Untangling Polymer Chains: Size, Topology, Processing, and Recycling

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Learn More CiteCitationCitation and abstractCitation and referencesMore citation options ShareShare onFacebookXWeChatLinkedInRedditEmailBlueskyJump toExpandCollapse ViewpointMarch 23, 2025Untangling Polymer Chains: Size, Topology, Processing, and RecyclingClick to copy article linkArticle link copied!Zhiqiang SunZhiqiang SunShenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaMore by Zhiqiang SunView BiographyZhen DongZhen DongShenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaMore by Zhen DongView BiographyFeng YuFeng YuShenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaMore by Feng YuView BiographySitong FengSitong FengShenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaMore by Sitong FengView BiographyZhong-Ren Chen*Zhong-Ren ChenShenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaGuangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055, China*Email: [email protected]More by Zhong-Ren ChenView Biographyhttps://orcid.org/0000-0002-2929-9566Open PDFAccounts of Materials ResearchCite this: Acc. Mater. Res. 2025, XXXX, XXX, XXX-XXXClick to copy citationCitation copied!https://pubs.acs.org/doi/10.1021/accountsmr.5c00057https://doi.org/10.1021/accountsmr.5c00057Published March 23, 2025 Publication History Received 20 February 2025Published online 23 March 2025article-commentary© 2025 Accounts of Materials Research. Co-published by ShanghaiTech University and American Chemical Society. All rights reserved. This publication is available under these Terms of Use. Request reuse permissionsThis publication is licensed for personal use by The American Chemical Society. ACS Publications© 2025 Accounts of Materials Research. Co-published by ShanghaiTech University and American Chemical Society. All rights reserved.Subjectswhat are subjects Article subjects are automatically applied from the ACS Subject Taxonomy and describe the scientific concepts and themes of the article. Copolymers Materials Plastics Polymers Recycling Over the past century, advances in polymer science have enabled the creation of materials that are strong, durable, versatile, and remarkably cost-effective to produce. These attributes have made polymers indispensable in nearly every aspect of modern life, from packaging and construction to healthcare and electronics. However, the success of synthetic polymers has led to unintended consequences. Their low cost and widespread availability have encouraged a culture of disposability, resulting in massive amounts of plastic waste that persist in the environment. Today, plastic pollution is a global crisis, with millions of tons of plastics entering oceans and ecosystems each year, threatening wildlife and human health.In response to this crisis, two primary strategies have emerged, i.e., plastics degradation and recycling, each targeting a different stage of the plastic lifecycle. One focuses on the end of the lifecycle, promoting biodegradable polymers that can break down naturally after disposal, thereby reducing environmental pressure. The other aims to create a closed-loop cycle by regulating disposal practices and establishing efficient recycling systems, ensuring that plastic waste is recovered and reused rather than entering the environment. While both aim to mitigate pollution, they follow divergent philosophies─degradation adopting a linear end-of-life solution and recycling striving for a circular lifecycle─often without a unified vision.At the heart of this entangled societal issue lies a fundamental question: How can we untangle the complex web of challenges surrounding plastic pollution? To find a solution, perhaps we should trace back to the essence of polymer science─the formation of synthetic macromolecules by polymerizing small molecules, or monomers. By understanding and reimagining the processes that create these materials, we may uncover new pathways to design polymers that are not only high-performing and easy for processing but also inherently sustainable.To accommodate the minimum mechanical properties of polymeric materials, the size of synthesized macromolecules must exceed the critical entanglement molecular weight, Mc. In semicrystalline polymers, molecular weight, Mw, plays a crucial role in crystallization, entanglements, and tie molecules, thus affecting the mechanical strength and ductility. (1) Tie molecules are chains that traverse multiple crystalline domains, effectively linking them together through the intercrystalline amorphous layers. A high fraction of tie molecules ensures the connectivity of crystalline regions, contributing to the overall strength, toughness, and durability of