var $el = $('div');
(function loop() {
  $el.animate({
    'border-top-left-radius': 100,
    'border-top-right-radius': 100,
    'border-bottom-left-radius': 100,
    'border-bottom-right-radius': 100
  }, 500, function() {
    $el.animate({
      borderTopLeftRadius: 0,
      borderTopRightRadius: 0,
      borderBottomLeftRadius: 0,
      borderBottomRightRadius: 0
    }, 500, loop);
  });
}());