Hmm, I think I can. Would have to scratch my brain a little.
Let me try, short on time. So one word reasons will be present.
Rouboute Guillimann(Ultramarines) - In stasis on Macragge after being poisoned by Fulgrim
Fulgrim(Emperors Children) - Demon Prince within the Eye of Terror
Alpharius(Alpha Legion) - Killed by Rouboute Guillimann
Ferrus Mannus(Iron Hands) - Killed by Fulgrim
Logar(Word Bearers) - Demon Prince within the Eye of Terror
Vulkan(Salamanders)- Disappeared after Istvaan 5
Corax(Raven Guard)- Disappeared after Istvaan 5
Rogal Dorn(Imperial Fists)- Disappeared during a boarding action, his hand was later found and he is presumed dead.
Leman Russ(Space Wolves)- Led a company into the warp never to be heard from again. Maybe with the 13th?
Jagathai Khan(White Scars)- Led a company into the warp after Dark Eldar never to be seen again.
Saguinius(Blood Angels)- KillPued by Horus on his Battle Barge
Lion'el Johnson(Dark Angels
)- Disappeared during the Horus Heresy, supposedly in stasis aboard the rock.
Mortarian(Death Guard)- Demon Prince within the Eye of Terror
Purturabo(ex cheese heads... aka Iron Hands)- Demon Prince in the Eye of Terror.
Angron(World Eaters)- Demon Prince
Konrad Kurze aka Night Haunter(Night Lords)- Allowed himself to be assassinated by the assassin M'Shen (got a whole funny story about this name and short story, but that is for another thread).
Magnus the Red(1k sons)- Force to flee to the Eye of Terror by the space wolves. Now a demon prince. This guy was really a good guy at first, read the second horus heresy book, shame... he could have tipped the balance.
Horus(Luna Wolves/Sons of Horus)- Had to scratch my head at first, how could I forget this guy! Died at the hands of the Emperor... damned arch heretic.
The other two Primarchs died or were never found during the Great Crusade (Sigmar anyone?)
Ok, so I ended up typing way more than I had intedned
You laid down the Gauntlet
I think I spelled a few names wrong, but that's it.
EDIT: Yup, made quite a few spelling mistakes. See if you can notice them. Also, it seems a few of the stories surrounding what happened to the Primarchs change quite a bit depending on the source.