Is it impossible to start a startup as a side project of a full-time job?

I think you can start a company as a side project while working full-time if you want to prove out the concept, prototype the idea, test out the market before you fully dive into the new business. But at a certain point, you will have to dedicate your full attention to the startup in order to give it the best chance to succeed. Also, it will be difficult to secure funding if you are not working on the startup full-time.