Spiked chain is already a weapon, just an exotic one.
If it is a homegame, you may be able to convince the gm to use it as a base and create a weapon: chain and shackles, and allow for a feat Exotic Weapon Proficiency (chain and shackles).
Otherwise, normal chain or shackles, that weresn't created to be weapon, can always be used as improvised weapon. So every feat or trait that affects improvised weapon fighting is applicable.
More generally, Catch Off Guard lets you improvise weapons from all sorts of things. Improvised Weapon Mastery makes it better, but isn't strictly necessary.
More generally, Catch Off Guard lets you improvise weapons from all sorts of things. Improvised Weapon Mastery makes it better, but isn't strictly necessary.
There's also lots of good stuff for Improvised Weapon Builds in the new Adventurer's Armory 2.
With the Weapon Specialist advanced weapon training, you can have them all work for a number of feats equal to your weapon training bonus. Difficulty: You do have to have weapon training, and it doesn't kick in until 9th level.
Before then, at 4th level a human can use the Martial Versatility feat to make one "choose a weapon" feat work with all improvised weapons. You could take it multiple times, then retrain them away (either with fighter retraining or regular retraining) once you hit 9.
^Why do you have to wait until 9th level? You could take the feat Advanced Weapon Training (Weapon Specialist) at 5th level (assuming that you don't need some other Advanced Weapon Training first), and it would work for 1 feat for which you have to choose a weapon. The problem I see is that "Improvised Weapons" is not a valid weapon group, so you have to figure out which improvised weapons fit into the actual weapon group you are going to use the most.
Didn't realize there was also a feat. That's...kind of confusing.
I'll be damned, though, I thought there was an improvised weapons group. Well, so much for that advice. -_-
^To be fair, it took a LONG time for the Advanced Weapons Training feat to appear on www.d20pfsrd.com (when the Weapon Master's Handbook came out, they were already starting to have problems updating and even maintaining the site . . . and the current rash of broken or otherwise mangled links has been going on for weeks) . . . and around this time, Archives of Nethys started having lnog stretches of not being updated . . . and as far as I can tell, this stuff STILL isn't on the Paizo PRD (and probably won't be, since the Weapon Master's Handbook is not one of the types of book they normally put on there).
An Improvised Weapons Group would be a really good idea for a remake of the Cad Fighter archetype.