the polymer.However, while ensuring excellent material properties, ultrahigh molecular weight polymers also bring about processing challenges. As depicted in Figure 1, above Mc, the viscosity, η, of entangled linear polymer chains increases sharply with increasing molecular weight, i.e., η ∼ Mw3.4, significantly increasing the difficulty of material processing.Figure 1Figure 1. Molecular weight, Mw, dependence of viscosity, η, for entangled and untangled linear polymer chains.High Resolution ImageDownload MS PowerPoint SlideThis trade-off between mechanical properties and processability is a universal challenge in polymer science, and it is particularly pronounced in the case of polyolefins, which account for the majority of global plastic production. As one of the most widely used synthetic polymers, polyolefins, such as polyethylene (PE) and polypropylene (PP), exemplify the delicate balance between achieving superior material performance and maintaining feasible processing conditions. For instance, ultrahigh molecular weight polyethylene (UHMWPE) fibers exhibit exceptional strength and modulus due to their extended chain structures with high crystallinity and tie molecules. However, the extremely high melt viscosity and poor flowability due to their high entanglements pose a bottleneck that hinders further development, highlighting the need for innovative solutions to bridge the gap between material performance and processability.To overcome processing difficulties, one promising approach is chain topology design, as depicted in Figure 2. Compared to linear chains, polymers with short chain branches (SCBs), e.g., linear low-density polyethylene (LLDPE) polymerized from direct copolymerization or chain walking polymerization, have little effect on the rheological properties of the melt and exhibit enhanced flexibility and impact resistance. On the other hand, polymers with long chain branches (LCBs) exhibit much lower melt viscosity and enhanced processability at the same Mw level. However, high branching density will somewhat reduce the crystallinity and mechanical strength. Achieving superior mechanical properties and processability through branching control remains an ongoing pursuit. (2)Figure 2Figure 2. Development of commercial production of polyolefins.High Resolution ImageDownload MS PowerPoint SlideAnother common strategy to facilitate manufacturing or enhance polymer properties is the incorporation of small molecule additives, such as nucleating agents, plasticizers, antioxidants, and flame retardants. While these additives can improve processability and performance, the cumulative chemical complexity introduced by these additives poses significant environmental challenges, including toxicity, migration, and difficulties in recycling due to poor compatibility with the polymer matrix. (3) These issues have prompted researchers to explore alternative approaches that can achieve similar benefits without relying on external additives.One such approach is the use of polymers with bimodal molecular weight distribution (MWD), e.g., bimodal PE, as depicted in Figure 3. By modulating the MWD from unimodal to bimodal, it is possible to simultaneously enhance multiple properties of semicrystalline polymers without the need for additional additives or changes to the chain structure or chemical composition. In bimodal PEs, the low molecular weight (LMW) fraction acts as a built-in processing aid, contributing to low melt viscosity and high crystallinity, while the high molecular weight (HMW) fraction provides a high fraction of tie molecules, resulting in improved mechanical properties. (4−6)Figure 3Figure 3. A schematic diagram to show how unimodal and bimodal MWD shapes affect the structure in crystalline and amorphous phases via nucleation and growth process, and their impact on the mechanical properties of semicrystalline polymers. The comparison of unimodal and bimodal PEs is at the same Mw. (a) Unimodal and bimodal MWD shapes. (b) Nuclei. (c) Structure in crystalline and amorphous phases. The bold chains are tie molecules for passing through n = 4 layers of lamellae. (d) Mechanical properties. Reproduced with permission from ref (5). Copyright 2023 Elsevier Ltd.High Resolution ImageDownload MS PowerPoint SlideThis design raises an intriguing possibility: Can the LMW fraction in bimodal polymers be chemically functionalized to replace traditional small molecule additives? By tailoring the LMW fraction to perform specific functions, such as nucleation, plasticization, antioxidation, or flame retardation, it may be possible to address the diffusion and compatibility issues associated with small molecule additives, reduce their required dosage, and facilitate recycling. Such an