollermaster
Targetmaster: Spanner