My PhD supervisor discourages me from initiating a collaboration with people outside of our group. He never explicitly told me that I could not do it, but I can realize it from his reactions (e.g., digressing the conversation, asking me to wait for now, or suggesting me to talk to someone in the group first) whenever I bring up a discussion about the possibility of external collaboration.
The fact is that my research topic is relatively new and is different from what other people are doing in our group, so none of the group members (including my supervisor) can actually help me, and I have to go all the way through my research on my own.
I was wondering how I can settle this? One time I also tried to discuss this issue via email. I sent him an email saying that I prefer to collaborate with researcher X from university Y for these reasons, but he never replied to me.
Any suggestions are very welcome.