The effect of me‐substituents of 1,4‐butanediol analogues on the thermal properties of biobased polyesters

Biobased 1,4‐butanediol analogues are used to tune the glass transition temperature and crystallization in a series of polyesters and allow for the formation of stereocomplexes.

INTRODUCTION The use of biomass for the production of chemicals and materials can reduce our dependence on fossil fuels. It also opens opportunities to develop novel materials, which are not easily accessible from petrochemicals.
These three butanediol analogues with varying numbers of methyl groups provide an interesting platform to prepare a series of derived (biobased) polyesters. To our knowledge, no systematic investigation on the influence of these methylgroups on polyester properties is reported.
It is, however, expected that these methyl-groups should have a profound effect on the crystallization behavior and the glass transition temperature (T g ) of derived polyesters, and thus could be used to tune polymer properties. This hypothesis is based on previously reported properties for succinate polyesters with 1,3-propanediol analogues: 1,3-propanediol So, for propanediol-analogues, it has been shown that methyl groups have a significant influence on the properties of succinate polyesters made from them. Clearly, the orientation of the methyl groups plays an important role, since only the 2-methyl-1,3-propanediol (irregular orientation of methyl groups after build-in in the polymer chain) displays no apparent T m .
For the 1,4-butanediols in our study, different enantiomers exist. It can be expected that optically pure derivatives of 2,5-hexanediol (Fig. 1) provide a regular orientation of methyl-groups, which might allow for crystallization of Additional Supporting Information may be found in the online version of this article. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. derived polyesters (as in the propanediol series, where regularity plays such an important role). These pure enantiomers can now be prepared via developed biochemical routes with excellent entiomeric excess, leading to the accessibility of optical pure (2R,5R)-hexanediol [18][19][20][21] and (2S,5S)-hexanediol. 18 It is expected that the use of optically pure 2,5-hexanediols should allow for crystallization due to the symmetry, and potentially the formation of the stereocomplex when (2R,5R)-hexanediol-and (2S,5S)-hexanediol-derived polyesters are mixed in a 1:1 ratio.
Here, we report on the systematic investigation of the influence of Me-groups on 1,4-butanediol-analogues in three series of polyesters. For these three series, a combination of the diols with (potentially) carbohydrate-derived diacids was chosen: succinic acid, adipic acid, 22-24 and 2,5-furandicarboxylic acid 25,26 (Fig. 2).
The influence of methyl-groups on 1,4-butanediol-analogues on the thermal properties was first investigated for the most flexible and aliphatic diacid building block, that is, adipic acid.
Poly(butylene adipate) is already a well-studied polyester, 27 and was therefore not prepared, but for comparison, the literature data on poly(butylene adipate) are shown in Table 1, entry 1. Poly(butylene adipate) is a polyester with a low T g (T g 5 268/260 8C) but, due to its linear structure, it is still able to crystalize, and therefore has a melting temperature of 54-60 8C (Table 1, entry 1). Next, 1,4-pentanediol (racemic) and 2,5-hexanediol (racemic) were prepared starting from 2-methylfuran and 2,5-dimethylfuran (see supporting information), and the corresponding adipate polyesters were prepared. Polymerizations were performed at small scale (1.5 g diol), by transesterification of dimethyl-adipate with the diols (30 mol % excess diol relative to diester). Both the materials were obtained as sticky yellow oils, and the materials were characterized on molecular weight (GPC) and thermal properties (DSC and TGA). The results of the measurements are summarized in Table 1, entries 2-3.
When an additional methyl-group into the 1,4-butanediol structure was introduced, thus poly(1,4-pentylene adipate), an increase in the T g (252 8C) and a loss of crystallinity was observed, as apparent from the absence of a T m ( Table 1, entry 2). Addition of two methyl-groups, thus poly(2,5-hexylene adipate), further increases the T g to 239 8C, and still no apparent T m was observed (Table 1, entry 3). These results indicate that the introduction of methyl-groups increases the T g , and that the racemic diols disturb the crystallization of the adipate-polyesters, making them fully amorphous.
Next, a shorter chain building block was investigated, that is, succinic acid (slightly less flexible and less aliphatic compared with adipic acid). As mentioned above, PBS is a commercial biodegradable polyester, with a T g of 233 8C and a melting temperature of 114-115 8C (Table 1, entry 4). The racemic methyl-analogues, poly(1,4-pentylene succinate) and poly(2,5-hexylene succinate), were prepared and the properties are listed in Table 1, entries 5-6. Introduction of 1 methyl-group, poly(1,4-pentylene succinate), increases the T g from 233 to 229 8C, and resulted in loss of T m (Table 1, entry 5). Addition of a second methyl-group, poly(2,5-hexylene succinate), further increases the T g to 214 8C, and still no apparent T m was observed (Table 1, entry 6). The results of the succinate-series are thus comparable to the results obtained in the adipate-series.
It was decided to investigate the effect of the stereo conformation of 2,5-hexanediols on the crystallinity of succinate polyesters. It was expected that for the succinate polyesters, which are slightly less flexible and less aliphatic compared with adipic acid, an effect should be visible. Therefore, the two optically pure 2,5-hexanediol-succinates, that is, poly(2R,5R-hexylene succinate) and poly(2S,5S-hexylene succinate), were prepared (Table 1, entries 7-8). Initially, both materials were transparent directly after polymerization, with a T g of 218 to 219 8C, and no apparent T m (Table 1, entries 7-8). When the samples were stored at 50 8C to induce crystallization, indeed one of the polymers (poly(2S,5S-hexylene succinate); Table 1, entry 8) turned opaque, indicating crystal formation. DSC analysis showed a T m of 200 8C, confirming the visual observation. However, the crystallization was very slow, since it took 3 months before the sample turned opaque.

