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BACKGROUND AND SUMMARY OF THE INVENTION
This invention relates to the preparation of biodegradable thermoplastic
graft copolymers. More particularly, this invention relates to an improved
process for converting a polysaccharide to a biodegradable thermoplastic
graft copolymer not only capable of forming homogeneous blends with
synthetic thermoplastic polymers itself, but further capable of
functioning as compatibilizing/stabilizing agents for blends of cellulose
and starch with synthetic plastics. Nucleophilically displaceable groups
are formed on the polysaccharide and are thereafter displaced by selected
polymers having anionic carboxylate or thiocarboxylate groups, to provide
high yields of well-defined, multifunctional biodegradable thermoplastic
graft copolymers.
Increasingly strident environmental concerns have put pressure on federal
and state legislatures to mandate plastics degradability. This is a push
that directly affects an annual 1.8 billion pounds of business, mostly
packaging, done by the plastics industry. Plastics take as much as 200
years to degrade in landfill. Plastic litter dumped into the oceans every
day cause heavy loss of marine and animal life. Naturally occurring
biopolymers like starch and cellulose are readily biodegradable (degrades
only in soil, sewage and marine environments where bacteria are
active--that is, biologically active environments only--precisely the
conditions where onset of degradation is desired). Incorporation of these
types of biopolymers plastics (styrenic plastics or polyethylene or
polymethyl methacrylate plastics) by blending or graft copolymerization
should lead to a new type of plastics having the trait of
biodegradability. However, preparing a new material system by mixing two
incompatible polymers as in the present case results in products with
reduced physical properties. Strength and toughness values are minimal and
are lower for the mixture than any of the pure components. This situation
arises from poor interfacial adhesion between the individual components
due to their inherent incompatibility. It is like trying to mix or
disperse oil and water. The solution to this incompatibility problem which
is widely practiced in the polymer industry, uses block or graft
copolymers of the form A-B as compatibilizers or interfacial agents to
improve adhesion between immiscible A rich and B-rich phases. To function
effectively as a compatibilizer, the following is true: (1) components of
the graft copolymers must be identical with the polymers in the two phases
(identical with the 2 dissimilar polymers which needs to be blended); (2)
molecular weight of the segments plays an important role, and control over
the molecular weights is essential; (3) molecular weights greater than
150,000 is generally a poor compatibilizer; (4) block or graft copolymers
segments containing 10-15 monomer units is an effective compatibilizing
agent for the corresponding higher molecular weight homopolymer.
Thus, the key to the incorporation of natural biopolymers like starch and
cellulose in a plastics materials system to make biodegradable/bio based
plastics is the ability to tailor cellulose/starch synthetic polymer graft
copolymer structures with control over the molecular weights of the graft,
degree (amount) of graft substitution and control over backbone graft
linkage. Current technology does not permit the making of cellulose/starch
(natural biopolymers)--synthetic polymer graft copolymers with precise
control over molecular weights, degree of substitution, backbone graft
linkage, etc., i.e., cannot make precise tailor made cellulose/starch
graft copolymers.
The present invention allows the preparation of tailor-made
cellulose/starch synthetic polymer graft copolymers with good control over
molecular weights degree of substitution, backbone-graft linkage. These
graft copolymers can function effectively as compatibilizing
agents/interfacial agents for compounding/blending of cellulose and starch
with synthetic polymers. The graft copolymer allows a fine dispersion of
the natural polymer into the plastic phase without detracting from the
excellent mechanical and thermal properties inherent in the plastic, while
incorporating a new trait of biodegradability.
The grafting of synthetic polymers onto polysaccharides and polysaccharide
derivatives has been described in the art. Preparation of cellulosic graft
polymers utilizing free radical polyerization methods has been reported by
McDowall, Gupta, and Stannett, Prog. Polym. Sci. 1984, 10, 1; Hebeish and
Guthrie, The Chemistry and Technology of Cellulosic Copolymers, Berlin,
1981; Arthur, Adv. Macromol. Chem. 1970, 2, 1 See also U.S. Pat. No.
