How do I put a simple transition on this button?

I've made this simple button with gradient, but how do I make the transition work? I tried using transition: .2s linear, but it isn't working.

.contact_button {
    color: white;
    font-size: 20px;
    width: 130px;
    height: 50px;
    border: solid 4px transparent;
    border-radius: 80px;
    background-image: linear-gradient(black, black), linear-gradient(to right ,#00AEAD, #72CC50);
    background-origin: border-box;
    background-clip: content-box, border-box;
    cursor: pointer;
}

.contact_button:hover {
    background-image: linear-gradient(transparent, transparent), linear-gradient(to right ,#00AEAD, #72CC50);
}
<button class="contact_button">Contact</button>

Do it differently using mix-blend-mode and you can apply a transition to background-color

.contact_button {
    color: white;
    font-size: 20px;
    width: 130px;
    height: 50px;
    border: solid 4px transparent;
    border-radius: 80px;
    background: 
      linear-gradient(to right ,#00AEAD, #72CC50) border-box,
      #000 content-box;
    background-blend-mode:darken;
    cursor: pointer;
    transition:0.5s;
}

.contact_button:hover {
    background-color:#0000;
}
<button class="contact_button">Contact</button>