C#

private void TestChunking()
{
    string testString = "aaabbbcccdddeeefffggghhh"; 
    int[] intary = new int[36];
    List<int> intlist = new List<int>(36);
    Stack<int> stack = new Stack<int>();
    Queue<int> queue = new Queue<int>();
    
    for (int i = 0; i < 36; i++)
    {
        intary[i] = i;
        intlist.Add(i);
        queue.Enqueue(i);
        stack.Push(i);
    }
    
    var result1 = intary.ToChunkedKvPList(9);
    var result2 = intlist.ToChunkedKvPList(6);

    // I reverse the Stack here so results would be in expected order
    var result3 = stack.Reverse().ToChunkedKvPList(9);

    var result4 = queue.ToChunkedKvPList(4);
    var result5 = testString.ToChunkedKvPList(3);
}

public static class IEnumerableExtensions
{
    public static IEnumerable<KeyValuePair<T1, List<T1>>> ToChunkedKvPList<T1>(this IEnumerable<T1> source, int chunksz)
    {
        if(source.Count() % chunksz != 0) throw new ArgumentException("Source.Count must equal ChunkSize modulo 0");
    
        int ndx = 0;
        int listsz = chunksz - 1;
    
        return source
            .GroupBy(x => (ndx++/chunksz))
            .Select(grp => grp.ToList())
            .Select(lst => new KeyValuePair<T1, List<T1>>(lst[0], lst.GetRange(1, listsz)));
    }
}