4,026,849. Polyisobutylene-grafted cellulose products have been prepared
by reacting anhydride-terminated polyisobutylene with sodium cellulosate.
Coleman-Kammula and Hulskers Wood and Cellulosics--Industrial Utilisation,
Biotechnology, Structure and Properties 1987, 195-202. The successful
grafting of the polyisobutylene onto cellulose involves the conversion or
activation of cellulose to cellulosates. Polystyrene has been grafted onto
cellulose acetate with a grafting yield of up to 83% using the acid
chloride of carboxylic acid-terminated polystyrene. Mansson and Westfelt,
J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1509. This method involves the
acylation of the free hydroxyl groups on the cellulose acetate by the
polystyrene acid chloride. Other known methods for preparing graft
copolymers include the simultaneous polymerization and grafting of an
ethylenically unsaturated monomer onto the molecule of a polysaccharide
and thereafter reacting the grafted polysaccharide, in the presence of a
catalyst, with an acylating agent to form a polysaccharide ester
derivative. See U.S. Pat. No. 3,332,897.
The known methods for synthesizing polysaccharide-synthetic polymer graft
copolymers have several disadvantages. For example, the molecular weights
of graft copolymers produced by free radical polymerization techniques are
very high and the molecular weight distribution in such copolymers is
polydisperse. The reproducibility of these polymerization methods is also
poor and there is little control over the grafting process. Thus, the
resultant graft copolymers exhibit a low level of graft substitution
typically with very high molecular weight graft molecules. Likewise,
products prepared by other prior art polymerization reactions have
considerable disadvantages. For example, products prepared by reacting a
polysaccharide and an acid anhydride cannot be molded easily, if at all.
Typically, such molded products are brittle, inflexible and entirely
unsuitable for commercial utilization. Moreover, to achieve high grafting
efficiencies, strictly anhydrous conditions must be used along with fairly
large amounts of acylation catalysts such as 4-(dimethylamino) pyridine
with reaction periods of up to 3 days. Thus, there is the need in the art
for an improved method of synthesizing biodegradable polysaccharide graft
copolymers.
It is an object of this invention to provide an improved method of
preparing polysaccharide-synthetic polymer graft copolymers.
A further object of this invention is to provide an economic, commercially
feasible procedure for achieving high yields of well-defined biodegradable
graft copolymers.
An additional object of this invention is to provide a grafting method
which allows greater control of the molecular weight distribution and the
number and nature of the grafted sidechains.
Still another object of this invention is the use of such novel
biodegradable thermoplastic copolymers alone and for blending with
synthetic thermoplastic polymers, with or without other added
biodegradable polysaccharides, to provide homogeneous, tough, high
strength biodegradable plastics.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates use of carbonionic nucleophilic graft copolymerization.
FIGS. 2-3 illustrate reaction schemes for preparation of biodegradable
graft copolymers via carboxylates in accordance with the invention.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method is provided for
converting a polysaccharide to a biodegradable thermoplastic graft
copolymer which is not only capable itself of forming uniform blends with
synthetic thermoplastic polymers but is also surprisingly effective for
compatibilizing blends of synthetic thermoplastic polymers and
biodegradable polysaccharides.
The method comprises the steps of chemically treating said polysaccharide
to form on said polysaccharide nucleophilically displaceable groups at a
substitution level of about 0.1 to about 0.75 nucleophilically
displaceable groups per anhydroglucose unit of said polysaccharide;
reacting said polysaccharide bearing nucleophilic displaceable groups with
an anionic thermoplastic polymer carboxylate or thiocarboxylate under
conditions conducive to nucleophilic displacement of said groups by said
anionic carboxylate or thiocarboxylate; and isolating the resulting
biodegradable thermoplastic graft copolymer.
Suitable polysaccharides include starch, chitin, lignin, cellulose and
derivatives thereof. Starch and cellulose and their commercially available
ethers and esters are preferred. Cellulose acetate has been found to be
particularly well suited as a starting material for use in the present
method.
