Synthesis of Poly(Lactic Acid):A Review
RAJEEV MEHTA,1VINEET KUMAR,1HARIPADA BHUNIA,1动员大会主持词
AND S.N.UPADHYAY 2
1Department of Chemical Engineering,Thapar Institute of Engineering and
Technology,Patiala,Punjab,India 2Department of Chemical Engineering and Technology,Institute of Technology,
Banaras Hindu University,Varanasi,India
Poly(lactic acid),a bio-degradable polymer,has been studied extensively during the
past 15years.This paper prents a review on poly(lactic acid)(PLA)with focus on
its stereochemistry,synthesis via ring-opening polymerization,reaction kinetics and裙子的英文
thermodynamics,synthesis of low molecular weight polymer,a continuous process
for production of PLA from lactic acid,and blends.The different polymerization
mechanisms,which have been propod in the literature,are also summarized.
Various catalyst systems,solvents,and reaction temperature and time give products
of an entire range of molecular weights,ranging from a few thousand to over a
million.Modeling and simulation of the ring-opening polymerization of PLA is also
discusd.
Keywords poly(lactic acid),bio-degradable polymer,ring-opening polymerization,
dehydratization,blends,modeling and simulation
Introduction
Worldwide production of high-volume consumer plastics continues to be dominated by petroleum-bad polymers.Two factors have made biodegradable polymers economically attractive.The are (i)environmental and economic concerns associated with waste disposal and (ii)rising costs of petroleum production resulting form the depletion of the most easily accessible rerves.Ea of deg
radability,versatility and anticipated price as well as performance of the new generation of polymer,polylactic acid (PLA),will enable it to displace a significant volume of fossil fuel-bad polymers.Poly(glycolic acid)(PGA)was the first biodegradable polymer synthesized (1).It was followed by poly(lactic acid)(PLA)and copolymers of the two (2).The a -polyesters have been investigated for u as sutures and as implant materials for repairing a variety of tissues
(3).‘Vicryl’suture,a blend of PGA (90%)and PLA (10%)was introduced in USA in 1974.PLA was discovered in 1932by Carothers (DuPont)who produced a low
Received 20May 2005,Accepted 1August 2005
Address correspondence to S.N.Upadhyay,Department of Chemical Engineering and Technology,Institute of Technology,Banaras Hindu University,Varanasi,India 221005.Tel.:þ91-542-2317179;Fax:þ91-542-2368428;E-mail:
Journal of Macromolecular Science w ,Part C:Polymer Reviews ,45:325–349,2005
Copyright #Taylor &Francis,Inc.
ISSN 1532-1797print /1532-9038online
on the way什么意思
DOI:
10.1080/15321790500304148
325
molecular weight product by heating lactic acid under vacuum.The inability,at that time to increa the molecular weight of the product,led to discontinuation of further studies.Later on,high molecular weight PLA was synthesized by ring-opening polymerization of lactide.
According to the European Society for Biomaterials (4)‘biodegradation’is a process in which biological agents (enzymes or microbes)play a dominant role in the degrada-tion process.Unlike other biodegradable polymers the degradation of PLA is not biodegradation as it is caud by hydrolytic cleavage of the polymer backbone,with little or no evidence of participation of enzymes.However,PLA is commonly termed as a biodegradable polymer.
PLA can be manufactured with a wide range of properties,becau of lactic acid being chiral with two
asymmetric centers existing in four different forms.It can be made into a polymer with molecular weight ranging from a few thousands to over a million.It can be easily converted into film,fiber,spun bond,and melt blown products on existing processing equipment.
PLA has practical medical applications as dissolvable sutures,as matrices for drug delivery,and bone fracture internal fixation devices in surgery (5).Other applications include its role as agricultural plant growth promoter,in textiles,and non-woven applications such as fiberfill,crop covers,geotextiles,wipes,diapers,binder fibers,etc.However,the total volume of PLA production remains small compared to other polymers (PE reached a global level of 54million tones in 2001,which can be compared to the figure of about 390,000tonnes for PLA which is expected to be produced by 2008).
