Synthesis of Functional Polyesters via Baylis-Hillman Polymerization

Aliphatic polyesters receive increasing attention over the last years driven by their application as biodegradable substitutes for conventional commodity thermoplastics and applications in the biomedical field, where amongst the family of biodegradable polymers, aliphatic polyesters possess the leading position as a consequence of the ready metabolization of the degradation products in most cases. This has led to a wide spread use of polyester homo- and copolymers in drug delivery, surgical implants, and functional materials in tissue engineering. In general, polyesters have a high level of commercial importance, and a variety of well known processing technologies are available. To tune the properties of these materials, e.g. their degradation behavior and to add further functionality, there is significant interest in synthetic strategies which can be used to prepare side-chain functionalized polyesters with variable architectures. This Thesis explores the feasibility of a novel approach towards functional polyesters that is based on the Baylis-Hillman reaction, which involves the base catalyzed condensation of an aldehyde and an acrylate building block to produce an α-methylene-β-hydroxycarbonyl compound. As the Baylis-Hillman polymerization does not require the use of side chain protected monomers, this route may represent an interesting alternative strategy for the preparation of functional polyesters. An attractive feature of the Baylis-Hillman polymerization is that it generate polymer with reactive hydroxyl and vinyl groups, which can be used in a subsequent step for post-modification or prepare functional crosslinked scaffolds. This Thesis consists of four chapters. The different synthetic strategies that have been used for the preparation of multi-functional polyesters and the Baylis-Hillman reaction are reviewed in Chapter 1. Chapter 2 investigates the feasibility of the Baylis-Hillman polymerization of diacrylates and dialdehydes to prepare side-chain functional polyesters and explores the subsequent post-polymerization modification of these polymers to further enhance their functionality. Using 1,3-butanediol acrylate and 2,6-pyridinecarboxaldehyde as monomers and DABCO as catalyst, polymers with a degree of polymerization of up to 25 were prepared. These polymers are attractive as they contain chemically orthogonal side-chain hydroxyl and vinyl groups that can be further modified. In first experiments, it was demonstrated that the side-chain hydroxyl and vinyl groups can be quantitatively post-modified with phenyl isocyanate, respectively, methyl-3-mercaptopropionate. Chapter 3 explores the feasibility of the Baylis-Hillman reaction for the synthesis of hyperbranched polyesters. This Chapter describes an efficient and novel approach to functional hyperbranched polyesters via an A2 + B3 approach based on the Baylis-Hillman reaction. Using trimethylolpropane triacrylate and 2,6-pyridinedicarboxaldehyde as monomers and 3-quinuclidinol as the catalyst, polymer swith a degree of branching between 0.36 and 0.8, and a molecular weight of up to 28,100 were prepared. The effects of monomer ratio and addition mode on the final molecular weight, polydispersity index and degree of branching were studied. In order to demonstrate the potential of this polyester as a reactive polymer, the hydroxyl, vinyl groups and pyridine residues were post-modified with phenyl isocyanate, methyl-3-mercaptopropionate and methyl iodide. Chapter 4 aims to further explore the scope and versatility of the Baylis-Hillman polymerization and studies the preparation of side-chain functional polyesters by polymerization of 1,3-butanediol diacrylate with meta- and para-phthalaldehyde, which are two commercially available dialdehyde building blocks.

Synthesis of Side-Chain Functional Polyesters via Baylis-Hillman Polymerization

Sanhao Ji, Harm-Anton Klok

Strategies that allow access to side-chain functional polyesters are valuable as they would enable to engineer the properties of these hydrolytically degradable materials. This contribution explores the feasibility of a novel approach toward side-chain functional polyesters that is based on the Baylis-Hillman reaction, which involves the base-catalyzed condensation of an aldehyde and an acrylate building block to produce an alpha-methylene-beta-hydroxycarbonyl compound. Using 1,3-butanediol acrylate and 2,6-pyridinecarboxaldehyde as monomers and DABCO as catalyst, polymers with a degree of polymerization of up to 25 could be prepared. These polymers are attractive as they contain chemically orthogonal side-chain hydroxyl and vinyl groups that can be further modified. In the first experiments, it was demonstrated that the side-chain hydroxyl and vinyl groups can be quantitatively postmodified with phenyl isocyanate and methyl-3-mercaptopropionate, respectively. As the Baylis-Hillman polymerization does not require the use of side-chain protected monomers, this route may represent an interesting alternative strategy for the preparation of side-chain functional polyesters.