approach would not only simplify the material composition but also minimize the environmental impact associated with additive migration and accumulation.The design of bimodal polymers, with their built-in processing aids and enhanced mechanical properties, offers a promising alternative to traditional small molecule additives. However, the challenges of plastic waste management extend beyond processing and performance; they also require innovative solutions for recycling.Branched polymers, particularly those with long-chain branches, have shown great potential in this regard. Their unique chain topology enhances compatibility between different polymers, making them ideal compatibilizers for the mechanical recycling of mixed plastic waste, e.g., polyethylene/isotactic polypropylene (PE/iPP) blends.Coates reported that only 1 wt % PE-graft-iPP copolymers, prepared using a grafting-through strategy, can achieve an efficient compatibilization of PE/iPP blends. (7) Recently, we developed a method using commercially viable ethylene propylene diene monomer (EPDM) rubber as compatibilizers for PE/iPP blends, as depicted in Figure 4. (8) The in situ reversible free radical reaction enables the grafting reaction between EPDM and PE (or iPP) at the interface, providing a facile approach for plastic recycling.Figure 4Figure 4. Schematic illustration of in situ grafting reaction for PE/iPP recycling. Reproduced with permission from ref (8). Copyright 2025 Elsevier Ltd.High Resolution ImageDownload MS PowerPoint SlideThe success of branched polymers in recycling applications highlights the importance of chain topology design in addressing plastic waste challenges. Beyond branching, another powerful approach lies in the use of block copolymers (BCPs). Unlike branched polymers, block copolymers consist of chemically distinct segments connected in a linear sequence, enabling precise tuning of their properties for specific recycling applications.Among block copolymers, the well-defined simplest diblock copolymers have long been investigated for polymer blend compatibilization. Symmetric diblock copolymers, in particular, are widely accepted as the most effective compatibilizers for polymer blends via one-pot mixing. (9,10) However, our recent work has revealed that asymmetric diblock copolymers can achieve even superior performance when used in sequential mixing strategies, as depicted in Figure 5. (11)Figure 5Figure 5. Schematic illustrations of the proposed mechanism of BCP distribution through sequential mixing (stages I and II) and the interfacial adhesion of vulcanized blends for (a) the neat PB/PI and (b) compatibilized blends with BCPs of longer PI block, (c) symmetric, and (d) shorter PI block. Block copolymer and homopolymer chains are identified with slightly different colors. Reproduced with permission from ref (11). Copyright 2023 American Chemical Society.High Resolution ImageDownload MS PowerPoint SlideBeyond the established diblock or triblock, multiblock copolymers, which combine multiple distinct domains, offer new opportunities for advanced material design and recycling applications. (12) Multiblock copolymers serving as compatibilizers demonstrate much enhanced performance for the mechanical recycling of PE/iPP blends. (13) However, the use of living polymerization for multiblock synthesis is not economically attractive due to its high catalyst usage, limiting their widespread adoption in industrial recycling processes.To address these challenges, significant progress has been made through innovative polymerization techniques. One such breakthrough is chain shuttling polymerization (CSP), which offers a practical and scalable approach to producing multiblock copolymers. Introduced by Dow Chemical in 2006, chain shuttling polymerization utilizes a pair of catalysts in the presence of a chain shuttling agent (CSA) to create copolymers with a statistical distribution of block lengths and numbers. (14) This method has revolutionized the industrial production of ethylene–octene multiblock copolymers, exemplified by the commercial product INFUSE olefin block copolymers (OBCs).Building on this success, our group recently discovered a new FI catalyst pair for ethylene–norbornene chain shuttling polymerization, leading to glassy cycloolefin block copolymers (COBCs), as depicted in Figure 6. (15) The tunability allows for the design of materials with tailored mechanical properties from plastics to elastomers, depending on the monomer concentration, catalyst ratio, and CSA ratio. Undeniably, COBCs demonstrate great potential as bulk materials in the elastomer market, or as compatibilizers in polymer waste recycling.Figure 6Figure 6. Schematic illustration of the synthesis of the cycloolefin