The major driver for PLA production is its high volume u as fibers.PLA was developed as an alternative binder for cellulosic non-wovens becau of its ea of hydrolysis compared with polyvinyl acetate or ethylene-acrylic acid copolymers.Spun-laid and melt-blown non-wovens bad on PLA were rearched at the University of Tenne,Knoxville in 1993(6).Kanebo (Japan)introduced Lactron w (poly L -lactide)fiber and spun-laid non-wovens in 1994claiming a capacity of 2000tons per annum,later expanded to 3000tons per annum.It targeted agricultural applic
ations to start with,and in 1998was re-launched for apparel end-us.At that time,Japane demand for PLA fibers was said to be 500–1000tons per annum.In order to improve the biode-gradability and reduce the cost of the non-wovens,blends with rayon were also developed.rh
Cargil Dow Polymers LLC,the prent leader in polylactic acid technology,is a 50-50joint-venture between Cargill and Dow and was formed in November 1997.In 2004,they started the world’s first full-scale PLA plant in Blair,Nebraska,capable of producing 140,000tons per annum.
Polylactic acid,first synthesized about 50years ago,has finally arrived as an alterna-tive to PET,HIPS,PVC,and cellulosics in some high-clarity packaging roles.PLA is being ud in candy wrap,optically enhanced films,and shrink labels.It is also showing up as the alant layer in form-fill-al coextrusions.The novel resin is forging roles in thermoformed cups and containers and is about to appear in single-rve drink bottles.A comparison of the mechanical and barrier properties of PLA and some other polymers is given in Tables 1and 2,and 3(7,8).Recent developments in the capability to manufacture the monomer economically from renewable feed stocks have placed the materials at the forefront of the emerging biodegradable plastics industry.Increasing realization of the intrinsic properties of the polymers,coupled with the knowledge of how such properties can be manipulated to achieve compatibility with thermoplastics pro-cessing,manufacturing,and end-u requirements,ha
ve fuelled the technological and commercial interests in PLA products.
R.Mehta et al.
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The objective of this review is to prent a comprehensive picture of PLA synthesis,with emphasis on the various catalysts and some of the polymerization mechanisms,ud for ring-opening polymerization.
Monomer
An important feature of lactic acid is that it exists in two optically active forms (Fig.1).Fermentation derived lactic acid consists of 99.5%of the L -isomer.Production of the cyclic lactide dimer intermediate results in three potential forms.The D -and L -forms are optically active,the D,L -or meso-form is optically inactive.The ratio of the three forms is readily controlled in the process.Ring opening polymerization of the lactide results in a “family”of polymers containing different isomer ratios and in a range of molecular weights.Polymers with high L -levels can be ud to produce crystalline products whereas the higher D -levels (.15%)result in an amorphous product.
The thermal,mechanical,and biodegradation characteristics of lactic acid polymers are known to depend on the choice and distribution of stereoisomers within the polymer chains.High-purity L -and D -lactides form stereo regular isotactic poly(L -lactide)
Table 1
Properties of biaxially oriented films (7)
PLA
PP PET Nylon Cellophane Density,g /cc
1.250.9 1.4 1.2 1.45Haze,%
2.11–42–52–31–2Tensile str.,psi,MD,
15,95027,550a 29,725a 36,25013,050a Tensile mod.,psi,MD
478,500348,000551,000a 264,625594,500a Ult.elongation,%,MD
160110a 140a 125a 23a Tear,g /mil,MD
流利说英语154–618134a Median of a range of values.
Table 2
Barrier performance of clear resins (7)
Polymer
MVTR a Permeation b
Oxygen CO 2PLA
2140183HIPS
10300–400NA Nylon 6
233NA PET
圆脸适合什么样的发型13–615–25PP
0.7150NA PVC
25–2020–50a
g-mil /100in.2-day.b cc-mil /100in.2-day-atm.
Synthesis of Poly(Lactic Acid)
327
(PLLA)and poly(D -lactide)(PDLA),respectively.The are mi crystalline polymers with a high melting point 1808C and a glass transition temperature in the 55–608C range.The degree of crystallinity depends on many factors,such as molecular weight,thermal and processing history,and the temperature and time of annealing treatments.The meso-and D,L -lactide,on the other hand,form atactic poly (D,L -lactide)(PDLLA)which is amorphous (9).The mechanical properties and degradation kinetics of the mi-crystalline PLLA are quite different from tho of completely amorphous PDLLA.
Table 3
Mechanical properties of PGA and PLA a
Polymer Glass
transition
(8C)
Melting temp.(8C)Tensile strength (MPa)Flexural modulus (MPa)Elongation
Yield (%)Break (%)PGA (MW:
50,000)
35210n /a n /a n /a n /a PLA
L -PLA
(MW:50,000)
54170281400 3.7 6.0L -PLA (MW:
1,00,000)
58159503000 2.6 3.3L -PLA (MW:
3,00,000)
59178483250 1.8 2.0D ,L -PLA
(MW:20,000)
爱词霸英语50—n /a n /a n /a n /a D ,L -PLA
(MW:1,07,000)
51—291950 4.0 6.0D ,L -PLA (MW:
5,50,000)
53—352350 3.5 5.0a Bad on data published by Engleberg and Kohn (8).
n /a ¼not available.教学评价方法
(—)¼not
applicable.
