<div class="button-container">
  <a href="#" class="btn"><span>Click Me</span></a>
</div>

@import "bourbon";

body, html {
  margin: 0;
  padding: 0;
  height: 100%;
  width: 100%;
  font-family: 'Open Sans', sans-serif;
  background-color: #17161e;
}

.button-container {
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

.btn {
  display: block;
  height: 50px;
  width: 186px;
  position: relative;
  overflow: hidden;
  text-decoration: none;
  text-transform: uppercase;
  letter-spacing: 2px;
  color: white;
  font-size: 14px;
  text-align: center;
  
  &:after {
    position: absolute;
    content:'';
    display: inline-block;
    background: rgba(59,173,227,1);
    background: linear-gradient(45deg, rgba(59,173,227,1) 0%, rgba(87,111,230,1) 25%, rgba(152,68,183,1) 51%, rgba(255,53,127,1) 100%);
    height: 50px;
    width: 372px;
    z-index: -1;
    transform: translateX(-280px);
    transition: transform 400ms ease-in;
  }
  
  &:hover {
    &:after {
      transform: translateX(-200px);
    }
  }
  span {
    position: relative;
    top: 16px;
  }
}