In the Laboratory
Polymerization and Characterization of PMMA
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Polymer Chemistry Laboratory Experiments for Undergraduate Students Caroline Duval-Terrié and Laurent Lebrun* UMR 6522 CNRS, Université de Rouen – Faculté des Sciences, 76821 Mont-Saint-Aignan Cedex–France, *
[email protected] In this experiment third-year undergraduate chemistry students (1) prepare polymethylmethacrylate (PMMA), one of the best-known industrial polymers. First, the free radical polymerization of methyl methacrylate (MMA) monomer with various ratios of initiator and monomer (I:M) are examined. By analyzing their polymeric products with size exclusion chromatography (SEC), the students can verify the kinetics scheme of the radical chain polymerization. Second, anionic chain polymerization is examined to show the influence of the polymerization process on the stereochemistry of the chain. The tacticity can be investigated by proton nuclear magnetic resonance spectroscopy (1H NMR) and the influence on the glass transition temperature (Tg ) can be measured by differential scanning calorimetry (DSC). PMMA belongs to the acrylic polymer family and is known by trademarks such as Lucite, Perspex, Oroglas, Goldglas, Altuglas, or Plexiglas. The production of PMMA in the world is over 6 × 105 tons per year (2) and is sold at ∼$5 per kg. This polymer is amorphous and gives thermoplastic materials that are hard, rigid, and brittle at room temperature. PMMA is more transparent and less brittle than conventional glass with a lighter weight and it is easy to mold into a variety of shapes. PMMA shows a wide variety of uses as glass replacement material in cars and aircraft (light clusters, dashboards, canopies, port-holes), in urban furnishing (verandas, antinoise walls, shop signs, display units, light diffusers, reflectors) and in medical applications (contact lenses, lenses, teeth, hospital incubators) (2). Surgical-grade PMMA
H3C R
H 3C R
H H
R CH3 H3C R
H3C R syndiotactic H H
H 3C R
H H
H3C R
R CH3
atactic H H
H H
Figure 1. Stereoregularity of PMMA: isotactic, syndiotactic, and atactic.
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Theory Chain addition polymerization occurs by the stepwise addition of monomer units to an active site on the growing chain. This is possible because of the opening of a double bond or a cycle on the monomer. The presence of an electron withdrawing group (⫺COOCH3) on MMA monomers stabilizes either the free radicals or the carbanions and thus favors either the free radical or the anionic polymerizations.
Radical Polymerization The chain reaction involves four steps: initiation, propagation, termination, and transfer. The initiation is illustrated with 2,2´-azo-bis-isobutyrylnitrile (AIBN) as initiator and the transfer reaction to the monomer is shown in Scheme I. Anionic Polymerization The steps of anionic polymerization are similar to those of free radical polymerization. The initiator is a strong base, such as n-butyllithium (n-BuLi), that reacts with the double bond of the monomer (nucleophilic addition) (Scheme II). At the end of the polymerization, each macromolecule keeps its living end as long as there are no protic impurities. This system is usually called “living anionic polymerization” because there is no termination step. At the end of the synthesis, HCl (or NaCl) is added to deactivate the active center. Tacticity Tacticity is the term used to describe the configuration of polymer chains. Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain (Figure 1). A syndiotactic polymer chain is composed of alternating stereocenters and an atactic polymer is a random combination of the groups.
H3C R
isotactic H H
is used as bone cement by self-polymerizing during curing (3). PMMA is also used in the manufacture of bathtubs, hand-wash basins, dashes, optical fibers, adhesives, and glues (PMMA兾MMA).
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Thermal Properties The type of tacticity has a great influence on the physical properties of the material. From a thermodynamic point of view, the glass transition temperature, Tg, is one of the most important parameters for characterizing a polymer (4). Tg can be interpreted as the temperature where the segments of the macromolecule chains begin to move. At Tg, short chain segments undergo rotation, translation, and diffusion motions. Below Tg, an amorphous polymer has the characteristics of a glass, while it becomes leathery and rubbery above Tg (4).
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443
In the Laboratory
Initiation CH3 H3C
C
CH3
N
N
C
C
ⴙ
C
H2C
ⴙ
N2
CN
CH3
CH3 H 3C
2 H3C
CH3
CH3
Scheme I. Free radical polymerization of MMA with AIBN as the initiator.