Figure 1.Three different lactides.
R.Mehta et al.
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In particular,PLLA is always preferred whenever higher mechanical strength and longer degradation time is required (10).角色扮演英文
The PLA lifecycle is shown in Fig.2.After formation of PLA oligomers from lactic acid by removal of water,depolymerization of oligomers takes place to give a thermodynamically favored lactide.This monomer is readily polymerized under vacuum distillation.It is ud to produce high molecular weight PLA by ring-opening polymerization.The high molecular weight PLA can be degraded to lactic acid in prence of water.
Synthesis of Polylactic Acid from Lactic Acid
Synthesis of Low Molecular Weight Polylactic Acid
by Condensation Polymerization
There are two major routes to produce polylactic acid from the lactic acid monomer (Fig.3).Route 1involves removal of water of condensation by the u of solvent under high vacuum and high temperature.Polycondensation of lactic acid is usually performed in bulk by distillation of condensation water,with or without a catalyst,while vacuum and temperature are progressively incre
ad.Although,high molecular weight polyesters with good mechanical properties are not easy to obtain,the properties of lactic acid oligomers,which can be further ud as intermediates in the synthesis of polyurethanes,can be controlled by the u of different catalysts and functionalization agents,as well as by varying the polymerization conditions (11–13).This approach was ud by Carothers and is still ud by Mitsui Toatsu manufacture a low to intermediate molecular weight polymer.
Addition of acidic catalysts,such as boric or sulfuric acid (14)accelerates the ester-ification and transterification process,but also catalyzes side reactions at temperatures above 1208C.Therefore,polycondensation of aqueous DL -lactic acid never gave M n ’s above 3000(14,15),as against M n ’s up to 6500reported for pure L -lactide.When the pre-condensates obtained by dehydratization up to 1208C are heated to 1808C in the prence of nonacidic transterification catalysts,such as PbO,a moderate yield of relatively high-molecular weight PLA may result (16).Yet,even the low molecular
rockweight Figure 2.PLA lifecycle.
Synthesis of Poly(Lactic Acid)329
precondensates may be of interest for the preparation of biodegradable glues or lacquers, becau the–OH and–COOH end groups allow cross-linking with suitable inorganic or organic multivalent additives(15).
Polycondensation method produces oligomers with average molecular weights veral tens of thousands lower than tho of PLA synthesized by one-step polycondensa-tion of lactic acid if appropriate azeotropic solvents are employed(17–19).This polymer can be ud as it is,or can be coupled with isocyanates,epoxides or peroxide to produce polymers having a range of molecular weights.An alternative method,route2,is discusd below.
Synthesis of High Molecular Weight Polylactic Acid
by Ring-Opening Polymerization of Lactide
Water is removed under mild conditions,without a solvent,to produce a cyclic inter-mediate dimer,referred to as lactide.This monomer is readily purified by vacuum distilla-tion accomplished by heating,again without the need for solvent.By controlling the purity of the dimer it is possible to produce a wide range of molecular weights.
PLA has been produced commercially worldwide since the last decade.The catalyst currently ud industrially is stannous octoate(zinc metal has been in u in France).Ring-opening polymerization of lactide can be carried out in melt,bulk,or in solution and by cationic,anionic,and coordination-inrtion mechanisms depending on the catalyst (20–22).The choice of initiator system,co-initiator as chain control agent,catalyst concentration,monomer-to-initiator ratio,and polymerization temperature and time significantly affect the polymer properties.The properties,such as the molecular weight,degree of crystallinity and residual monomer content,in turn affect the physical-mechanical properties and range of temperature for u of the polylactide and its copolymers(23–29).The role of the racemization and the extent of transterification in the polymerization and copolymerization process are also decisive for the enantiomeric purity and chain microstructure of the resulting polymer(30–33).Many current PLA polymerization methods employ stannous octoate as the catalyst.It has been shown to be very effective,caus a low degree of racemization at high temperature(34),has low toxicity,and is accepted by the US Food and Drug Administration.
A lective summary of literature on PLA synthesis is given in the Table4 (35–51).Other recent studies include dimeric aluminum chloride complexes of N-alkoxy-alkyl-b-ketoimines(activated with propylene oxide)(52),alkoxy-amino-bis(phenolate)
