Set up an RSS campaign in MailChimp and use the rss feed from your wordpress site for the campaign content. If you don’t want to include every post that would make it into your site’s rss feed, you’ll need to create an rss feed that’s limited to just the posts you do want to go out. how to create a limited rss feed should be a separate question, if you needed help with that See this page for a more details.
