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Macromolecular design and architecture of aliphatic polyesters
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0002-5850-8873
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Public health care has reached a level where tissue or organ reconstruction by means of biodegradable short-term implants via e.g. tissue engineering will be practicable in the near future. The vital issue now is to be able to reproducibly fabricate and design new materials with the appropriate properties and three-dimensional shape, and to facilitate their sterilization. In this thesis, macromolecular design and polymer architecture techniques are used to synthesize well-defined polymers with narrow molecular weight distributions (MWD), and to control the reactions upon sterilization, and the degradation rate and profile.

A model system for the ring-opening polymerization of L-lactide (LLA) initiated by a spirocyclic tin initiator was developed to synthesize star-shaped polymers. It was shown that an increase in temperature and a decrease in the dielectrical constant of the solvents increased the reaction rate. The versatility of the spirocyclic tin initiator system was subsequently established, by homopolymerization of ε-caprolactone (CL) and 1,5-dioxepan-2-one (DXO) and this system was compared with a conventional system using stannous octoate (Sn(Oct)2) and pentaerythritol ethoxylate. Two different strategies were assessed for the two initiator systems for the synthesis of copolymers that are random or blocky in their nature. Random copolymers with distinct sequence lengths were synthesized using both initiator systems, together with block poly(DXO-co-LLA) and poly(CL-co-LLA) with narrow MWD.

Three different types of copolymers of LLA, CL and DXO were synthesized using Sn(Oct)2 and ethylene glycol. A solvent casting and particulate leaching technique was developed and applied to construct porous scaffolds of the copolymers. The porous scaffolds were subsequently sterilized using electron beam or γ-irradiation and it was shown that the reactions induced by radiation can be used to tailor the end-properties of the materials.

Homo- and copolymers of CL and DXO with different macromolecular designs (triblock and multiblock) and different polymer architectures (linear and cross-linked) were synthesized and degraded in a phosphate buffer solution for up to 364 days. By altering the network composition, the release pattern of acidic degradation products was controlled, where an increase in DXO content led to an increase in the release of both monomeric hydroxy-acids. Varying the distribution of the more hydrolysis-susceptible DXO-sequences in the linear copolymers also enabled the amount of monomeric hydroxy acids released to be controlled, where the triblock copolymer showed the highest release.

Place, publisher, year, edition, pages
Stockholm: KTH , 2008. , 62 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2008:11
Keyword [en]
Ring-opening polymerization, L-lactide, 1, 5-dioxepane-2-one, ε-caprolactone, Polymerization kinetics, Star-shaped architecture, Spirocyclic tin initiator
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4657ISBN: 978-91-7178-883-2 (print)OAI: oai:DiVA.org:kth-4657DiVA: diva2:13274
Public defence
2008-03-20, F3, KTH, Lindstedtsvägen 26, Stockholm, 09:00
Opponent
Supervisors
Note
QC 20100901Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2010-09-01Bibliographically approved
List of papers
1. Versatile and controlled synthesis of resorbable star-shaped polymers using a spirocyclic tin initiator: Reaction optimization and kinetics
Open this publication in new window or tab >>Versatile and controlled synthesis of resorbable star-shaped polymers using a spirocyclic tin initiator: Reaction optimization and kinetics
2006 (English)In: Journal of Polymer Science Part A: Polymer Chemistry, ISSN 0887-624X, E-ISSN 1099-0518, Vol. 44, no 1, 596-605 p.Article in journal (Refereed) Published
Abstract [en]

A spirocyclic tin initiator was synthesized from pentaerythritol ethoxylate and dibutyltin oxide and used to polymerize L-lactide with dichloromethane, chloroform, toluene, and chlorobenzene as solvents. The reactions were performed at different temperatures and it is concluded that neither the temperature nor the solvent affects the molecular weight or the molecular weight distribution of the star-shaped polymers. The reaction rate was significantly increased by raising the reaction temperature or choosing a solvent with a low dielectric constant. All polymers showed a molecular-weight distribution below 1.19 and a molecular-weight determined by the initial monomer to initiator concentration ([M](0)/[1]). No induction period was seen for the polymerizations. They were all first order in initiator and the degree of aggregation in toluene at 110 degrees C was found to be 4/5. The glass transition temperature and the melting temperature of the star-shaped polymers increase with increasing arm length.

