Adding multiple Products to cart via php

I want to add multiple product variations to my cart via ajax at once. I call this function via ajax for each product variation:

WC()->cart->add_to_cart( $product_id, $quantity, $variation_id, $variation, null )

After the loop has finished, only one products is added to the cart.
I think the problem is related to the last variable, I am passing.

$cart_item_data

What is wrong there, or what do I have to pass there, tha I cann ad multiple variations at one?

https://wordpress.org/plugins/woocommerce/