Recently me and an acquaintance have been talking about working together on a side project. I know he operates his own LLC and probably wants to release the side project under the LLC which is fine. What I am more concerned with is ownership of the code base. I intend to reuse whatever me and him come up with on a separate project of my own independently. If me and him work on our common project without signing any contract, is it legally possible for me to just grab the code and publish another app under my own name which uses that code? If not what kind of contract should I prepare that would give me that kind of ownership (I m thinking some kind of shared ownership of the codebase type of arrangement)?
he operates his own LLC and probably wants to release the side project under the LLC which is fine
If the LLC is listed as the only copyright holder then, absent any shares of that LLC in your portfolio, you are far from being "fine": proving your rights to the codebase would then be a challenge, especially if you did not object for the LLC to be the only copyright holder.
When the code is released, make sure your name is on the list of copyright holders, regardless of what license it is released under. Additionally, if the country you operate in supports copyright registration (like the US does), having your name on the register against the codebase will be a very good idea.
is it legally possible for me to just grab the code and publish another app under my own name which uses that code?
This depends on the jurisdiction. In the US you'll be able to do whatever you want with the code without asking the other copyright owners (however, if you make profits, they will be able to force you to allow them to wet their beaks). In the UK it is the opposite: one joint owner may not exploit the copyright without the explicit consent of the other coowners.
Therefore, in countries like the UK you would need an agreement between all the copyright holders to allow each other to do anything with the code; in the US no such agreement needed as the copyright law provides it by default.
It seems you will both be working on this project, and neither of you will be paid for the work, so you should end up as joint copyright holders.
In order to distribute a copyrighted work, or create a derivative work (which is exactly what you want to do), you need permission of all copyright holders, in this case both of you. Without any license, each of you can stop the other from doing anything useful with your work, not a good situation if you ever fall out.
So I would recommend that you create an agreement (you can call it a license) that allows each of you to continue using your codebase. What exactly should be in that agreement is tricky.