Production of Bimodal Molecular Weight Poly(hydroxyalkanoate)

Abstract

A poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. The composition includes a first portion and a second portion of poly(hydroxyalkanoate). The first portion has a first weight-average molecular weight, and the second portion has a second weight-average molecular weight that is at least 50% less than the first weight-average molecular weight. The poly(hydroxyalkanoate) is composed of at least 10 mol% of 3-hydroxypropionate monomer repeat units. A method for preparing the composition is also disclosed.

Description

Technical Field

[0001] The present disclosure relates to biodegradable polymer compositions. More specifically, the present disclosure relates to poly(hydroxyalkanoate) compositions having a bimodal molecular weight distribution and methods of making such compositions. Background Art

[0002] Poly(hydroxyalkanoates) can be prepared by ring-opening polymerization processes. In such polymerizations, the physical properties of the polymer are generally a function of the molecular weight. To obtain the desired physical properties, it is generally desirable to produce polymers having the highest possible molecular weight.

[0003] However, not all physical properties improve with increasing molecular weight. For example, melt viscosity is an important function of molecular weight, and if the melt viscosity is too high, it may be difficult to thermally process the polymer in conventional conversion equipment.

[0004] Accordingly, there is a need to provide a poly(hydroxyalkanoate) composition that has a high molecular weight and exhibits many of the favorable physical properties associated with higher molecular weights, while exhibiting a lower melt viscosity typically associated with lower molecular weights. Summary of the Invention

[0005] In response to the foregoing and other needs, the present disclosure provides poly(hydroxyalkanoate) compositions having a bimodal molecular weight distribution. In other words, the molecular weight distribution of the poly(hydroxyalkanoate) in the composition exhibits two distinct molecular weight peaks.

[0006] Generally, the composition includes a first portion of poly(hydroxyalkanoate) and a second portion of poly(hydroxyalkanoate). The first portion and the second portion each have a different weight-average molecular weight. Thus, the first portion of poly(hydroxyalkanoate) has a first weight-average molecular weight and the second portion of poly(hydroxyalkanoate) has a second weight-average molecular weight. According to the present disclosure, the second weight-average molecular weight is at least 50% less than the first weight-average molecular weight.

[0007] More specifically, as determined by ASTM D5296, the first weight-average molecular weight is generally at least 100,000 Daltons. Preferably, as determined by ASTM D5296, the first weight-average molecular weight is at least 200,000 Daltons. Even more preferably, as determined by ASTM D5296, the first weight-average molecular weight is at least 300,000 Daltons.

[0008] Conversely, as determined by ASTM D5296, the second weight-average molecular weight is preferably less than about 25,000 Daltons.

[0009] Due to this molecular weight difference, the composition typically has a polydispersity of at least 1.9 as determined by ASTM D5296. More preferably, the composition typically has a polydispersity of from about 1.9 to about 3.5 as determined by ASTM D5296.

[0010] Typically, the composition comprises a first portion of poly(hydroxyalkanoate) of from about 90 wt% to about 50 wt% and a second portion of poly(hydroxyalkanoate) of from about 50 wt% to about 10 wt%. More preferably, the composition comprises a first portion of poly(hydroxyalkanoate) of from about 85 wt% to about 60 wt% and a second portion of poly(hydroxyalkanoate) of from about 40 wt% to about 15 wt%.

[0011] In some embodiments, the weight ratio of the first portion of poly(hydroxyalkanoate) to the second portion of poly(hydroxyalkanoate) is from about 1:1 to about 9:1.

[0012] The poly(hydroxyalkanoate) of the composition of the present invention comprises at least 10 mol% of 3-hydroxypropionate monomer repeat units. In certain embodiments, the poly(hydroxyalkanoate) may comprise at least 25 mol%, at least 50 mol% or at least 75 mol% of 3-hydroxypropionate monomer repeat units.

[0013] Broadly speaking, the poly(hydroxyalkanoate) of the composition can be a homopolymer, a copolymer or a terpolymer. Thus, in certain embodiments, the poly(hydroxyalkanoate) comprises a homopolymer, namely poly(3-hydroxypropionate).