The anionic polymer carboxylate or thiocarboxylate can be derived, for
example, by acid or base hydrolysis of corresponding polymer esters. A
preferred anionic polymer of the formula
##STR1##
wherein R.sub.1 is C.sub.1 -C.sub.6 alkyl or a group of the formula
##STR2##
can be prepared by (1) anionic polymerization of a compound of the formula
R--CR'.dbd.CH.sub.2 and (2) treatment with a compound of the formula
X.dbd.C.dbd.Z, wherein in the above formulas R and R' are independently
hydrogen, C.sub.1 -C.sub.6 alkyl, phenyl, substituted phenyl or
methoxycarbonyl; X and Z are each independently O or S; M.sup.+ is an
alkali metal cation; and n is an integer such that the group
##STR3##
has a molecular weight between about 3000 and about 25,000, more
preferably between about 5000 and 18,000; reacting said anionic
thermoplastic polymer with said polysaccharide bearing nucleophilically
displaceable groups under conditions conducive to nucleophilic
displacement of said groups by said anionic polymer; and isolating the
resulting biodegradable thermoplastic graft copolymer. The molecular
weight of such anionic polymers can be controlled by the nature of the
base used to catalyze the anionic polymerization reaction and the molar
ratio of base to the polymerizable monomer (RCR'.dbd.CH.sub.2) For
example, the molecular weight of polystyrylcarboxylate obtained by the n
butyl lithium initiated polymerization of polystyrene can be estimated by
the formula:
##EQU1##
Suitable nucleophilic displaceable groups are halo, sulfate, haloacetoxy,
lower alkane (methyl, ethyl, propyl, butyl)sulfonyloxy, benzylsulfonyloxy
and aryl (e.g., phenyl and substituted phenyl)sulfonyloxy, e.g.,
benzenesulfonyloxy, p toluenesulfonyloxy and the like. Skilled
practitioners will readily appreciate what conditions/reagents are
appropriate to introduce such displaceable groups on the polysaccharide
chain.
Preferred DS (degree of substitution) for the polysaccharide copolymer is
somewhat dependent on the DP (degree of polymerization) of the anionic
polymer. Where the anionic polymer has a DP such that its average
molecular weight is in a preferred range of 5000 to 18,000, the DS of the
graft copolymers preferably should be in the range of 1 polymer
substituted to every 15 to 100 glucose units, more preferably 1
substituent to every 15 to 50 glucose units, in the copolymerized
polysaccharide chain.
The present method avoids the problems encountered with prior art
preparations of polysaccharide based plastics. The method employed in this
invention has been found to provide unprecedented control over the
grafting mechanism. Because the reaction chemistry involves an SN.sub.2
type nucleophilic displacement reaction by the synthetic polymer, there is
no uncertainty in the nature of the backbone-graft linkage. Homopolymer
formation is minimized and, if formed, is easily extractable. The
resultant graft polymer composition, including structure, molecular
weight, molecular weight distribution, and sequence distribution are well
defined and readily reproducible. The degree of substitution (DS) of the
graft copolymer is controlled by controlling the ratio between the
reactive sites on the polysaccharide backbone and the synthetic
carboxylate polymer.
The present invention is based on the discovery that by modifying
carbonations to form a more controllable, less reactive group, better
control of the grafting process can be obtained with negligible side
reactions. Carboxylate-terminated or thiocarboxylate synthetic polymers
are sufficiently nucleophilic to displace better leaving groups such as
halo, haloacetoxy, lower-alkane methane sulfonyloxy, or arylsulfonyloxy,
with the concomitanant formation of an ester linkage between the synthetic
polymer and the polysaccharide. A further advantage of the direct use of
the carboxylate-terminated polymer is that water does not interfere with
the grafting reaction.
Polymers produced by art-recognized anionic polymerization techniques are
favored intermediates for carboxylate terminated polymers since the later
can be derived simply by addition of carbon dioxide (or the sulfur
analogues thereof: COS and CS.sub.2) immediately following the anionic
polymerization reaction is complete.
