Article pubs.acs.org/Macromolecules
Catalytic Ring-Opening (Co)polymerization of Semiaromatic and Aliphatic (Macro)lactones Mark P. F. Pepels,† F. van der Sanden,† E. Gubbels,† and Rob Duchateau*,‡ †
Laboratory of Polymer Materials, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 Eindhoven, MB, The Netherlands ‡ SABIC T&I, STC-Geleen, SABIC Europe B.V., Urmonderbaan 22, 6160 AH Geleen, The Netherlands S Supporting Information *
ABSTRACT: The ring-opening polymerization (ROP) of cyclic butylene terephthalate oligomers (cBT) using an aluminum salen catalyst was investigated. The kinetic analysis of the ROP of cBT in tetrachloroethane (TCE) revealed a firstorder dependence of the rate constant in catalyst as well as monomer concentration. Comparison with the ROP in other solvents showed that TCE significantly retards the reaction by short-term reversible catalyst deactivation and by long-term permanent deactivation. The apparent activation energy for the ROP of cBT and ε-caprolactone (CL) were determined independently, and the difference in reactivity was used to investigate the possibility to synthesize blocky copolymers. Sequence distribution analysis of the copolymers obtained over a period of time from a single feed copolymerization revealed that over the full reaction range the degree of randomness was high. A sequential feed approach, in which CL was added after the polymerization of cBT was completed, showed that the propagation reaction was not fast enough to avoid transesterification, yielding copolymers with an increasing degree of randomness over time. Additionally, the copolymerization of cBT with ωpentadecalactone (PDL) was performed in bulk at temperatures above the crystallization temperature of poly(butylene terephthalate) (PBT), yielding random copolymers over the full composition range between polypentadecalactone (PPDL) and PBT. Copolymerizations using aluminum salen as catalyst were rapid and high molecular weights could be achieved. The melting temperature of both PPDL and PBT decreased upon the introduction of either counit, reaching a eutectic point between 50 and 70 mol % PDL. Furthermore, a combination of thermal analysis and X-ray diffraction shows that PDL units are fully excluded from the PBT crystal lattice, while butylene terephthalate units are partially incorporated into the PPDL crystal lattice.
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yields random copolymers.10−13 On the other hand, employing a sequential feed strategy or copolymerization with branched lactones can be used to obtain block copolymers.10,14−18 The ROP of cyclic esters having semiaromatic moieties in the main chain has been studied considerably less intensive compared to their aliphatic counterparts. One of the reasons for this is the scarcity of the available monomers.19 Next to some recent exotic examples,20 most studies are focused on cyclic oligomers of poly(alkylene terephthalate)s,21,22 such as poly(ethylene terephthalate) (PET),23−27 poly(propylene terephthalate) (PPT),25,28 poly(butylene terephthalate) (PBT),29−34 and poly(hexamethylene terephthalate) (PHT).35,36 More recently, also poly(ethylene furandicarboxylate) and poly(butylene furandicarboxylate) have been obtained via ROP.37 Among the semiaromatic macrolactones, the ROP of cyclic butylene terephthalate (cBT) has been studied most intensively.19 Cyclic butylene terephthalate oligomers are commercially available and more easily produced compared to their PET counterpart,
INTRODUCTION The ring-opening polymerization (ROP) of cyclic esters (lactones) has been investigated intensively in recent years, since it offers the opportunity to tune polymer composition, sequence distribution, and architecture in order to obtain the desired polymer properties. Especially in the biomedical field, this method is used to produce biodegradable and biocompatible copolymers of, for instance, polylactide (PLA) and polycaprolactone (PCL).1−4 Moreover, ROP reactions are also investigated for engineering applications such as (nano)composites, which combine the advantage of high reactivity and low viscosity of monomers, resulting in a better dispersion of fillers, better wetting, and a higher final molecular weight compared to the use of presynthesized polymer.5−7 A special class of lactones which has recently received considerable attention are macrolactones, e.g., ω-pentadecalactone (PDL), which are cyclic esters having a large ring size, resulting in negligible ring strain. The obtained (co)polymers combine the advantages of polyethylene-like properties,8,9 with the versatility belonging to polymers synthesized by ROP. Because of the lack of ring strain, a single feed copolymerization of macrolactones with unbranched lactones © XXXX American Chemical Society
Received: April 10, 2016 Revised: May 30, 2016
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DOI: 10.1021/acs.macromol.6b00744 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Scheme 1. Ring-Opening Polymerization of Cyclic Butylene Terephthalate and Ring-Opening Copolymerization of Cyclic Butylene Terephthalate with Either ε-Caprolactone or ω-Pentadecalactone
Figure 1. SEC traces of samples taken at different polymerization time intervals (a). Example of a measured chromatogram with the fitted cBT peak underneath (b, top) and the obtained chromatogram after subtraction of the cBT from the reaction mixture (b, bottom). Conversion vs time plot of the calculated cBT conversions from the fitted SEC traces (c). Reaction conditions [cBT] = 1.0 M, [1] = 0.01 M, T = 100 °C.