[0014] In other embodiments, the poly(hydroxyalkanoate) comprises a copolymer or a terpolymer, which comprises

[0015] a first repeat unit, namely (3-hydroxypropionate),

[0016] a second repeat unit as shown in Formula I

[0017]

[0018] Optionally, a third repeat unit as shown in Formula II,

[0019]

[0020] wherein R1 and R2 are each independently selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

[0021] In some embodiments, this poly(hydroxyalkanoate) is more preferably a copolymer and R1 is methyl. In such embodiments, the poly(hydroxyalkanoate) copolymer preferably comprises from about 10 mole % to about 90 mole % of a first repeating unit and from about 90 mole % to about 10 mole % of a second repeating unit.

[0022] In other embodiments, the poly(hydroxyalkanoate) is more preferably a terpolymer, where R1 is methyl and R2 is propyl. In such embodiments, the poly(hydroxyalkanoate) terpolymer preferably comprises from about 15 mole % to about 75 mole % of a first repeating unit, from about 75 mole % to about 15 mole % of a second repeating unit, and from about 1 mole % to about 5 mole % of a third repeating unit.

[0023] Advantageously, it has been found that this bimodal poly(hydroxyalkanoate) composition provides good physical properties associated with a higher molecular weight while also having a reduced melt viscosity.

[0024] The present disclosure also provides various articles incorporating the poly(hydroxyalkanoate) compositions described above. In one embodiment, the present disclosure provides a film composed of a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. In another embodiment, the present disclosure provides a fiber composed of a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution.

[0025] On the other hand, the present disclosure provides a method for preparing a composition having a first portion of poly(hydroxyalkanoate) and a second portion of poly(hydroxyalkanoate). According to one embodiment, the method comprises polymerizing a first feed of at least one substituted lactone to produce a first portion of poly(hydroxyalkanoate) having a first weight-average molecular weight. The method further comprises polymerizing a second feed of at least one substituted lactone to produce a second portion of poly(hydroxyalkanoate) having a second weight-average molecular weight. The second weight-average molecular weight is at least 50% less than the first weight-average molecular weight.

[0026] The method further comprises blending the first portion and the second portion to produce the final poly(hydroxyalkanoate).

[0027] According to the present disclosure, at least one substituted lactone comprises β-propiolactone and the poly(hydroxyalkanoate) comprises 3-hydroxypropionate monomer repeating units.

[0028] According to certain embodiments of the method, the poly(hydroxyalkanoate) is a homopolymer, namely poly(3-hydroxypropionate).

[0029] In other embodiments of the method, the poly(hydroxyalkanoate) is a copolymer or terpolymer composed of:

[0030] A first repeating unit, namely (3-hydroxypropionate),

[0031] A second repeating unit, as shown in Formula I

[0032]

[0033] Optionally, a third repeating unit, as shown in Formula II,

[0034]

[0035] wherein R1 and R2 are each independently selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

[0036] In some embodiments of the method, the first feed is preferably polymerized in a first loop reactor. In some cases, the second feed is also preferably polymerized in a second loop reactor.

[0037] According to some embodiments of the method, blending of the first part and the second part is carried out by polymerizing the second feed in the presence of the first part to form the second part. Detailed Description

[0038] The present disclosure provides a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. The poly(hydroxyalkanoate) composition is industrially compostable and / or home compostable as determined by ASTM D6400.

[0039] According to one embodiment, the composition consists of a first part of poly(hydroxyalkanoate) and a second part of poly(hydroxyalkanoate). The first part of poly(hydroxyalkanoate) has a first weight-average molecular weight, and the second part of poly(hydroxyalkanoate) has a second weight-average molecular weight that is at least 50% less than the first weight-average molecular weight. In addition, according to the present disclosure, the poly(hydroxyalkanoate) consists of at least 10 mol% of 3-hydroxypropionate monomer repeating units.

[0040] Generally, the poly(hydroxyalkanoate) of the composition can be a homopolymer, copolymer, or terpolymer. Thus, in certain embodiments, the poly(hydroxyalkanoate) is a homopolymer, i.e., poly(3-hydroxypropionate).

[0041] In other embodiments, the poly(hydroxyalkanoate) is a copolymer or terpolymer consisting of

[0042] a first repeating unit, i.e., (3-hydroxypropionate),

[0043] a second repeating unit, as shown in Formula I

[0044]

[0045] Optionally, a third repeating unit, as shown in Formula II,

[0046]

[0047] wherein R1 and R2 are each independently selected from the group consisting of linear alkyl groups or branched alkyl groups having 1 to 22 carbon atoms.