FIG. 1 depicts a three-step procedure for the preparation of cellulose
graft polystyrene by reaction with a polystyryl carbanion. Cellulose
acetate is first tosylated for example, by reaction with tosyl chloride in
the presence of a proton acceptor. Styrene is polymerized utilizing
n-butyllithium in THF, typically at dry ice/acetone temperature. Step 2
also includes capping the anionic form of the resulting polystyrene with
1,1-diphenylethylene to stabilize the anionic polymer prior to its use in
the Step 3 reaction with cellulose tosylate. The reaction carbanions with
polysaccharides having displaceable groups have been found much more
difficult to control than those with the carboxylate derivatives.
FIG. 2 illustrates the grafting of polystyrylcarboxylate anion 1 onto
mesylated cellulose acetate. FIG. 3 illustrates the formation of the
polystyryldicarboxylate anion 2 by carbon dioxide quenched, sodium
napthalene induced, anionic polymerization of styrene, and the use of that
dicarboxylate intermediate to form, upon reaction with mesylated cellulose
acetate, a controlled crosslinked graft copolymer.
The graft copolymers in accordance with this invention are useful for the
construction of articles of manufacture, for example, food packaging
articles, which not only have the favorable structural characteristics of
synthetic plastics but also are biodegradable. The graft copolymers can be
blended with art-recognized thermoplastic resins, such as polyethylene,
polypropylene, polystyrene, polymethylmethacrylate or polyacrylonitrile in
a ratio of about 1:8 to about 5:2, respectively, to provide structurally
functional yet biodegradable plastic blends. Increased graft copolymer
content corresponds to faster rates of biodegradation.
The present graft copolymers are perhaps most advantageously employed as
agents to compatibilize synthetic thermoplastic polymers and biodegradable
polysaccharide fillers or additives. Thus biodegradable blends having
functional characteristics very similar to that of the unfilled synthetic
thermoplastic polymer can be prepared by blending the synthetic
thermoplastic polymer with about 5 to about 40 weight percent (based on
wt. of blend) of polysaccharide, for example, granular starch or cellulose
acetate, and about 5 to about 30 weight percent of a graft copolymer
(preferably of a polysaccharide of a structure closely related to that of
the polysaccharide added to the blend). The blend can then be subject to
extrusion processing or it can be utilized in other art recognized
manufacturing procedures to form biodegradable articles of manufacture.
Detailed Description of the Invention
EXAMPLE 1
Grafting of Polystyrene onto Cellulose Acetate
Polystyrylmono- and -dicarboxylate anions were prepared in tetrahydrofuran
by using n-butyllithium and sodium naphthalene as the initiators,
respectively, at -78.degree. C. The carbanions were reacted with dry
carbon dioxide. The products were precipitated in methanol, filtered,
washed with water and methanol, and dried. GPC analysis established
molecular weight values of 6200 for the polystyrylmonocarboxylate and
10,900 for the polystyryldicarboxylate products.
Cellulose acetate (Eastman Kodak, 40% acetyl, DS 2.5) (10g) was mesylated
by the procedure of Wolfrom et al. (Wolfrom, M. L., Sowden, J. C., and
Metcalf, E. A., J. Am. Chem. Soc. 1941, 63, 1688) with 6 ml of
methanesulfonyl (mesyl) chloride in 200 ml of pyridine for four days at
room temperature. Elemental analysis gave 45.61% C, 5.52% H, and 4.90% S
corresponding to mesyl substitution of 0.46 mesyl groups per
anhydroglucose unit. Cellulose acetate with a lower mesyl contact was
prepared by reacting 50 g of cellulose acetate in 500 ml of pyridine and
3.50 ml of mesyl chloride for one day at room temperature. Elemental
analysis gave 46.70% C, 5.68% H, and 2.42% S, corresponding to a DS of
0.21 for the mesyl groups.
