Synthesis and Degradation of Biodegradable Polyurethanes

Abstract

A series of biodegradable star poly(ε-caprolactone) (PCL)-based polyols and PCL-based diisocyanate prepolymers were synthesised and fully characterised. Biodegradable polyurethanes (PUs) were synthesised using star PCL-based polyols and either biodegradable diisocyanate prepolymers 4,4’-methylenebis(phenyl isocyanate) (MDI) or 2,4-toluene diisocyanate (TDI). The resulting polyols, diisocyanate prepolymers and PUs were subjected to enzymatic degradation using lipase for up to 30 days.

Chapter 1 is a general introduction to the reactions involved in the syntheses of PU foams and the ring-opening polymerisation of cyclic esters. The general components used in PU formulations including biodegradable polyols and diisocyanate prepolymers are discussed. Furthermore, polymer biodegradation testing methods and analytical methods to monitor degradation are investigated.

Chapter 2 includes the syntheses and enzymatic degradation of a series of biodegradable four- and six-arm star PCL polyols. This was achieved through the tin(II)octoate (SnOct2) catalysed ring opening polymerisation (ROP) reaction of ε-caprolactone (ε-CL) using pentaerythritol, di(trimethylolpropane) and dipentaerythritol initiators. Furthermore, a series of six-arm star poly[(ε-caprolactone)-co-(β-butyrolactone)] were synthesised in a similar manner. Star PCL and star poly[(ε-CL)-co-(β-BL)] both exhibited almost 100% mass loss after 15 days of enzymatic degradation at a constant rate. Generally, an initial increase in % crystallinity (%χc) is seen for star PCL and star poly(ε-CL)-co-(β-BL) in the first few days (0-3 days) of enzymatic degradation. This was followed by a decrease in crystallinity (%χc), indicating amorphous regions of the polymer were preferentially degraded. This was supported by scanning electron microscopy (SEM) analyses showing surface pitting and occurrence of crystal spherulite structures within the first few days of enzymatic degradation.

Chapter 3 concerns the synthesis and enzymatic degradation of a series novel four-arm dumbbell-shaped copolymers (PCL)2-(PEG)-(PCL)2 bridged with 2,2’-bis(hydroxymethyl)propionic acid (bisMPA) moieties. This was achieved by the synthesis of a tetra-hydroxyl PEG macro-initiator through a coupling reaction of poly(ethylene glycol) (PEG) and acetyl-protected bisMPA using dicyclocarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP). Subsequently, the four-arm star structure was synthesised with the SnOct2 catalysed ROP of ε-CL using tetra-hydroxyl PEG macroinitiator. Contact angle and %water uptake (%WU) indicated the copolymers containing a higher %PEG showed increased hydrophilic nature and surface wetting. A dispersity (Ð) of 1.32-1.51 and a small molecular weight shoulder were seen due to using a polydisperse PEG macro-initiator and the high viscosity of the reaction mixture under bulk conditions. The novel star copolymers showed >90% mass loss within 7 days and an increase in %χc within the first few days of enzymatic degradation.

Chapter 4 entails the synthesis and enzymatic degradation of biodegradable seven arm star PCL with a central acetylated β-cyclodextrin (β-CD) moiety. This was achieved through a four step synthetic route involving the protection of 1° OH groups on the β-CD moiety, acetylation of the 2° OH groups on the β-CD moiety, removal of protecting group on the 1° OH group and subsequent ROP of ε-CL catalysed by SnOct2. Contact angle and %WU analyses showed minimal surface wetting and high hydrophobicity. A very low rate of enzymatic degradation was seen with 7% mass loss and a general increase in %χc in 20 days.

Chapter 5 involves the synthesis and enzymatic degradation of a series of biodegradable diisocyanate prepolymers containing a central PCL or PCL-b-PEG-b-PCL moiety and capped with either MDI or TDI moieties. This was achieved through the reaction of two molar equivalents of the diisocyanate moiety, MDI or TDI, and either PCL diol or PCL-PEG-PCL triblock copolymer. The diisocyanate prepolymers showed absorbances attributing to the C=N stretch in the NCO group as well as N-H and C-N in the urethane group in FT-IR spectra. Contact angle and %WU measurements of diisocyanate prepolymers with higher %PEG showed increased surface wetting and hydrophilicity. Generally the TDI-based diisocyanate prepolymer showing 100% mass loss in 4 days, degraded at a faster rate than MDI-based diisocyanate prepolymer of 23% in 40 days. Furthermore, the lower Mn MDI-based prepolymer showed a significantly faster rate of enzymatic degradation of 79% mass loss in 40 days than the higher Mn MDI-based prepolymer.

Chapter 6 concerns the synthesis and enzymatic degradation of a series of biodegradable PUs using biodegradable PCL-based star polyols and diisocyanate prepolymer components synthesised in Chapter 2-5. PU gels were produced as a result of a minimal amount of dichloromethane (DCM) solvent to ensure complete mixing. Generally, PUs showed the disappearance of the C=N stretch in fourier transform infrared spectroscopy (FT-IR) analyses, indicating all NCO groups successfully reacted to give urethane groups. PUs showed up to 18.5% mass loss over the 30 days of enzymatic degradation. Comparisons of the PU degradation behaviour were made to demonstrate the effects of polyol type, diisocyanate type, and ratio of NCO:OH used, on the rate of PU enzymatic degradation.

Chapter 7 surmises and concludes the work covered in Chapter 2-6 and suggests further work.