shanghai to hainan

Unfortunately, your itinerary is not eligible for 144 hour visa free transit because if you land in Shanghai you can travel just within Shanghai transit area that consist of Shanghai city and Jiangsu and Zhejiang provinces. Hainan is part of mainland China, so a travel there from Shanghai makes you immediately in breach of 144 hour visa free transit rules.

Also, you should be aware that you will not be able to stay in Shanghai area from August 31 until September 8 without a visa because your 144 hour visa free transit expires on September 6. The only way how you could cover yourself with visa free transit is the following:

Country X - Shanghai - Macau or Hong Kong or Taipei
Macau or Hong Kong or Taipei - Shanghai - Country Y

In this way, you can use 144 hour visa free transit in Shanghai twice without any problem because Hong Kong, Taiwan and Macau are eligible territories for 144 hour visa free transit, but Hainan, being a part of mainland China is not.