попробуйте
NSString *str2=@"Hi,How r u";
NSMutableArray *arary = [[NSMutableArray alloc] initWithArray:[str2 componentsSeparatedByCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@", "]]];
NSLog(@"%@",arary);
если хотите, как объект
NSString *str2=@"Hi,How r u";
str2 = [str2 stringByReplacingOccurrencesOfString:@"," withString:@" , "];
NSMutableArray *arary = [[NSMutableArray alloc] initWithArray:[str2 componentsSeparatedByString:@" "]];
NSLog(@"%@",arary);