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and they can be used for reactive injection molding of (nano)composite structures,31,33,34,38,39 as processing aid, and as additive carrier.40 The advantages of producing these polymers via ROP compared to the conventional process, i.e., polycondensation, include the fact that high molecular weight is easily attainable without the need for condensate removal to increase conversion and the easy controllability of the molecular weight by the addition of chain transfer agent.19 Furthermore, ROP allows rapid and easy copolymerization with other lactones, in bulk or in solution. Block copolymers of semiaromatic polyesters have recently been obtained by the ROP of their cyclic oligomers from a low molecular weight polyol.41,42 Furthermore, the single feed bulk copolymerization of cBT and CL as well as cyclic hexamethylene terephthalate with CL has been investigated, which in both cases yielded random copolymers.32,35 This study describes the kinetics of the ROP of cBT in solution using an aluminum−salen catalyst, for which it is shown that the used solvent plays an important role. Furthermore, the kinetics are used to study both the single and sequential feed copolymerization of cBT with CL, for which special attention is paid to the sequence distribution over time. Finally, the bulk copolymerization of cBT with PDL is investigated to elucidate the copolymer properties vs composition, ranging from polyethylene-like to PBT properties.
RESULTS AND DISCUSSION
Before the polymerization studies were conducted, the composition of the cyclic butylene terephthalate (cBT) was investigated by means of low-MW-SEC and DSC. As reported before,22,43 commercial cBT consists of a mixture of cyclic oligomers (dimer to heptamer) showing a complex melting behavior in which each oligomer appears to have a different melting temperature (Supporting Information, Figures S1 and S2). In order to investigate the kinetics of the bulk polymerizations, it is desired that the polymerization is performed at temperatures above the crystallization temperature of the formed polymer. For the polymerization of cBT, this means that reaction temperatures of 180 °C or higher are required. As will be shown later, the polymerization of cBT using aluminum salen (1) as catalyst proceeds extremely fast at this temperature which makes kinetic analysis very difficult. Therefore, in order to get a good picture of the ROP kinetics, the solution polymerization of cBT was investigated. General Considerations of Polymerization Kinetics. Since cBT consists of cyclic oligomers of different sizes, kinetic analysis is less straightforward than the ROP of discrete lactones having a uniform ring size. In this work the assumption is made that all rings present have negligible ring strain, since B
DOI: 10.1021/acs.macromol.6b00744 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Figure 2. Logarithmically transformed conversion vs time plot. Circles indicate experimental values, and dotted lines are the corresponding fits using eq 2a. kapp vs [cat] showing the (linear) first-order relation between catalyst concentration and reaction rate. T = 100 °C, [cBT] = 1.0 M in TCE. kapp = kp[cat].
observed (Figure 1c). Therefore, the low conversion data (
