*2.1. Materials*

p-Anisic acid sodium salt (sodium p-anisate, 4-methoxybenzoic acid sodium salt for synthesis, Merck KGaA, Darmstadt, Germany) was used without further treatment. LC-MS grade methanol, chloroform, acetonitrile and 0.1% formic acid in water were purchased from Merck (Merck KGaA, Darmstadt, Germany). (*R,S*)-β-Butyrolactone (β-BL, Sigma-Aldrich Chemie GmbH, Steinheim, Germany) was distilled over CaH2 and the fraction boiling at 56 ◦C (9 mmHg) was collected. The 4-Methoxybenzoyloxymethythylpropiolactone (p-AA-CH2-PL) was synthesized at the Institute of Organic Chemistry, Polish Academy of Sciences by carbonylation of the respective epoxide under ambient CO pressure [26].

The oligo(3-hydroxy-3-(4-methoxybenzoyloxymethyl)propionate) (p-AA-CH2-HP)n oligoesters (sample 1; Mn,GPC = 700, Mw/Mn = 1.15) and oligo[(3-hydroxy-3-(4-methoxybenzoyloxymethyl) propionate)-co-(3-hydroxybutyrate)], [(p-AA-CH2-HP)x-co-(HB)y] (co)oligoesters (sample 2; Mn,GPC = 900 Mw/Mn = 1.23) were obtained via anionic ring-opening oligomerization of (p-AA-CH2-PL) and via anionic ring-opening (co)oligomerization of β-BL with p-AA-CH2-PL, respectively. The synthesis was performed according the procedure described in [25]. The resulting (co)oligoesters were purified and characterized by 1H nuclear magnetic resonance (1H NMR), gel permeation chromatography (GPC), and multistage mass spectrometry (ESI-MSn).