Keyword
narrow molecular-weight distribution, poly(L-lactide), polymerization kinetics, spirocyclic tin initiator, star-shaped architecture
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-8048 (URN)10.1002/pola.21192 (DOI)000234091800058 ()2-s2.0-30344470475 (Scopus ID)
Note
QC 20100901Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2017-12-14Bibliographically approved
2. Precision synthesis of microstructures in star-shaped copolymers of epsilon-caprolactone, L-lactide, and 1,5-dioxepan-2-one
Open this publication in new window or tab >>Precision synthesis of microstructures in star-shaped copolymers of epsilon-caprolactone, L-lactide, and 1,5-dioxepan-2-one
2008 (English)In: Journal of Polymer Science Part A: Polymer Chemistry, ISSN 0887-624X, E-ISSN 1099-0518, Vol. 46, no 4, 1249-1264 p.Article in journal (Refereed) Published
Abstract [en]

Star-shaped homo- and copolymers were synthesized in a controlled fashion using two different initiating systems. Homopolymers of E-caprolactone, L-lactide, and 1,5-dioxepan-2-one were firstly polymerized using (I) a spirocyclic tin initiator and (II) stannous octoate (cocatalyst) together with pentaerythritol ethoxylate 15/4 EO/OH (coinitiator), to give polymers with identical core moieties. Our gained understanding of the versatile and controllable initiator systems kinetics, the transesterification reactions occurring, and the role which the reaction conditions play on the material outcome, made it possible to tailor the copolymer microstructure. Two strategies were used to successfully synthesize copolymers of different microstructures with the two initiator systems, i.e., a more multiblock- or a block-structure. The correct choice of the monomer addition order enabled two distinct blocks to be created for the copolymers of poly(DXO-co-LLA) and poly(CL-co-LLA). In the case of poly(CL-co-DXO), multiblock copolymers were created using both systems whereas longer blocks were created with the spirocyclic tin initiator.

Keyword
copolymerization, microstructure, polyesters, polymerization, ring-opening, star polymers
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-8049 (URN)10.1002/pola.22466 (DOI)000253221300010 ()2-s2.0-38849122042 (Scopus ID)
Note
20100901Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2017-12-14Bibliographically approved
3. Elastomeric hydrolyzable porous scaffolds: Copolymers of aliphatic polyesters and a polyether-ester
Open this publication in new window or tab >>Elastomeric hydrolyzable porous scaffolds: Copolymers of aliphatic polyesters and a polyether-ester
2005 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 6, no 5, 2718-2725 p.Article in journal (Refereed) Published
Abstract [en]

Porous scaffolds of 1,5-dioxepan-2-one (DXO), L-lactide (LLA), and epsilon-caprolactone (CL) were prepared by a solvent casting, salt particulate leaching technique in which the composites were detached from their mold using a novel methanol swelling procedure. By incorporating DXO segments into polymers containing LLA or CL, an increase in hydrophilicity is achieved, and incorporating soft amorphous domains in the crystalline sections enables tailoring of the mechanical properties. The porosities of the scaffolds ranged from 89.2% to 94.6%, and the pores were shown to be interconnected. The materials were synthesized by bulk copolymerization of 1,5-dioxepan-2-one (DXO), L-lactide (LLA), and epsilon-caprolactone (CL) using stannous 2-ethylhexanoate as catalyst. The copolymers formed varied in structure; poly(DXO-co-CL) is random in its arrangement, whereas poly(DXO-co-LLA) and poly(LLA-co-CL) are more blocky in their structures.