The grafting reaction was carried out by adding 0.50 g of mesylated
cellulose acetate and 1.00 g of polystyrylcarboxylate to a 25-ml
Erlenmeyer flask with 20 ml of solvent (4:1 dimethylformamide,
dimethylsulfoxide:THF, or, in a few reactions, other solvents). When
dimethylsulfoxide was used as the solvent, some THF was added in order to
dissolve the polystyrene.
Initial grafting reactions with the mesylated cellulose acetate and lithium
polystyrylcarboxylate were carried out at room temperature for three days
in acetone and methylene chloride. Analysis of these samples showed that
about 5-15% grafting yields were realized. Modifying the reaction
conditions to 50.degree. C. in dimethylsulfoxide/THF for three days gave
grafting yields of 45%. Optimum conditions with a temperature of
75.degree. C. for a reaction time of 20 h were used in the remaining
grafting reactions.
Allowing the reaction to proceed for 72 h in dimethylsulfoxide at
75.degree. C. did not significantly change the yield. This evidence
indicates that the grafting reaction is essentially complete after 20 h at
75.degree. C. and that the grafting yield is limited only by the
efficiency of the carboxylation reaction of polystyrene. The graft yields
obtained in the reactions the polystyrylmonocarboxylate anions under the
optimum conditions were close to the expected efficiency of carboxylation
of 78-90%.
The graft polymers were precipitated with 200 ml of 4:1 methanol:water,
filtered, washed with water and methanol, and dried. The product was then
extracted with 100 ml of toluene with gentle shaking for 24 h to remove
any unreacted polystyrene and polystyrylcarboxylte salt. The product was
filtered, washed with toluene, dried, and weighed. The toluene extract and
washings were combined and the amount of unreacted homopolymer present in
each of the toluene extracts was determined after evaporation of the
toluene.
The results of the grafting reactions are shown in Table 1. The grafting
yield was calculated as the weight percent of the polystyrene (PS) that
attaches to the cellulose backbone according to the following equation:
##EQU2##
UV measurements in CH.sub.2 Cl.sub.2 at 260 nm of the THF-soluble graft
polymers were used to determine the polystyrene contents of the graft
polymers. (Polystyrene homopolymer and mesylated cellulose acetate
dissolved in CH.sub.2 Cl.sub.2 were used to make a standard curve.)
The reaction of the mesylated cellulose acetate with the
polystyrylmonocarboxylate anion resulted in graft polymer product soluble
in THF. However, reaction with the polystyryldicarboxylate anion resulted
in the formation of a solid gel, indicative of cross linking.
Cross-linking was expected because both ends of the polystyrene chain
could potentially react with the mesylate groups on the cellulose
backbone.
The increase in weight of the toluene-extract product obtained, compared to
the original weight of mesylated cellulose acetate, showed that grafting
occurred. The fact that the toluene-extracted polystyrene decreased
dramatically with increasing reaction temperature also indicated that a
grafting reaction was occurring. Analysis of the products obtained from
the reaction of the mesylated cellulose acetate with a
polystyrylmonocarboxylate anion by GPC gave peaks corresponding to a
molecular weight higher than the mesylated cellulose acetate, with no
peaks in the range of the molecular weight of the homopolymer.
TABLE 1
__________________________________________________________________________
Results of Grafting Experiments
at 75.degree. C. and 20 h
PS
PS Content
AGU
Grafting
Content
By UV,
per PS
Copolymer Product
Product No.
Solvent
Yield, %
wt % wt % Chain
__________________________________________________________________________
Polystyrylcarboxylate/cellulose
1 DMF 68 57.6 58.2 17.0
acetate graft copolymer
Polystyrylcarboxylate/cellulose
2 Me.sub.2 SO/THF
60 54.5 59.9 19.3
acetate graft copolymer
Polystyryldicarboxylate/cellulose
3 DMF 90.5 64.4 22.5
acetate graft copolymer
Polystyryldicarboxylate/cellulose
4 Me.sub.2 SO/THF
88.5 63.9 23.0
acetate graft copolymer
__________________________________________________________________________
Some of the products were further subjected to mild alkaline hydrolysis.