[0048] In some embodiments, the poly(hydroxyalkanoate) is more preferably a copolymer and R1 is methyl. In such embodiments, the poly(hydroxyalkanoate) copolymer typically consists of from about 10 mole % to about 90 mole % of a first repeating unit and from about 90 mole % to about 10 mole % of a second repeating unit.

[0049] In other embodiments, the poly(hydroxyalkanoate) is more preferably a terpolymer, wherein R1 is methyl and R2 is propyl. In such embodiments, the poly(hydroxyalkanoate) terpolymer typically consists of from about 15 mole % to about 75 mole % of a first repeating unit, from about 75 mole % to about 15 mole % of a second repeating unit, and from about 1 mole % to about 5 mole % of a third repeating unit.

[0050] According to certain embodiments, the composition comprises from about 90 wt % to about 50 wt % of a first portion of poly(hydroxyalkanoate) and from about 50 wt % to about 10 wt % of a second portion of poly(hydroxyalkanoate).

[0051] In some embodiments, the weight ratio of the first portion of poly(hydroxyalkanoate) to the second portion of poly(hydroxyalkanoate) is from about 1:1 to about 9:1.

[0052] In some cases, the first weight average molecular weight is at least 100,000 Daltons as determined by ASTM D5296, and the second weight average molecular weight is less than about 25,000 Daltons as determined by ASTM D5296.

[0053] In certain embodiments, the composition has a polydispersity of at least 1.9 as determined by ASTM D5296.

[0054] The bimodal poly(hydroxyalkanoate) composition according to the present disclosure has good physical properties associated with a higher molecular weight while also having a reduced melt viscosity.

[0055] The present disclosure also provides various articles incorporating the above poly(hydroxyalkanoate) composition.

[0056] For example, in one embodiment, the present disclosure provides a film composed of a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. The film structure can be formed by blown film extrusion, cast film extrusion, or biaxial orientation after cast film extrusion. Such films can be used in end applications such as bags, pouches, food packaging, agricultural mulch films, and protective packaging.

[0057] In another embodiment, the present disclosure provides fibers composed of a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. These fibers can be formed into staple fibers, monofilaments, or nonwoven fiber webs by extrusion. The fibers can be used in end applications such as textiles, wipes, diapers, hygiene products, yarns, or geotextiles.

[0058] On the other hand, the present disclosure also provides a method for preparing a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution. As described above, the poly(hydroxyalkanoate) composition includes a first portion of poly(hydroxyalkanoate) and a second portion of poly(hydroxyalkanoate), each portion having a respective different weight-average molecular weight, and the second weight-average molecular weight (i.e., of the second portion) is significantly lower than the first weight-average molecular weight (i.e., of the first portion).

[0059] Generally, according to the method, both the first portion and the second portion of poly(hydroxyalkanoate) are formed by ring-opening polymerization of substituted lactones.

[0060] Thus, the method includes a first step of polymerizing a first feed of at least one substituted lactone to produce a first portion of poly(hydroxyalkanoate) having a first weight-average molecular weight.

[0061] The method also includes polymerizing a second feed of at least one substituted lactone to produce a second portion of poly(hydroxyalkanoate) having a second weight-average molecular weight.

[0062] The method also includes blending the first portion and the second portion to produce the final poly(hydroxyalkanoate).

[0063] Various different forms of poly(hydroxyalkanoate) having a bimodal molecular weight distribution can be prepared according to the method of the present disclosure. The properties of the poly(hydroxyalkanoate) are determined by at least one substituted lactone used as the starting material.

[0064] Generally, the at least one substituted lactone includes at least β-propiolactone, and the resulting poly(hydroxyalkanoate) includes 3-hydroxypropionate monomer repeat units formed by ring-opening of β-propiolactone.

[0065] In some cases, β-propiolactone is the only substituted lactone starting material and the resulting poly(hydroxyalkanoate) is a homopolymer, i.e., poly(3-hydroxypropionate).

[0066] In other embodiments of the method, the poly(hydroxyalkanoate) is a copolymer or terpolymer composed of:

[0067] a first repeating unit, namely (3-hydroxypropionate),

[0068] a second repeating unit, as shown in Formula I

[0069]

[0070] optionally, a third repeating unit, as shown in Formula II,

[0071]

[0072] wherein R1 and R2 are each independently selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

[0073] In some embodiments, the poly(hydroxyalkanoate) is preferably a copolymer in which R1 is methyl. In such embodiments, the substituted lactone starting materials are β-propiolactone and β-butyrolactone. For such poly(hydroxyalkanoate) copolymers, the copolymer preferably comprises from about 10 mole% to about 90 mole% of the first repeating unit and from about 90 mole% to about 10 mole% of the second repeating unit.