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succinate) was dissolved in chloroform, and the solvent was allowed to slowly evaporate. When the sample was dry, immediately an opaque structure was visible, indicating crystallinity. The material was still sticky, indicating a low degree of crystallinity. DSC analysis up to 200 8C did not show a T m , and therefore could not confirm the formation of the stereocomplex (Table 1, entry 9). However, the T m of the stereocomplex is expected to be higher compared with the T m (200 8C) of the poly(2S,5S-hexylene succinate). Also the speed of crystallization was much faster, which are both in accordance with the theory on stereo complex formation (higher T m and faster crystallization). 30 However, the opaque color of the sample is a very strong indication that the stereocomplex has been formed. The T g of the stereo-complex was 216 8C, which is in the same range as the parent 2R,5Rand 2S,5S-(hexylene succinate)polyesters (T g of 219 to 218 8C) ( Table 1, entries 7-9).
All three series showed an increase in the T g with increasing numbers of methyl-groups. Surprisingly, the slopes of all series (furanote-, succinate-, and adipate-) display the same trend in a parallel direction. This indicates that the found correlation (increasing T g with increasing number of Megroups) might be a universal feature, and that this knowledge could be used to predict T g s for other polyester series.
In addition, the same feature is reported for polyesters with 1,3-propanediol analogues, where an an increase in Megroups resulted in an increase in T g . [15][16][17] To summarize, a systematic investigation on the influence of methyl-groups of 1,4-butanediol-analogues of adipate-, succinate-, and furanoate-polyesters was investigated. All series showed an increase in T g with an increasing number of methyl-groups. The chirality of these methyl-groups was also found to influence the crystallization behavior. For the first time, we have a strong indication that a stereocomplex of poly(2R,5R-hexylene succinate) and poly(2S,5S-hexylene succinate) can be formed (visual observation of crystallization). The use of these carbohydrate based 1,4-butanediol-analogues provides a nice tool for further tuning of other (co)polyesters.

EXPERIMENTAL
A description of the materials, analytical equipment, and experimental methods can be found in the Supporting Information.