Mika Härkönen

Lactic acid based poly(ester‐urethanes): Use of hydroxyl terminated prepolymer in urethane synthesis

We studied a two step process for lactic acid polymerization: in the first step, the lactic acid is condensation polymerized to a low molecular weight hydroxyl terminated prepolymer and then the molecular weight is raised by joining prepolymer chains together using diisocyanate as the chain extender. The resulting polymer is a thermoplastic poly(ester-urethane). The polymer samples were carefully characterized with 13C-NMR, GPC, DSC, and IR. The results indicate that high conversions of lactic acid can be achieved, as well as independent control of the stereostructure, long chain branches, molecular weight average, and molecular weight distribution. Lactic acid is converted into a poly(ester-urethane) with a weight average molecular weight as high as 390,000 g/mol and a glass transition temperature of 53.7°C. The analyzed content of the monomer in the prepolymer is less than 1 mol % and the lactide content 2.4 mol %, while the final poly(ester-urethane) is essentially monomer and lactide free. The mechanical properties of the poly(ester-urethane) are comparable to those of polylactides.

Lactic acid based poly(ester-urethane)s: The effects of different polymerization conditions on the polymer structure and properties

A two-step process for lactic acid polymerization is studied: in the first step the lactic acid is condensation-polymerized to a low-molecular-weight hydroxyl-terminated prepolymer; and then the molecular weight is raised by joining prepolymer chains together using diisocyanate as the chain extender. The resulting polymer is a thermoplastic poly(ester-urethane). In this study, we synthesized three different prepolymers and used three different diisocyanates as chain extenders. All of the prepolymers were hydroxyl-terminated, and their weight average molecular weights were 5,500 g/mol, 11,900 g/mol, and 26,000 g/mol. One of the diisocyanates was aliphatic, and the other two were stiff cycloaliphatic diisocyanates. The results indicate that of the tested diisocyanates, high weight average molecular weight can be achieved only by using aliphatic 1,6-hexamethylene diisocyanate. The cycloaliphatic diisocyanates produced poly(ester-urethane)s with weight average molecular weights which were quite low, but due to the stiffness of the polymer chains the glass transition temperatures can be as high as 60°C. The 1,6-hexamethylene diisocyanate (HDI) results also indicate that the molecular weight and network formation can be controlled independently by the amount of diisocyanate used and the polymerization conditions. Only the poly(ester-urethane)s which were produced with HDI had good mechanical properties, while the stiff diisocyanates produced very brittle polymers.

The modification of lactic acid based poly(ester-urethane) by copolymerization

Copolymers of L-lactic acid (LLA) with DL-mandelic acid (DLMA), 4-hydroxybenzoic acid, 4-acetoxybenzoic acid, DL-malic acid, or anhydrous citric acid were synthesized via direct copolycondensation in the presence of 1,4-butanediol, using stannous octoate as catalyst. The effect of the comonomer and the comonomer ratio on polycondensation and the glass transition temperature were investigated. The glass transition temperature of amorphous poly(L-lactic acid-co-DL-mandelic acid) increased linearly from 33° to 56°C as the mandelic acid composition was increased from 0 to 45 mol %. For urethane synthesis, prepolymers of LLA and DLMA were condensation polymerized with compositions of 100/0, 90/10, and 80/20 (mol % in feed). The preparation of poly(ester-urethane) (PEU) was carried out in a stirred glass reactor, using 1,6-hexamethylene diisocyanate and isophorone diisocyanate in melt. The glass transition temperature of poly(L-lactic acid-co-DL-mandelic acid-urethanes) showed a marked increase with increased mandelic acid composition. The molecular weights of these urethanes were lower than for PEU based on poly(L-lactic acid). Such a depression in the degree of polymerization is attributed to the steric hindrance of the bulky phenyl group as a side chain of mandelic acid.