[0074] In other embodiments, the poly(hydroxyalkanoate) is more of a terpolymer in which R1 is methyl and R2 is propyl. In such embodiments, the substituted lactone starting materials are β-propiolactone, β-butyrolactone and β-valerolactone. For such poly(hydroxyalkanoate) terpolymers, the terpolymer preferably comprises from about 15 mole% to about 75 mole% of the first repeating unit, from about 75 mole% to about 15 mole% of the second repeating unit and from about 1 mole% to about 5 mole% of the third repeating unit.

[0075] The polymerization of the first feed for producing the first part of the poly(hydroxyalkanoate) and the polymerization of the second feed for producing the second part of the poly(hydroxyalkanoate) can be carried out batchwise or continuously.

[0076] For example, a batch of the first part can be polymerized in a first batch reactor, and a batch of the second part can be polymerized in a second batch reactor. Then the first part and the second part of the poly(hydroxyalkanoate) can be melt blended, for example, in an extruder or other melt mixing device.

[0077] However, more preferably, both the first and second portions of the poly(hydroxyalkanoate) can be produced continuously. A preferred reactor for this continuous polymerization process is a loop reactor, where the reactants flow and recycle through a pipe loop. The monomers, catalyst, and fresh solvent are fed into the loop at one point, and the reacted polymer is removed continuously at a different point in the loop.

[0078] In a particularly preferred embodiment, the method of the present disclosure can be carried out using two loop reactors operating in series. A first feed of lactone is added to the first loop reactor together with a catalyst and a solvent and polymerized therein to produce a first portion of poly(hydroxyalkanoate) having a first (higher) weight average molecular weight. Once polymerized, the first portion of poly(hydroxyalkanoate) is transferred to the second loop reactor. A second feed of lactone is added to the second loop reactor together with a catalyst and a solvent and polymerized therein to produce a second portion of poly(hydroxyalkanoate) having a second (lower) weight average molecular weight.

[0079] Thus, according to this embodiment, the second feed is polymerized in the presence of the first portion to form the second portion. It can then be seen that, according to this embodiment, the step of blending the first and second portions is carried out by polymerizing the second feed in the presence of the first portion to form the second portion.

[0080] For both polymerization steps, suitable solvents for carrying out the reaction include tetrahydrofuran and methyl (tert-butyl ether). It is also preferred to use a catalyst or initiator. Suitable catalysts or initiators include quaternary ammonium salts. The content of the catalyst or initiator can be different for the polymerization of the first feed and the polymerization of the second feed. For the polymerization of the first feed to produce the first portion of poly(hydroxyalkanoate), the content of the catalyst or initiator is preferably about 1 part of catalyst or initiator relative to about 5000 parts of the first feed. For the polymerization of the second feed to produce the second portion of poly(hydroxyalkanoate), the content of the catalyst or initiator is preferably about 1 part of catalyst or initiator relative to about 500 parts of the second feed.

[0081] The present disclosure is further illustrated by the following embodiments:

[0082] Embodiment 1, A composition comprising:

[0083] A first portion of poly(hydroxyalkanoate) and a second portion of poly(hydroxyalkanoate),

[0084] wherein the first portion of poly(hydroxyalkanoate) has a first weight average molecular weight and the second portion of poly(hydroxyalkanoate) has a second weight average molecular weight that is at least 50% less than the first weight average molecular weight, and

[0085] wherein the poly(hydroxyalkanoate) comprises at least 10 mol% of 3-hydroxypropionate monomer repeat units.

[0086] Embodiment 2. The composition according to Embodiment 1, wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate).

[0087] Embodiment 3. The composition according to Embodiment 1, wherein the poly(hydroxyalkanoate) is a copolymer or terpolymer that comprises

[0088] a first repeat unit, namely (3-hydroxypropionate),

[0089] a second repeat unit, as shown in Formula I

[0090]

[0091] optionally, a third repeat unit, as shown in Formula II;

[0092]

[0093] wherein R1 and R2 are each independently selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

[0094] Embodiment 4. The composition according to Embodiment 3, wherein the poly(hydroxyalkanoate) comprises a copolymer and R1 is methyl.