block copolymers (COBCs) via chain shuttling polymerization and their mechanical properties from plastics to elastomers. Reproduced with permission from ref (15). Copyright 2024 American Chemical Society.High Resolution ImageDownload MS PowerPoint SlideThe emergence of OBCs and COBCs highlights a paradigm shift in elastomer design. Traditional elastomers, such as vulcanized rubber, rely on chemical cross-linking to achieve their mechanical properties. While effective, this approach involves complex processing and the use of various additives and results in materials that are difficult to recycle or reuse. By replacing chemical cross-linking with physical networks formed by the self-assembly of block copolymers, OBCs and COBCs not only simplify processing but also enable easier recycling, addressing one of the major limitations of traditional elastomers. Furthermore, their tunable properties open new possibilities for multifunctional materials that can meet the demands of diverse applications while minimizing environmental impact.So, what is the next direction for polymer design─chain untangling? To achieve polymers that integrate performance, processability, and recyclability, we need to address the challenges posed by chain entanglements. Untangled polymers demonstrate much lower melt viscosity even at ultrahigh molecular weight due to their low entanglements, as depicted in Figure 1.Untangling chains directly during polymerization seems viable. By optimizing polymerization conditions such as temperature, pressure, and catalyst types, Rastogi and Mecking achieved disentangled UHMWPE with reduced chain entanglements while maintaining high molecular weight. (16,17) Our group is also exploring this approach using single-chain supported catalysts, as depicted in Figure 7. (18)Figure 7Figure 7. Schematic diagrams of possible mechanisms leading to the formation of petal-like crystals during polymerization. Reproduced with permission from ref (18). Copyright 2024 Elsevier Ltd.High Resolution ImageDownload MS PowerPoint SlideAlternatively, exploring new methods through dynamic chains during processing, e.g., reversible bonds, (19) appears to be another workable solution, as depicted in Figure 8. Under processing conditions, reversible dynamic bonds can temporarily break to untangle the ultrahigh Mw linear chains into oligomers, reducing the effective chain length and viscosity, thereby improving processability. After processing and during practical use, these bonds can reform and combine oligomers, restoring the high molecular weight and mechanical properties of the materials. This method not only allows for the formation of high molecular weight polymers but also minimizes chain entanglements during processing, thereby enhancing both material properties, processability, and recyclability.Figure 8Figure 8. Schematic illustration of chain untangling for oligomer–polymer transformation via reversible bonds.High Resolution ImageDownload MS PowerPoint SlideLooking forward, the future of polymeric materials must not lose sight of their original design goals: high performance and ease of synthesis. At the same time, material design must evolve to prioritize not only performance but also environmental and circular friendliness. The ultimate solution to plastic pollution remains an entangled societal issue. Should we focus on biodegradation, which risks encouraging a throwaway culture and introducing microplastic toxicity? Or should we prioritize recycling, whether through chemical methods that recover monomers or physical methods that reuse polymers? While chemical recycling offers the promise of high-quality raw materials, it often comes with high energy costs and inefficiencies. Mechanical recycling, though simpler and more energy-efficient, faces challenges such as waste sorting and performance deterioration.To untangle this complex web, we must prioritize mechanical recycling as the first line of defense, resorting to chemical recycling only when physical methods are unfeasible. More importantly, we must return to the very essence of material design. By considering performance, processability, and recyclability from the first-principles of polymer physics─from chain size and topology design to dynamic bond engineering─we can create polymers that minimize processing and recycling challenges without compromising on performance. This approach not only optimizes the plastic lifecycle but also aligns with the principles of sustainable circularity.Collaborative efforts between academia and industry will be essential in translating these scientific breakthroughs into practical solutions. Together, we can untangle the societal issues surrounding plastic pollution by untangling the polymer chains