Pulverized product (200 mg) was added to 50 ml of 15% aqueous ammonia at
room temperature for three days with mixing. The residue was extracted
with THF to remove released polystyrene, mesylate, and acetate.
Elemental analysis of the reaction products in the mesylated cellulose
acetate (mesyl-DS 0.46) is shown in Table 2. There was a marked increase
in the carbon and hydrogen percentages with a corresponding decrease in
the oxygen and sulfur percentages of the reaction product as compared to
the starting mesylated cellulose acetate. Thus, elemental analysis data is
also in conformity with the grafting of polystyrene onto the cellulose
backbone by displacement of the mesylate groups. In the case of the
reactions performed in DMF, little nitrogen was incorporated.
The IR spectra was also in conformity with the grafting of polystyrene onto
the cellulose backbone by displacement of the mesylated groups.
Characteristic peaks of polystyrene were present such as the aromatic CH
vibration (above 3000 cm.sup.-1) and the aromatic ring vibrations
(1500-1600 cm .sup.-1), as well as the strong carbonyl band (1740
cm.sup.-1) of the cellulose acetate. Because toluene extraction removed
over 98% of the polystyrene homopolymer from the cellulose
acetate-polystyrene lens, the presence of the polystyrene bands in the IR
Spectra of the toluene-extracted graft polymers confirms the formation of
a covalent link between polystyrene and the cellulose backbone.
The off-resonance proton-decoupled C NMR spectrum at 50.degree. C. in
dimethylformamide of the toluene-extracted graft polymer clearly showed
signals corresponding to both cellulose and polystyrene components.
Because this graft polymer product was extensively extracted with toluene
to remove any polystyrene homopolymer, the presence of well-resolved
intense polystyrene peaks in the NMR spectrum supported the covalent
attachment of polystyrene to the cellulose backbone. Polystyrene peaks
were readily discernible at: (1) 146 ppm with a multiple splitting pattern
due to quaternary aromatic ring carbon; (2) 125 ppm corresponding to the
other aromatic ring carbons; (3) the typical methylene (CH.sub.2)
resonance splittings centered around 48 ppm; and (4) single methane (CH)
resonance at 41 ppm. The chemical shifts and the splitting patterns
observed are in complete agreement with those reported for polystyrene.
The signals due to the cellulose were: (1) the 107 ppm peak corresponding
to the C-1 carbon of the anhydroglucose unit; (2) a group of signals
between 70 and 80 ppm due to the C-2, C-3, C-4, and C-5 carbons; (3) the
C-6 carbon signal at 63 ppm; and (4) the carbonyl carbon signal of the
acetate groups on the cellulose at 170 ppm with the methyl of the acetate
group appearing at 20 ppm. Each of the polystyrene peaks was much more
intense than the ring C-1 to C-6 carbons of the anhydroglucose units
because for every one anhydroglucose carbon there are 3.5 styrenic
carbons.
The number of anhydroglucose units (AGU) per grafted polystyrene chain
(Table 1) was calculated based on the molecular weight of polystyrene and
the polystyrene content of the graft. A high degree of substitution
corresponding to one polystyryl chain per 17-23 anhydroglucose units was
obtained. One polystyrene chain per 17 anhydroglucose units in product 1
corresponds to about 16 polystyrene chains per cellulose acetate molecule
based on the molecular weight of 7500 for the cellulose acetate. Because
the reaction is slow and proceeds by second-order nucleophilic
displacement, the grafting of polystyrene chains is limited to the primary
carbon atoms. Because there were more free primary hydroxyl groups in
cellulose acetate than in methyl cellulose (DS=1.7), upon mesylation,
there were more mesyl groups on primary carbon atoms in cellulose acetate
than in methyl cellulose. Thus, reaction of the polystyrylcarboxylate ion
with mesylmethylcellulose proceeded much slower and gave lower graft yield
(20%) than reaction with mesylcellulose acetate.