[0095] Embodiment 5. The composition according to Embodiment 4, wherein the poly(hydroxyalkanoate) copolymer comprises from about 10 mol% to about 90 mol% of the first repeat unit and from about 90 mol% to about 10 mol% of the second repeat unit.

[0096] Embodiment 6. The composition according to Embodiment 3, wherein the poly(hydroxyalkanoate) comprises a terpolymer, R1 is methyl, and R2 is propyl.

[0097] Embodiment 7. The composition according to Embodiment 6, wherein the poly(hydroxyalkanoate) terpolymer comprises from about 15 mol% to about 75 mol% of the first repeat unit, from about 75 mol% to about 15 mol% of the second repeat unit, and from about 1 mol% to about 5 mol% of the third repeat unit.

[0098] Embodiment 8. The composition according to any one of the preceding embodiments, wherein the composition comprises a first portion of poly(hydroxyalkanoate) from about 90 wt% to about 50 wt% and a second portion of poly(hydroxyalkanoate) from about 50 wt% to about 10 wt%.

[0099] Embodiment 9. The composition according to any one of the preceding embodiments, wherein the first weight-average molecular weight is at least 100,000 Daltons as determined by ASTM D5296, and the second weight-average molecular weight is less than about 25,000 Daltons as determined by ASTM D5296.

[0100] Embodiment 10. The composition according to any one of the preceding embodiments, wherein the composition has a polydispersity of at least 1.9 as determined by ASTM D5296.

[0101] Embodiment 11. The composition according to any one of the preceding embodiments, wherein the weight ratio of the first portion of the poly(hydroxyalkanoate) to the second portion of the poly(hydroxyalkanoate) is from about 1:1 to about 9:1 as determined by ASTM D5296.

[0102] Embodiment 12. A film comprising the composition according to any one of the preceding embodiments.

[0103] Embodiment 13. A fiber comprising the composition according to any one of the preceding embodiments.

[0104] Embodiment 14. A method for preparing a composition having a first portion of poly(hydroxyalkanoate) and a second portion of poly(hydroxyalkanoate), the method comprising the steps of:

[0105] Polymerizing a first feed of at least one substituted lactone to produce a first portion of poly(hydroxyalkanoate) having a first weight-average molecular weight;

[0106] Polymerizing a second feed of at least one substituted lactone to produce a second portion of poly(hydroxyalkanoate) having a second weight-average molecular weight; and

[0107] Blending the first portion and the second portion to produce a final poly(hydroxyalkanoate),

[0108] wherein at least one substituted lactone comprises β-propiolactone and the poly(hydroxyalkanoate) comprises 3-hydroxypropionate monomer repeat units, and

[0109] wherein the second weight-average molecular weight is at least 50% less than the first weight-average molecular weight.

[0110] Embodiment 15. The method according to Embodiment 14, wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate).

[0111] Embodiment 16. The method according to Embodiment 14, wherein the poly(hydroxyalkanoate) is a copolymer or terpolymer comprising

[0112] The first repeating unit, i.e., (3-hydroxypropionate),

[0113] The second repeating unit, as shown in Formula I

[0114]

[0115] Optionally, a third repeating unit, as shown in Formula II

[0116]

[0117] wherein R1 and R2 are each independently selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

[0118] Embodiment 17. The method according to any one of Embodiments 14 - 16, wherein the first feed is polymerized in a first loop reactor.

[0119] Embodiment 18. The method according to any one of Embodiments 14 - 17, wherein the second feed is polymerized in a second loop reactor.

[0120] Embodiment 19. The method according to any one of Embodiments 14 - 18, wherein the step of blending the first part and the second part is carried out by polymerizing the second feed in the presence of the first part to form the second part.

[0121] The foregoing description of the preferred embodiments of the present invention has been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments were chosen and described in order to best illustrate the principles of the invention and its practical application, thereby enabling one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications suitable for the particular purposes contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when fairly, legally, and equitably construed in accordance with the breadth to which they are fairly entitled.

Claims

1. A poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution, the poly(hydroxyalkanoate) composition comprising: a first portion of poly(hydroxyalkanoate) and a second portion of the poly(hydroxyalkanoate), wherein the first portion of the poly(hydroxyalkanoate) has a first weight-average molecular weight and the second portion of the poly(hydroxyalkanoate) has a second weight-average molecular weight that is at least 50% less than the first weight-average molecular weight, and wherein the poly(hydroxyalkanoate) comprises at least 10 mol% of 3-hydroxypropionate monomer repeat units.