that shape our world.Author InformationClick to copy section linkSection link copied!Corresponding AuthorZhong-Ren Chen - Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, China; Guangdong Provincial Key Laboratory of Catalysis, Southern University of Science and Technology, Shenzhen 518055, China; https://orcid.org/0000-0002-2929-9566; Email: [email protected]AuthorsZhiqiang Sun - Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaZhen Dong - Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaFeng Yu - Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaSitong Feng - Shenzhen Grubbs Institute and Department of Chemistry, Southern University of Science and Technology, Shenzhen 518055, ChinaNotesThe authors declare no competing financial interest.BiographiesClick to copy section linkSection link copied!Zhiqiang SunDownload MS PowerPoint SlideZhiqiang Sun is now a M.S. student at the Southern University of Science and Technology (SUSTech). He obtained his B.S. degree from SUSTech in 2022. His research focuses on the structure–property relationships of soft materials formed by the self-assembly of multiblock copolymers.Zhen DongDownload MS PowerPoint SlideZhen Dong received his Ph.D. degree at the Southern University of Science and Technology (SUSTech) in 2024 and M.Eng. and B.Eng. degrees from Harbin Institute of Technology in 2019 and 2017. His research focuses on olefin polymerization using tailored polymer-supported catalysts.Feng YuDownload MS PowerPoint SlideFeng Yu is now a Ph.D. student at the Southern University of Science and Technology (SUSTech). He obtained his M.S. degree from Ningbo University in 2017 and B.S. degree at the Wuhan Donghu University in 2014. His research focuses on novel compatibilizer design and mechanical recycling of polyolefin blends.Sitong FengDownload MS PowerPoint SlideSitong Feng is now a Ph.D. student at the Southern University of Science and Technology (SUSTech). He obtained his M.S. and B.S. degrees from Harbin Institute of Technology in 2020 and 2018. His research focuses on the construction of a covalent adaptable polymer network through ring-opening metathesis polymerization.Zhong-Ren ChenDownload MS PowerPoint SlideZhong-Ren Chen received his B.Eng. and M.Eng. degrees from Zhejiang University in 1984 and 1987 and Ph.D. degree in Chemical Engineering and Chemistry in 1998 from the California Institute of Technology (Caltech) under the joint supervision of Julia A. Kornfield and Robert H. Grubbs. Then he pursued postdoctoral research in chemistry at Stanford University with Robert M. Waymouth. From 2000 to 2011, he joined Bridgestone Americas as a research scientist in the U.S. Currently, he has been a Chair Professor in the Department of Chemistry at the Southern University of Science and Technology (SUSTech) since 2016. His group focuses on polyolefin research guided by polymer physics.AcknowledgmentsClick to copy section linkSection link copied!This work was financially supported by the State Key Research Development Programme of China (Grant Nos. 2021YFB3800702 and 2021YFB3800705), the National Natural Science Foundation of China (22075124) and Shenzhen Special Fund (Grant Nos. JCYJ20190809115013348 and JCYJ20210324103811030), and Guangdong Provincial Key Laboratory of Catalysis (No. 2020B121201002). The authors acknowledge the assistance of SUSTech Core Research Facilities.ReferencesClick to copy section linkSection link copied! This article references 19 other publications. 1Flory, P. J.; Yoon, D. Y. Molecular morphology in semicrystalline polymers. Nature 1978, 272 (5650), 226– 229, DOI: 10.1038/272226a0 Google Scholar1Molecular morphology in semicrystalline polymersFlory, Paul J.; Yoon, Do YeungNature (London, United Kingdom) (1978), 272 (5650), 226-9CODEN: NATUAS; ISSN:0028-0836. The circumstances were obsd. under which crystn. of randomly coiled long chain polymers proceeded. The first sequences of a mol. to undergo crystn. may be remote from one another along the length of the mol., the sequences entering the same or different layers at the edge of the lamella. A mol. is affixed to the crystal by incorporation of several sequences, distributed over the length of the mol., before the mol. undergoes crystn. The majority of sequences acquired by the cryst. layers are contributed by mols. which are already engaged in crystal lamellae and which prevents overall mol. rearrangement. Large irreversible deformations entail melting which is immediately followed by recrystn., a different array of sequences being incorporated in the regenerated crystallite in a pattern compliant with the prevailing stress. >> More from SciFinder ®https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaE1cXkslGmsbk%253D&md5=5927913e7ebd378bea76d41568ea8b162Froese, R. D.; Arriola, D. J.; den Doelder, J.; Hou, J.; Kashyap, T.; Lu, K.; Martinetti, L.; Stubbert, B. D. A commercially viable solution process to control long-chain branching in polyethylene. Science 2024, 383 (6688), 1223– 1228, DOI: 10.1126/science.adn3067 Google ScholarThere is no corresponding record for this reference.3Law, K. L.; Sobkowicz, M. J.; Shaver, M. P.; Hahn, M. E. Untangling the chemical complexity of plastics to improve life cycle outcomes. Nat. Rev. Mater. 