2011

Shape memory polymers with controlled time-dependent shape recovery

Charly David Azra

The aim of this thesis has been to investigate the use of heat-activated shape memory polymers (SMPs) as novel smart actuators for the controlled delivery of fluids and their suitability for applications such as low-cost drug delivery. On heating to temperatures in the vicinity of their glass transition temperature, Tg, amorphous SMPs are able to recover their equilibrium shape from a “programmed” temporary shape generated by mechanical deformation, and fixed by cooling to below Tg. This phenomenon is linked to the substantial increase in molecular mobility on heating through the glass transition, and the memory of an equilibrium shape associated a crosslinking network. While SMPs have received considerable attention in the literature, there has to date been relatively little focus on the time-dependence of shape recovery in these materials. There are nevertheless many potential biomedical applications, for example, in which it is necessary to control the actuation rate in SMP-based devices activated by natural heat sources such as the human body, whose temperature may be subject to significant fluctuations. The present work has therefore focused on the time-dependent shape memory effect under isothermal conditions and its sensitivity to temperature variations. The first part of the study involved commercial thermoplastic (physically cross-linked) and thermoset (chemically cross-linked) shape memory polyurethanes (SMPUs). This provided insight into the physical mechanisms underlying the time-dependence of the shape memory effect, which was found to correlate strongly with the viscoelastic behaviour of a given material and the thermomechanical variables associated with the shape memory cycle. It also revealed the importance of the width of the α-transition, ∆Tα , as determined by dynamic mechanical analysis (DMA), and its position on the temperature axis relative to the recovery temperature, Tr. In particular, it was shown that controlled recovery rates could be obtained if Tr corresponded to the onset of the α-transition, and a broad transition led to a reduction in the sensitivity of the shape recovery rate to thermal fluctuations about a given Tr. The next part of the study was devoted to the development of materials optimized for controlled shape recovery under well-defined conditions. The commercial formulations were found to be unsuitable for this purpose, because they offered relatively limited possibilities for tailoring Tg and ∆Tα . Thermoset SMPUs with tunable Tg and ∆Tα were therefore produced in house, based on polytetrahudrofuran (PTHF), 4,4’-diphenylmethane diisocyanate (MDI) and 1,1,1-trimethylolpropane (TMP). The starting point was a literature formulation, which was used to optimize a reactive injection molding (RIM) set-up designed to produce void-free parts with a molding time of less than 3 minutes. These SMPUs were characterized by a PTHF network linked by rigid domains made up of crosslinked MDI and TMP. This made it possible to vary Tg from -60 to 80 °C and ∆Tα from 35 to 100 °C, by varying the molecular weight of the PTHF from 2000 to 650 g/mol, and hence the cross-link density for a given stoichiometric ratio PTHF:MDI:TMP. In order to adjust ∆Tα independently of Tg, flexible segments were introduced to the rigid domains and to the junctions between the rigid domains and the PTHF, using a fourth component, 1,4-butanediol (BDO), or by replacing the TMP with ethoxylated TMP (eTMP). This provided a highly flexible platform for optimizing the shape memory performance. Moreover, the resulting SMPUs showed equilibrium moduli, Ee, of up to 13 MPa, which is relatively high for unreinforced thermosets, and clearly beneficial for fluid displacement where high recovery stresses may be important. To increase Ee further, alumina nanoparticles were dispersed in the SMPUs. This was achieved through ultrasonic, solvent-assisted dispersion of the alumina in PTHF and subsequent curing with the MDI and TMP. Reproducible, void-free, SMPU nanocomposite sheets with alumina contents as high as 15 wt% could be produced in this way within the 3 minute pot-life of the reactive mixtures. These showed a homogeneous dispersion of the alumina (aggregates less than 50 nm in diameter), as confirmed by transmission electron microscopy, leading to a modest increase in Ee to about 21 MPa. This indicated reinforcement to be essentially a geometrical effect (strain amplification), suggesting relatively limited matrix/filler interactions. On the other hand, Rr remained high in these nanocomposites (a decrease of only 5 % on addition of 15 wt% alumina), in contrast with reports in the literature of a significant degradation in shape memory performance for SMPUs containing highly interacting fillers. A linear thermo-viscoelastic model was used to simulate the small strain shape memory effect. Continuous relaxation and retardation spectra, H(τ) and L(τ) respectively, were used to model the time-dependence, and a shift factor, aT(T), was used to model the temperature dependence. To streamline the procedure, a novel characterization method was developed for the determination of aT (T ) from a constant frequency DMA temperature sweep, based on the establishment of an approximate relationship between the storage modulus, the loss modulus and aT (T ). It was hence possible to generate continuous data for aT (T ) from a single DMA run, avoiding the time-consuming and operator-dependent construction of a mastercurve from multiple experiments. Using the Havriliak-Negami parametric functions, H(τ) and L(τ) could be subsequently obtained over a large number of decades of time from the modulus and compliance data. Comparison with results from experimental shape memory data for different model SMPUs, showed this approach to provide a reliable indication of the long-term shape recovery rate. Moreover, this method has great potential for providing rapid insight into the temperature-dependent viscoelastic properties of thermorheologically simple polymers in general, regardless of whether they are being considered specifically for shape memory applications. In the final part of the study, a tailored SMPU nanocomposite formulation was used to produce demonstrators in the form of active shape memory reservoirs, capable of expelling a liquid content under isothermal conditions. The reservoirs consisted of circular membranes (the equilibrium shape), inflated at elevated temperature to form 5 ml blisters and subsequently stabilized in this form by cooling (the programmed shape), using a biaxial deformation set-up incorporating temperature control. This innovative programming procedure made it possible to generate active SMPU reservoirs that could be activated at the skin temperature (32 °C), resulting in low average flow rates ranging between 1 and 5 μL/min over several hours. Such as a device has important potential applications, e.g. skin-temperature activated, low-cost transdermal drug delivery patches.

EPFL2013

Shape memory polymers with controlled time-dependent shape recovery

Charly David Azra

EPFL2013