Theoretical and Experimental Approach to Accurately Predict the Complex Molecular Weight Distribution in the Polymerization of Strainless Cyclic Esters

Abstract

Polyesters obtained by the catalytic ringopening polymerization of macrolactones in many aspects resemble the properties of polyethylene. However, the molecular weight distribution is intrinsically different and equals the molecular weight distribution observed for a stepgrowth process even though macrolactone ring-opening polymerization follows a chain-growth mechanism. The concurrent occurrence of transesterification reactions leading to the formation of cyclic polymers is responsible for the deviation from the molecular weight distribution characteristic for chain-growth polymerization. To explain and extent on the theoretical principles forming the basis of this peculiar molecular weight distribution, in this work the cyclization process during the polymerization of the 17-membered macrolactone ambrettolide has been analyzed. Liquid chromatography under critical conditions has been applied to semiquantitatively analyze the fractions of cyclic and linear products. In addition, low molecular weight size exclusion chromatography has been used to independently quantify the fractions of the smallest cyclics. Using the combination of these techniques it has been shown that cyclics are present during the whole polymerization process. Furthermore, the thermodynamics of the polymerization reaction were determined. The negligible ΔH ⊖ p = 0.9 ± 1.9 kJ•mol −1 and a positive ΔS ⊖ p = 38.5 ± 6.5 J•mol −1 •K −1 clearly demonstrate the absence of significant ring strain and proofs that the polymerization is driven by entropy. Individual equilibrium concentrations of the cyclics, from monomer to pentamer, were determined and these values were used in combination with the Jacobson and Stockmayer theory to calculate the effective molarity of the cyclic monomer, B = 0.087 M. This value subsequently yields a critical monomer concentration of 0.155 M, for which it was also experimentally determined that polymerizations having a monomer concentration below this value only yield cyclic polymers. Finally, B was used in combination with the monomer and initiator concentration to successfully predict the molecular weight distribution, which shows that real M n 's are far lower and dispersities far higher than predicted from often-applied theories.

Block Copolymers of “PE-Like” Poly(pentadecalactone) and Poly(l-lactide): Synthesis, Properties, and Compatibilization of Polyethylene/Poly(l-lactide) Blends

“…These cyclic oligomers are present in excess compared to their low molecular weight linear counterparts. 15,34 However, it is peculiar that this transesterification does lead to cyclic PPDL and PLLA, but not to cyclic copolymers consisting of a combination of PDL and LLA. For example, a peak is present at 1706.1 g·mol , which corresponds to cyclic (PDL) 7 + Na + , while there is no peak present at 1706.1 + x72.06 g·mol −1 (x lactidyl units).…”

“…9 The formed macrocyclic PPDL will also undergo ring-opening again and it is therefore the balance between cyclization reactions and ROP reactions that will yield the equilibrium concentration of the cyclic chains. 15 After the addition of lactide, the linear PPDL chains initiate the polymerization of LLA forming the poly(PDL-block-LLA) copolymer. From the moment one LLA monomer has been inserted, only LLA monomers can be polymerized or PLLA fragments transesterified.…”

“…9−11 Copolymerization of macrolactones and small-ring (strained) lactones has been reported by several authors. 9−14 Even though control in these types of polymerization is generally difficult due to the low ring strain of the macrolactone and the associated entropic nature of the polymerization, 15 stable block copolymer architectures could be synthesized using a sequential feed polymerization approach, 14,16 or even by a single-feed polymerization.…”

Mimicking (Linear) Low-Density Polyethylenes Using Modified Polymacrolactones

“…In earlier work it was shown that the conversion of Amb could quantitatively be determined using low-MW-SEC analysis. 40 Therefore, here we followed a similar approach for the polymerization of Amb, S1, and S2. Using this method, the conversion is defined as the disappearance of the monomer and not as the fraction of opened macrolactone rings.…”

“…Consequently, the vast majority of the chains in the low molecular weight regime will be cyclics. 40 As a result of the lower average molecular weight of P(S2), besides cyclics, a number of distributions corresponding to linear chains can be identified. All distributions have a repeating unit of 386 g mol −1 equivalent to the molecular weight of S2.…”