Chemical synthesis gives at least a 99% base coupling efficiency. So, for example, if you ordered a 40mer without purification, you will have about 70% full-length product and 30% truncated product/deletion mutants. This is why we recommend purification for longer oligos.
Our new Ultramer® oligo synthesis involves a higher fidelity synthesis with a higher coupling efficiency. Please see our Ultramer Oligos Catalog Page for more information.