Complex sentence: Serge was able to address manuscripts to the French writer Romain Rolland, who was sympathetic to Stalin but was against Serge's repression, but many manuscripts that Serge tried to send were lost.
Split into simpler sentences: Serge was able to address manuscripts to the French writer Romain Rolland, who was sympathetic to Stalin but was against Serge's repression. However, many manuscripts that Serge tried to send were lost.