I think we have to analyse Spock's Physiology in order to make more sense of it....
Vulcan's are physically superior in almost every way to human's, they are stronger, they live longer, they age slower and as McCoy has mentioned on several occasions, Vulcan's have a particulary unique "Metabolism"
We can gather that as soon as the Reactor Room started leaking Radiation Scotty went inside in order to "Take The Mains Offline", in this short space of time he succumbed to Radiation Poisoning and passed out (even while wearing his Engineers Isolation Suit), Also from what we can gather, Spock was one of the only (if not only) people onboard who could withstand dangerous levels of Radiation for prolonged periods of time. We've seen Spock in several situations where he has recovered from injuries and conditions that would be fatal to most Humans, etc
When Spock enters the Reactor Room he is initially unfazed by the leaking Radiation, its not until he opens the Cylinder Appliance and releases the build up that it starts to affect him, we can see in camera cuts, as he works on the systems that he is straining his eyes in a way that looks as if he is fighting to resist the effects of the Radiation
When Kirk comes down to Engineering we see the effects that the Radiation has had on Spock, he is covered in Radiation Burns (although his uniform is somehow in flawless condition :P ) he is extremely weak and it would appear that he is blind, now Spock repaired the Warp Drive in about 5 minutes, it was then, roughly another 5 minutes that it took for Kirk to get down to Engineering and for Spock to say his dying words, thus we can gather that Spock survived through exposure for 10 minutes
Now if we assume that Scotty wore a helmet when he entered the chamber to turn off the Warp Drive, then it would show that even wearing the full isolation suit would be useless against high levels of Radiation, therefore Spock reasoned that it would still be fatal even if he isolated himself (we can assume that he wore the gloves in order to give some small protection to his hands so his hands would continue to function for as long as possible, until he could finish fixing the Warp Drive) therefore Spock reasoned that he should just go in unprotected, since he knew how long he could survive and it would be enough time for him to complete the repairs, rather than spending a couple of minutes putting on an isolation suit (when it wouldn't protect him anyway)