2. The poly(hydroxyalkanoate) composition according to claim 1, wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate).

3. The poly(hydroxyalkanoate) composition according to claim 1, wherein the poly(hydroxyalkanoate) is a copolymer that comprises a first repeat unit, namely (3-hydroxypropionate), a second repeat unit as shown in Formula I wherein R1 is selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

4. The poly(hydroxyalkanoate) composition according to claim 3, wherein the poly(hydroxyalkanoate) is a copolymer that further comprises a third repeat unit as shown in Formula II, wherein R2 is selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

5. The poly(hydroxyalkanoate) composition according to claim 3, wherein the poly(hydroxyalkanoate) comprises a copolymer and R1 is methyl.

6. The poly(hydroxyalkanoate) composition according to claim 5, wherein the poly(hydroxyalkanoate) copolymer comprises 10 mol% to 90 mol% of the first repeat unit and 90 mol% to 10 mol% of the second repeat unit.

7. The poly(hydroxyalkanoate) composition according to claim 4, wherein the poly(hydroxyalkanoate) comprises a copolymer, R1 is methyl, and R2 is propyl.

8. The poly(hydroxyalkanoate) composition according to claim 7, wherein the poly(hydroxyalkanoate) copolymer comprises 15 mol% to 75 mol% of the first repeat unit, 75 mol% to 15 mol% of the second repeat unit, and 1 mol% to 5 mol% of the third repeat unit.

9. The poly(hydroxyalkanoate) composition according to claim 1, wherein the composition comprises 90 wt% to 50 wt% of the first portion of the poly(hydroxyalkanoate) and 50 wt% to 10 wt% of the second portion of the poly(hydroxyalkanoate).

10. The poly(hydroxyalkanoate) composition according to claim 1, wherein, As determined by ASTM D5296, the first weight-average molecular weight is at least 100,000 Daltons, and as determined by ASTM D5296, the second weight-average molecular weight is less than 25,000 Daltons.

11. The poly(hydroxyalkanoate) composition according to claim 1, wherein, As determined by ASTM D5296, the composition has a polydispersity of at least 1.

9.

12. The poly(hydroxyalkanoate) composition according to claim 1, wherein the weight ratio of the first part of the poly(hydroxyalkanoate) to the second part of the poly(hydroxyalkanoate) is from 1:1 to 9:

1.

13. A film comprising the poly(hydroxyalkanoate) composition according to claim 1.

14. A fiber comprising the poly(hydroxyalkanoate) composition according to claim 1.

15. A method for preparing a poly(hydroxyalkanoate) composition having a bimodal molecular weight distribution, the poly(hydroxyalkanoate) composition having a composition of a first part of a poly(hydroxyalkanoate) and a second part of the poly(hydroxyalkanoate), the method comprising the steps of: polymerizing a first feed of at least one substituted lactone to produce a first part of a poly(hydroxyalkanoate) having a first weight average molecular weight; polymerizing a second feed of at least one substituted lactone to produce a second part of the poly(hydroxyalkanoate) having a second weight average molecular weight; and blending the first part and the second part to produce a final poly(hydroxyalkanoate), wherein the at least one substituted lactone comprises β-propiolactone and the poly(hydroxyalkanoate) comprises monomeric repeat units of 3-hydroxypropionate, and wherein the second weight average molecular weight is at least 50% less than the first weight average molecular weight.

16. The method according to claim 15, wherein the poly(hydroxyalkanoate) comprises poly(3-hydroxypropionate).

17. The method according to claim 15, wherein the poly(hydroxyalkanoate) is a copolymer comprising a first repeat unit, namely (3-hydroxypropionate), a second repeat unit as shown in formula I wherein R1 is selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

18. The method according to claim 17, wherein the poly(hydroxyalkanoate) is a copolymer further comprising a third repeat unit as shown in formula II, wherein R2 is selected from the group consisting of straight-chain alkyl groups or branched-chain alkyl groups having 1 to 22 carbon atoms.

19. The method according to claim 15, wherein the first feed is polymerized in a first loop reactor.

20. The method according to claim 15, wherein the second feed is polymerized in a second loop reactor.

21. The method according to claim 15, wherein the step of blending the first part and the second part is carried out by polymerizing the second feed in the presence of the first part to form the second part.

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