External silane donors in Ziegler-Natta catalysis : a two-site model simulation of the effects of various alkoxysilane compounds

Abstract The results of experiments with a supported Ziegler-Natta catalyst were interpreted by a two-site model of the active sites, in order to enhance the understanding of multiple types of catalyst active sites and in particular the role of the external donor. The applicability of the two-site model is discussed and the stochastic parameters of the model are correlated with both the experimental and molecular modelling data. The two-site model satisfactorily explains the major part of the active sites of this highly isospecific catalyst and is a suitable tool for analysing the effects of the donors on catalyst active sites. Nevertheless, it does not completely describe the behaviour of Mg-supported Ziegler-Natta Ti catalyst. There is probably more than one kind of active site producing the part of polypropylene that is soluble in boiling heptane. This study also supports the earlier suggestion that some of the changes in the population of active sites can be explained only through the formation of new sites, and more specifically through the existence of fluctuating sites that can be stabilized by external donors. The stability of these new sites depends on both the structural and electronic properties of the external donors.

PROPERTIES AND POLYMERIZATION OF BIODEGRADABLE THERMOPLASTIC POLY(ESTER-URETHANE)

Abstract Aliphatic polyesters, such as poly(lactic acids), need high molecular weight for acceptable mechanical properties. This can be achieved through ring-opening polymerization of lactides. The lactide route is, however, relatively complicated, and alternative polymerization routes are of interest. In this paper we report the properties of a polymer made by a two-step process: first a condensation polymerization of lactic acid and then an increase of the molecular weight with diisocyanate. The end product is then a thermoplastic poly(ester-urethane). The hydroxylterminated prepolymer was made with condensation polymerization of L–lactic acid and a small amount of 1,4-butanediol. The polymerization was performed in the melt under nitrogen and reduced pressure. The preparation of poly(ester-urethane) was done in the melt using aliphatic diisocyanates as the chain extenders reacting with the end groups of the prepolymer. The polymer samples were carefully characterized, including preliminary degradation ...

The biodegradation of lactic acid-based poly(ester-urethanes)

The biodegradability of lactic acid based poly(ester-urethanes) was studied using the headspace test method, which was performed at several elevated temperatures. The poly(ester-urethanes) were prepared using a straight two-step lactic acid polymerization process. The lactic acid is first condensation polymerized to a low molecular weight hydroxyl-terminated telechelic prepolymer and then the molecular weight is increased with a chain extender such as diisocyanate. In the biodegradation studies the effect of different stereostructures (different amounts of D-units in the polymer chain), the length of ester units, and the effect of crosslinking on the biodegradation rate were studied. The results indicate that poly(ester-urethanes) do not biodegrade at 25‡C, but at elevated temperatures they biodegrade well. The different stereostructures and crosslinking have a strong influence on the biodegradation rate. The length of ester units in the polymer chain also affects the biodegradation rate, but much less than crosslinking and stereostructure.

Effects of the Structure of External Alkoxy Silane Donor in High Activity Ziegler-Natta Catalyst on the Microstructure of Polypropylene

Correlations were sought between the structure of external alkoxy silane donors and the microstructure of the obtained polymer chain. Propylene was polymerized in liquid monomer with a heterogeneous high activity Ziegler-Natta catalyst. Fifteen different alkoxy silanes of structure R n Si(OR′) 4-n , where n=1–3, R = Ph or alkyl and R′ = C 1–3 alkyl, were used as external donors. Polymers were fractionated by boiling heptane extraction. Microstructures of the polymers were studied by means of 13 C NMR.

Special Issue on Recent Advances in Coordination Polymerization of Olefins. Asymmetric Markovian Process in the Polypropylene Obtained by Ziegler-Natta Catalyst with External Alkoxysilane Donors.

重合が非対称マルコフ過程に従う場合の条件つき確率をNMR三連子強度から求める式を導き, これをアルコキシシラン添加チーグラー-ナッタ触媒で得られたポリプロピレンに適用した. 未分別試料, 熱ヘプタン不溶分ともに, 対称中心から (fluctuateしている) 非対称中心への転換に際し, 一方 (Lとする) の中心の転換に比て, D中心への転換は生じていないか, 生じていても生じ方が少ないこと, 外部ドナーの濃度に伴うイソタクチシチーの単調増加はPLLが大きくなるためであること, 芳香族系のドナーの方がイソタクチシチーの向上に有効であること, および外部ドナーの添加によるイソタクチシチーの向上はL中心, D中心の両方がプラスに作用しているのではなく, 前者による向上が後者による低下を凌駕していることによることを明らかにした.