Keyword
biodegradable polymeric scaffolds, epsilon-caprolactone, l-lactide, mechanical-properties, phase-separation, 1, 5-dioxepan-2-one, degradation, poly(epsilon-caprolactone), poly(l-lactide), polylactones
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-14414 (URN)10.1021/bm050190b (DOI)000231899200042 ()2-s2.0-25844458172 (Scopus ID)
Note
QC 20100805Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
4. Finalizing the properties of porous scaffolds of aliphatic polyesters through radiation sterilization
Open this publication in new window or tab >>Finalizing the properties of porous scaffolds of aliphatic polyesters through radiation sterilization
2006 (English)In: Biomaterials, ISSN 0142-9612, E-ISSN 1878-5905, Vol. 27, no 31, 5335-5347 p.Article in journal (Refereed) Published
Abstract [en]

Porous scaffolds made of various L,L-lactide (LLA), 1,5-dioxepane-2-one (DXO) and epsilon-caprolactone (CL) copolymers were sterilized by EB- and gamma-irradiation. Differences in the comonomers, composition and the microstructure of the starting materials were used to influence the degradation mechanism and susceptibility towards irradiation and by this means to achieve sterilized scaffolds with predicted end-properties. The chemical changes and the formation of low-molecular-weight products were determined by SEC, H-1 nuclear magnetic resonance (NMR), C-13 NMR and gas chromatography-mass spectrometry (GC-MS). The degradation mechanism changed from random chain scission to cross-linking depending on the choice of monomers, the copolymer composition and the monomer sequences. Copolymerization of LLA with small amounts of CL or DXO increased the stability compared to that of the LLA homopolymer. Changing DXO to CL in a LLA copolymer also increased the stability. The type of radiation and the microstructure of the copolymer chains determined which of the monomer sequences were more prone to degrade. The most abundant low-molecular-weight product identified after sterilization was DXO monomer. Traces of LLA and CL monomers were also identified. Modification of the copolyester microstructure changed the degradation mechanism and the susceptibility towards irradiation. This allows the use of radiation sterilization to finalize the scaffold properties.

Keyword
electron beam, gamma irradiation, scaffold, copolymer, polycaprolactone, polylactic acid
National Category
Polymer Chemistry Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-14416 (URN)10.1016/j.biomaterials.2006.06.024 (DOI)000240611000001 ()2-s2.0-33746216928 (Scopus ID)
Note

QC 20150720

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
5. The influence of composition of porous copolyester scaffolds on reactions induced by irradiation sterilization
Open this publication in new window or tab >>The influence of composition of porous copolyester scaffolds on reactions induced by irradiation sterilization
2008 (English)In: Biomaterials, ISSN 0142-9612, E-ISSN 1878-5905, Vol. 29, no 2, 129-140 p.Article in journal (Refereed) Published
Abstract [en]

In our previous work regarding radiation sterilization of porous scaffolds we have concluded that the composition and microstructure of the polymer chain are a key factor influencing the degradation reactions occurring upon irradiation. In this work we in contrast reported on the effects of high-energy irradiation on the thermal and mechanical properties. Electron beam (EB)- and T-irradiation sterilization were used in order to finalize the properties of a series of porous scaffolds comprised of different aliphatic polyester copolymers. The results presented here show that, for both sterilization methods, the crystallinity increased for all copolymers of 1,5-dioxepan-2-one (DXO) and L,L-lactide (LLA) at the minimum sterilization dose. The same was true of the P.-caprolactone (CL)- and LLA-containing copolymers upon EB sterilization, while a reduction in crystallinity were found upon gamma-irradiation. As was anticipated, it was shown that crystallinity also is a characteristic of the copolymer influencing the effects of the irradiation-induced reactions. Both the onset temperature and the temperature corresponding to the maximum rate of weight loss increased after irradiation and hence the thermal stability was increased. This is a result of a simultaneous lengthening of the chains by cross-linking reactions and a shortening by random chain-scissions occurring throughout the molecule, which lead to the formation of new endgroups with higher thermal stability. Scaffolds of crystalline polymers retained more of their initial tensile properties after irradiation compared to amorphous materials. The result previously published, showing that the composition was a key factor influencing the degradation reactions occurring upon irradiation, was augmented here.

