Elsevier

Carbohydrate Polymers

Volume 77, Issue 2, 10 June 2009, Pages 267-275
Carbohydrate Polymers

Synthesis of poly-(ε)-caprolactone grafted starch co-polymers by ring-opening polymerisation using silylated starch precursors

https://doi.org/10.1016/j.carbpol.2008.12.032Get rights and content

Abstract

Poly-(ε)-caprolactone grafted corn starch co-polymers were synthesized using a hydrophobised silylated starch precursor. The silylation reaction was performed using hexamethyl disilazane (HMDS) as the reagent in DMSO at 70 °C. Silylated starch with a degree of substitution (DS) between 0.45 and 0.7 was obtained. ε-Caprolactone is grafted to silylated starch by a ring-opening polymerisation catalysed by Al(OiPr)3 in THF at 50 °C. The grafting efficiency varies between 28% and 58%, the remainder being homopolymers of ε-caprolactone. The DS of the polycaprolactone graft is between 0.21 and 0.72. The poly-(ε)-caprolactone side chains consist of 40–55 monomer units and is a function of the reagent intakes. Experiments with native starch under similar conditions do not result in the desired poly-(ε)-caprolactone grafted corn starch co-polymers and unreacted starch was recovered after work-up. Removal of the silyl groups of the poly-(ε)-caprolactone grafted starch co-polymers is possible using a mild acid treatment with diluted hydrochloric acid in THF at room temperature.

Introduction

Worldwide, 245 million tons of plastics are produced per year, and this value increases with about 10% per year (PlasticsEurope, 2008). These plastics are mainly synthetic polymers from fossil resources, which are known to degrade with difficulty and cause serious environmental problems (Yavuz & Babac, 2003). The development of green biodegradable polymers for e.g. the future generation of packaging materials is highly desirable.

Starch, a natural biopolymer, is one of the potential candidates for future biodegradable polymer products. Starch is abundantly available. Global production of starch is 60 million ton per year in 2004 (International Starch Institute, 2008). Starch is present in the body of many plants (tubers, roots) as granules or cells with typical particle sizes between 1 and 100 μm. The polymeric structure of starch consists of repeating anhydroglucose units. There are two types of biopolymer in starch, amylose (a linear polymer of anhydroglucoses with α-d-1,4-glucosidic bonds) and amylopectin (a branched polymer with α-d-1,6-glucosidic bonds besides α-d-1,4-glucosidic bonds). The content of amylose in starches depends on the plant and typically varies between 18% and 28%. The amylose–amylopectin ratio in native as well as modified starches has a strong impact on the product properties.

Starch films are known to have good oxygen barrier properties. However, as starch is highly hydrophilic, it is water sensitive, and the mechanical properties of starch-based films are generally inferior to those derived from synthetic polymers (Krochta and De Mulder-Johnston, 1997, Tsiapouris and Linke, 2000). Starch modification is therefore needed to meet the product properties in a number of application areas. Various modification strategies have been explored, for instance grafting of monomers (like styrene and methyl methacrylate) to the starch backbone (Bagley et al., 1977, Beliakova et al., 2004). However, in almost all cases, the used monomers and the corresponding grafted chains are not easily biodegradable. Starch has also been thermoplasticized with the help of plasticizers such as glycerol and other polyalcohols (Weber, 2000). However, the product properties are in most cases still not up to standards and blending with other polymers is required (Wang, Yang, & Wang, 2003).

A wide variety of synthetic biodegradable polymers have been prepared. Well known examples are polyesters derived from cyclic lactones (polycaprolactone, polyvalerolactone and polybutyrolactone). Polycaprolactone (PCL) is easily degraded by micro-organisms (Karlsson & Albertsson, 1998). Aerobic soil-burial experiments showed that the mechanical properties of PCL films decreased rapidly in time and were fully degraded after 4 weeks (Goldberg, 1995). PCL has gained much interest for possible applications in the medical field as well as in the area of packaging materials (Chandra and Rustgi, 1998, Chiellini and Solaro, 1996).

Several studies to combine the properties of starch and PCL have been performed to obtain fully biodegradable materials with improved product properties. Blends of thermoplastic starch and PCL are not fully miscible, resulting in undesirable phase separation (Averous, Moro, Dole, & Fringant, 2000). To increase the miscibility of starch and polycaprolactone, it has been proposed to chemically graft caprolactone onto the hydroxyl groups of starch using ring-opening polymerisation (Dubois, Krishnan, & Narayan, 1999). Common Ring-Opening Polymerization (ROP) catalysts such as tin octoate or aluminium isopropoxide gave low grafting efficiencies (GE, 0–14%). The highest GE (90%) was achieved using triethylaluminium as catalyst (Dubois et al., 1999). This catalyst is extremely air and water sensitive, therefore difficult to handle and releases ethane, a very flammable by-product, during the reaction. All available data indicate that the presence of water reduces the GE. This is rationalised by assuming that water competes with the hydroxyl groups of starch in the initiation step of the polymerization reaction, thus leading to the formation of PCL homopolymers rather that starch-g-PCL (Dubois et al., 1999). Another possible cause for the low grafting efficiencies is the heterogenous nature of the reaction. Starch is insoluble in the typical organic solvents used for ROP (such as toluene or THF), leading to a liquid–solid system. This is expected to lead to reduced reaction rates between starch and CL compared to CL homopolymerisation, thus to a reduction in the GE.

In this paper, an alternative method to synthesize poly-(ε)-caprolactone grafted starch co-polymers (starch-g-PCL) is reported. The starch source is made less hydrophilic and thus more soluble in organic solvents by substituting part of the OH groups of starch by a bulky silyl group (Klemm and Einfeldt, 2001, Petzold et al., 2002, Petzold et al., 2003). In this way, the ring-opening polymerisation occurs solely in the liquid phase and this is expected to lead to higher GE values. This approach has also been applied successfully to graft PCL and polylactide on dextran (Nouvel et al., 2004, Ydens et al., 2000).

Section snippets

Materials

Corn starch (Sigma) was dried under high vacuum (∼1 mbar) at 100 °C for one day before use. Hexamethyldisilazane (HMDS, Acros) and methanol (Labscan) were used as received. DMSO (Acros) and toluene (Labscan) were dried overnight over molecular sieves 3 Å (Merck) and stored under a protective nitrogen atmosphere. Dry tetrahydrofuran (THF) and toluene for polymerization experiments were obtained in closed vessels from Aldrich and were used as received. Hydrochloric acid (HCl) 1 N was prepared from

Results and discussion

The overall procedure to synthesize poly-(ε)-caprolactone grafted starch co-polymers (3) consists of three steps and includes hydrophobization of starch by silylation of part of the hydroxyl groups of starch using hexamethyl disilazane (HMDS), followed by an in-situ Ring-Opening Polymerization (ROP) of ε-caprolactone monomer on the hydrophobized starch and subsequent silyl group removal by a mild acid treatment. Although all steps have been investigated, the focus of this paper will be on the

Conclusions

The successful synthesis of poly-(ε)-caprolactone grafted corn starch co-polymers using a three step approach is reported. The key feature is the use of a homogeneous reaction mixture for the ROP of starch with ε-CL. This was achieved by making the starch more hydrophobic by partial substitution of the OH groups by trimethylsilyl groups. Silylated starch with a low-medium DS (0.46–0.68) was obtained using a DMSO/toluene mixture as the solvent and HMDS as the silylating agent. The ROP with ε-CL

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