The presence of some water in the reaction medium did not have any
deleterious effects on the coupling reaction. The only possible effect of
the polystyrylcarboxylate anion, in suitable solvents, was to displace the
mesylate group. Another important consideration is that complete
mesylation is not required. Two trials with mesylcellulose acetate of
lower mesyl content (mesyl DS 0.21) in DMF at 75.degree. C. gave a
grafting yield of 40% after 20 h and 65% after 96 h.
TABLE 2
______________________________________
Results of Grafting Experiments at 75.degree. C. and 20 h
Pro-
duct Elemental Analysis
Polymer Product
No. C H S N O
______________________________________
Mesylated cellulose
acetate (mesyl
DS 0.46) 45.61 5.52 4.90 43.97
Polystyrylcarboxylate/
cellulose acetate
graft copolymer
1 70.07 6.90 2.83 0.37 19.83
Polystyrylcarboxylate/
cellulose acetate
graft copolymer
2 68.62 6.90 2.52 22.16
Polystyryldicarboxylate/
cellulose acetate
graft copolymer
3 73.37 6.84 2.52 0.16 17.11
Polystyryldicarboxylate/
cellulose acetate
graft copolymer
4 71.64 8.85 1.68 17.83
Polystyrylcarboxylate/
cellulose acetate
(hydrolyzed) 1 65.98 6.76 2.11 25.15
Polystyryldicarboxylate/
cellulose acetate
(hydrolyzed) 2 71.10 6.71 1.57 20.62
______________________________________
EXAMPLE 2
Grafting of Partially Hydrolysed Poly(methyl methacrylate) onto Mesylated
Cellulose Acetate
Poly(methyl methacrylate) (PMMA) (20 g) (molecular weight 12000) was added
to 0.5M or 0.2M KOH in ethanol (125 ml) and dissolved upon warming to
refluxing temperature. Refluxing was continued for 24 h. Under these
conditions, the 0.5M KOH solution gives about 8% hydrolysis of the methyl
ester, while the 0.2M KOH gives about 3% hydrolysis corresponding to an
average of 9.6 and 3.6 carboxylate groups per chain, respectively. The
hydrolysed polymer was isolated by decanting the KOH/ethanol solution at
room temperature. The polymer was then recrystallized in ethanol (200 ml)
with about 80% recovery being achieved in both cases.
Cellulose acetate having a degree of substitution (DS) of 2.5 was mesylated
as described previously in Example 1 to give one product having a mesyl DS
of 0.46 and a second product of DS 0.21. O-methyl cellulose (DS 1.7) was
also permesylated.
Each grafting reaction was carried out using mesylated cellulose acetate or
O-methyl cellulose (1.00 g) and hydrolysed PMMA (1.00 g) dissolved in
N,N-dimethyl formamide (DMF) (20 ml). The solutions were heated at
75.degree. C. for 20 h. The time required for gelation (due to cross
linking) is noted in Table 3.
The resultant graft copolymers were precipitated in methanol, washed,
dried, and weighed. The products were then extracted with refluxing
ethanol to remove ungrafted PMMA homopolymer. Grafting yields were
calculated as the wt % of the PMMA attaching to the cellulose backbone
according to the following formula:
##EQU3##
No grafting was observed if the unhydrolysed PMMA was used.
PMMA was partially hydrolysed with 0.2M KOH to form a carboxylate anion
having an average of 3.6 carboxylate groups. A second partially hydrolysed
product was prepared by hydrolysing PMMA with 0.5M KOH to form a
carboxylate anion having 0.6 carboxylate groups. Each of these partially
hydrolysed products were grafted onto mesylated cellulose acetate and
O-methyl cellulose by a nucleophilic displacement (SN.sub.2) reaction.
Grafting yields were calculated as the wt % of PMMA covalently linked to
the cellulose backbone, determined after ethanol (refluxing) extraction to
remove PMMA homopolymer present in the reaction product. Quantitative
yields of the graft copolymer were obtained at 75.degree. C. in a very
short reaction period (35-45 min).
