SCSS/SASS: How to dynamically generate a list of classes with commas separating them

I'm working with the SCSS syntax of SASS to create a dynamic grid system but I've hit a snag.

I'm trying to make the grid system completely dynamic like this:

$columns: 12;

then I create the columns like this:

@mixin col-x {
  @for $i from 1 through $columns {
  .col-#{$i} { width: $column-size * $i; }
  }
}

Which outputs:

.col-1 {
    width: 4.16667%;
  }

.col-2 {
    width: 8.33333%;
}
etc...

This works well but what I want to do next is dynamically generate a long list of column classes separated by commas based on the number of $columns chosen - e.g I want it to look like this:

.col-1,
.col-2,
.col-3,
.col-4,
 etc... {
float: left;
}

I've tired this:

@mixin col-x-list {
  @for $i from 1 through $columns - 1 {
  .col-#{$i}-m { float: left; }
  }
}

but the output is this:

.col-1 {
  float: left;
}
.col-2 {
  float: left;
}
etc...

I'm a little stuck on the logic here as well as the SCSS syntax required to create something like this.

Does anyone have any ideas?

Thanks