Keyword
copolymer, sterilization, electron beam, gamma irradiation, POLY-EPSILON-CAPROLACTONE, ALIPHATIC POLYESTERS, THERMAL-DEGRADATION, L-LACTIDE, POLY(LACTIDE-CO-GLYCOLIDE) PLGA, STRUCTURAL INVESTIGATIONS, RADIATION STERILIZATION, POLY(L-LACTIDE) PLLA, MASS-SPECTROMETER, CYCLIC DIMER
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-13610 (URN)10.1016/j.biomaterials.2007.08.046 (DOI)000251492200001 ()2-s2.0-35548945835 (Scopus ID)
Note
QC 20100622Available from: 2010-06-22 Created: 2010-06-22 Last updated: 2017-12-12Bibliographically approved
6. Tuning the release rate of acidic degradation products through macromolecular design of caprolactone-based copolymers
Open this publication in new window or tab >>Tuning the release rate of acidic degradation products through macromolecular design of caprolactone-based copolymers
2007 (English)In: Journal of the American Chemical Society, ISSN 0002-7863, E-ISSN 1520-5126, Vol. 129, no 19, 6308-6312 p.Article in journal (Refereed) Published
Abstract [en]

Macromolecular engineering is presented as a tool to control the degradation rate and release rate of acidic degradation products from biomedical polyester ethers. Three different caprolactone/1,5-dioxepan-2-one (CL/DXO) copolymers were synthesized: DXO/CL/DXO triblock, CL/DXO multiblock, and random cross-linked CL/DXO copolymer. The relation of CL and DXO units in all three copolymers was 60/40 mol %. The polymer discs were immersed in phosphate buffer solution at pH 7.4 and 37 degrees C for up to 364 days. After different time periods degradation products were extracted from the buffer solution and analyzed. In addition mass loss, water absorption, molecular weight changes, and changes in thermal properties were determined. The results show that the release rate of acidic degradation products, a possible cause of acidic microclimates and inflammatory responses, is controllable through macromolecular design, i.e., different distribution of the weak linkages in the copolymers.

Keyword
epsilon-caprolactone, aliphatic polyesters, hydrolytic degradation, l-lactide, 1, 5-dioxepan-2-one, polymers, implants, polymerization, glycolide, blends
National Category
Polymer Chemistry Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-16623 (URN)10.1021/ja0702871 (DOI)000246415100047 ()2-s2.0-34249041978 (Scopus ID)
Note

QC 20150720

Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
7. Controllable Degradation Product Migration from Cross-Linked Biomedical Polyester-Ethers through Predetermined Alterations in Copolymer Composition
Open this publication in new window or tab >>Controllable Degradation Product Migration from Cross-Linked Biomedical Polyester-Ethers through Predetermined Alterations in Copolymer Composition
2007 (English)In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 8, no 6, 2025-2032 p.Article in journal (Refereed) Published
Abstract [en]

Uniformly degrading biomaterials with adjustable degradation product migration rates were customized by combining the advantages of cross-linked poly(epsilon-caprolactone) with the hydrophilic character of poly(1,5-dioxepan-2-one). Hydrolytic degradation of these random cross-linked networks using 2,2'-bis-(epsilon-caprolactone-4-yl) propane (BCP) as the cross-linking agent was studied for up to 546 days in phosphate buffer solution at pH 7.4 and 37 degrees C. The hydrophilicity of the materials was altered by varying the copolymer compositions. After different hydrolysis times the materials were characterized, and the degradation products were extracted from the buffer solution and analyzed. Fourier transform infrared spectroscopy, differential scanning calorimetry, atomic force microscopy, scanning electron microscopy, and gas chromatography-mass spectrometry were used to observe the changes taking place during the hydrolysis. From the results it was concluded that degradation profiles and migration of degradation products are controllable by tailoring the hydrophilicity of cross-linked polyester-ether networks.

Keyword
Biomaterials; Crosslinking; Degradation; Ethers; Hydrolysis; Hydrophilicity; Polyesters; Hydrolytic degradation; Product migration
National Category
Polymer Chemistry Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-8054 (URN)10.1021/bm070292x (DOI)000247107900035 ()2-s2.0-34347335743 (Scopus ID)
Note

QC 20100812

Available from: 2008-03-03 Created: 2008-03-03 Last updated: 2017-12-14Bibliographically approved

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