The results of the grafting reactions are shown in Table 3. PMMA product 1
is unhydrolysed PMMA homopolymer. Product 2 is 0.2M KOH hydrolysed PMMA
homopolymer and product 3 is 0.5M KOH hydrolysed PMMA homopolymer. MCA
denotes mesylated cellulose acetate. MMC denotes mesylated O-methyl
cellulose.
Because there are several active sites on the PMMA polymer as well as on
the mesylated cellulose acetate, the products form gels. The carboxylate
groups of PMMA are on tertiary carbon atoms, making them "hindered" acids.
The nucleophilic displacement occurred relatively quickly, however, as
monitored by the gelation times. Others have also shown that "hindered"
carboxylate nucleophiles react about as quickly as "unhindered"
carboxylate nucleophiles. Liotta, Harris, McDermott, Gonzalez, and Smith,
Tetrahedron Lett. 1974, 28, 2417; Dursh, Tetrahedron Lett. 1974, 28, 2421;
Akabori and Ohtomi, Bull. Chem Soc. Jpn. 1975, 48, 2991.
The gelation times for PMMA/mesylated cellulose derivatives graft
copolymers were much shorter than for the sodium salt of
dicarboxy-terminated polystyrene. This may have been due to the presence
of more active sites per chain, and also due to the use of large cations
such as potassium as the counter ion of the carboxylate anion.
The IR spectrum of the graft copolymer product showed all of the unique
peaks attributable to the PMMA and mesylated cellulose acetate. In
addition, strong carbonyl stretching vibrations in the 1720-1750 cm.sup.-1
region and enhanced --CH stretching vibrations just below 3000 cm.sup.-1
were evident. This established that the ethanol-extracted reaction
products had PMM grafted onto the cellulose backbones.
TABLE 3
______________________________________
Gelation
PMMA.sup.a
Time Yields (%)
Substrate Product (min) Crude Graft
______________________________________
MCA.sup.b (DS 0.46)
3 20 100 100
MCA (DS 0.46)
3 23 101 99
MCA (DS 0.21)
3 36 88 98
MCA (DS 0.21)
3 35 90 99
MCA (DS 0.46)
2 42 100 98
MCA (DS 0.46)
2 44 98 98
MCA (DS 0.21)
2 -- 88 86
MCA (DS 0.21)
2 -- 85 84
MMC.sup.c 3 29 95 99
MMC 3 36 99 99
______________________________________
EXAMPLE 3
Grafting of Preformed Polyamide onto Cellulose Acetate
Mesylation of 10g of cellulose acetate (Eastman Kodak, 40% acetyl, DS 2.5,
MW 60000) was carried out by the procedure set forth in Example 1.
Elemental analysis gave 45.61% C, 5.52% H, and 4.90% S corresponding to
mesyl substitution of 0.46 mesyl groups per anhydroglucose unit.
A commercially available polyamide resin (Aldrich) was used. The polyamide
was formed by condensation of polyamine with dibasic carboxylic acids
produced from unsaturated fatty acids. GPC analysis established the
molecular weight of the polyamide as 18000. The dibasic acids have
complicated structures with bulky hydrocarbon side chains. An aliguot of
the resin was dissolved in THF and slowly precipitated in water. The pH
was 8.1, assuring that the carboxylic acids occurred in their salt forms.
Elemental analysis gave 77.80% C, 11.48% H, and 3.80% N.
Polyamide was grafted onto mesylated cellulose acetate in DMF:THF solvent
by substitution nucleophilic biomolecular (SN.sub.2) reaction. The
grafting reaction was carried out by adding 0.50 g (product 1) or 1.00
(product 2) g of mesylated cellulose acetate and 1.00 g of polyamide
carboxylate to a 25 mL Erlenmeyer flask with 20 mL of 2:1
dimethylformamide (DMF):tetrahydrofuran (THF) and reacting the mixture at
80.degree. C. for 20 hours. Polyamide homopolymer was subjected to
identical reaction conditions as a control. A mixture of unheated
polyamide and mesylated cellulose acetate (1 g each | | |