2024, 9 (9), 657– 667, DOI: 10.1038/s41578-024-00705-x Google ScholarThere is no corresponding record for this reference.4Long, C.; Dong, Z.; Liu, X.; Yu, F.; Shang, Y.; Wang, K.; Feng, S.; Hou, X.; He, C.; Chen, Z.-R. Simultaneous enhancement in processability and mechanical properties of polyethylenes via tuning the molecular weight distribution from unimodal to bimodal shape. Polymer 2022, 258, 125287 DOI: 10.1016/j.polymer.2022.125287 Google Scholar4Simultaneous enhancement in processability and mechanical properties of polyethylenes via tuning the molecular weight distribution from unimodal to bimodal shapeLong, Chuanjiang; Dong, Zhen; Liu, Xiaoqing; Yu, Feng; Shang, Yuxuan; Wang, Keqiang; Feng, Sitong; Hou, Xunan; He, Chaobin; Chen, Zhong-RenPolymer (2022), 258 (), 125287CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.) Semicryst. polymers with bimodal mol. wt. distribution (MWD) have captured broad interest in academia and industry. Current efforts to understand the effect of bimodal MWD shape compared with unimodal shape are mixed with factors such as mol. wt., chain branching or phase sepn. The collective effect is difficult to decouple in order to elucidate the independent contribution of MWD shape on the properties of polymers. In this work, a chain transfer polymn. method has been utilized to prep. well-defined linear unimodal and bimodal polyethylenes (PEs), to serve as a model system for the study of the impact of the MWD shape on the crystn., rheol. and mech. properties of semicryst. polymers. It has been demonstrated that PEs with bimodal shape display simultaneously enhanced crystallinity, processability, Young's modulus and tensile strength while ductility is unaffected. Compared to unimodal PEs with comparable mol. wts., bimodal PEs show about 40% lower processing viscosity, while exhibiting up to 30% greater tensile strength. This represents the first systematic comparative investigation of MWD shape-property relationship at the same mol. wt. over a wide mol. wt. range. This work indicates that semicryst. polymers with bimodal MWD shape have the merit to overcome the trade-off between processing and performance without altering av. mol. wt., chain structure or chem. compn. >> More from SciFinder ®https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB38Xitl2ksb7K&md5=4a4a8f758f8793b6f981edd4187d61335Long, C.; Dong, Z.; Wang, K.; Yu, F.; He, C.; Chen, Z.-R. Molecular weight distribution shape approach for simultaneously enhancing the stiffness, ductility and strength of isotropic semicrystalline polymers based on linear unimodal and bimodal polyethylenes. Polymer 2023, 275, 125936 DOI: 10.1016/j.polymer.2023.125936 Google ScholarThere is no corresponding record for this reference.6Long, C.; Dong, Z.; Yu, F.; Wang, K.; He, C.; Chen, Z.-R. Molecular Weight Distribution Shape Dependence of the Crystallization Kinetics of Semicrystalline Polymers Based on Linear Unimodal and Bimodal Polyethylenes. ACS Appl. Polym. Mater. 2023, 5 (4), 2654– 2663, DOI: 10.1021/acsapm.2c02236 Google Scholar6Molecular Weight Distribution Shape Dependence of the Crystallization Kinetics of Semicrystalline Polymers Based on Linear Unimodal and Bimodal PolyethylenesLong, Chuanjiang; Dong, Zhen; Yu, Feng; Wang, Keqiang; He, Chaobin; Chen, Zhong-RenACS Applied Polymer Materials (2023), 5 (4), 2654-2663CODEN: AAPMCD; ISSN:2637-6105. (American Chemical Society) The manipulation of the crystn. kinetics and crystallinity of polymers without altering their chem. compn., chain structure, or av. mol. wt. is challenging, while little attention is paid to the role of mol. wt. distribution (MWD) shape. In this work, a series of well-defined linear unimodal and bimodal polyethylenes (PEs) with mol. wts. ranging from 300 k to 1200 k g/mol were synthesized to serve as a model system to study the impact of bimodal MWD shape on the crystn. kinetics of semicryst. polymers in comparison with unimodal MWD shape at the same wt.-av. mol. wt. (Mw). It is shown that PEs with a bimodal shape exhibit a faster nucleation rate and crystn. rate with a smaller lamellar width at low isothermal temps. A higher crystn. enthalpy of bimodal PEs is shown in non-isothermal expts. The mechanism behind this is elucidated, which