CH3
CH3
CH3
C
H3C C
CN
C
OCH3
CH3
C C H2
CN
O
C
OCH3
O
Propagation CH3 C
H3C
CH3
CH3
C C H2
CN
COOCH3
n
CH3
CH3 ⴙ
C C H2
H2C
C
H3C C
COOCH3
OCH3
CH3
C
CH3
C C H2
CN
C C H2
COOCH3
O
n+1
COOCH3
Termination Combination CH3
CH3
C C H2
CH2
COOCH3
CH3 ⴙ
C COOCH3
m
CH3
C C H2
CH2
COOCH3
CH3
C
C C H2
COOCH3
n
COOCH3
m+n+2
Disproportionation CH3
CH3
C C H2
CH2
COOCH3
CH2
C C H2
COOCH3
m
CH3
CH3 ⴙ
C
COOCH3
C COOCH3
n
CH3
CH3
CH3
C C H2
CH2
COOCH3
m
ⴙ
CH
CH3
C C H2
CH
COOCH3
COOCH3
C COOCH3
n
Transfer CH3 C C H2 COOCH3
CH3 CH2 n
CH3
CH3 ⴙ
C
H2C
C COOCH3
COOCH3
CH3
C C H2
CH2
COOCH3
CH3 ⴙ
CH
HC
C COOCH3
COOCH3
n
Initiation H3C
H3C
C C C Li H2 H2 H2
C C C H2 H2 H2
Scheme II. Anionic polymerization of MMA with n-BuLi as the initiator.
ⴙ Li
CH2
CH3 H3C
C C C H2 H2 H2
ⴙ H 2C
H3C
C COOCH3
C C C C C H2 H2 H2 H2 COOCH3
Propagation CH3 H 3C
C C C H2 H2 H2
C C H2 COOCH3
CH3 C C H2 n
CH3
CH3 ⴙ H 2C
COOCH3
C
H3C COOCH3
C C C H2 H2 H2
CH3
C C H2 COOCH3
C C H2 n+1
COOCH3
Termination CH3 H3C
C C C H2 H2 H2
C C H2 COOCH3
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CH3 C C H2 n
COOCH3
Journal of Chemical Education
CH3
CH3 ⴙ H
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H3C
C C C H2 H2 H2
C C H2
C CH H2
COOCH3
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n
COOCH3
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In the Laboratory
Experimental Procedure
Table 1. Yield and SEC Analysis of the Free Radical MMA Polymerizations
The students work in pairs. The instructor chooses the polymerization reaction to be performed (radical or anionic) and the technique for characterizing the PMMA, depending on the apparatus available in the laboratory. The explicit details of the experimental procedures for the polymerization and for the characterization of the polymers are given in the Supplemental Material.W Each group of students uses a different I:M ratio for the radical polymerization and a different quantity of THF for the anionic polymerization. The students analyze a dry polymer previously synthesized by another group during an earlier experiment. This analysis is performed during the waiting time for the polymerization (2 hours for the radical polymerization and 1.5 hours for the anionic polymerization). The average molecular weight (Mave,n and Mave,W)1 and the polydispersity D of the synthesized PMMA (D = Mave,W兾Mave,n) are determined with SEC. The number of units per chain (xave,n) is deduced from the relationship Mave,n = M0(xave,n), where M0 = 100 g mol᎑1 for PMMA. The tacticity of the PMMA samples are investigated with 1H NMR. The influence of the tacticity on the thermal properties (Tg) of the polymer is assessed by DSC. At the end of the class, the results are compared. Hazards MMA, toluene, THF, deuterated chloroform, and heptane are toxic irritants and noxious. HCl is irritating to eyes and skin. Toluene, heptane, and THF are highly flammable. AIBN is explosive. n-BuLi causes burns, is spontaneously flammable in air, and reacts violently with water, liberating extremely flammable gases. The experiments must be done under a fume hood protected by a pane of glass. The students must use lab coats, safety goggles, and protective gloves (nitrile or heat protective). Safety and hazards are discussed further in the Supplemental Material.W Calculations and Comments
Free Radical Polymerization Typical results obtained by groups of students are shown in Table 1. The quantity of AIBN initiator [I] influences the yield of free radical polymerization. For the same monomer concentration, the higher the initiator concentration, the higher the yield of the polymerization. Logically, high initiator concentrations give low molecular weights, a small number of units per chain (xave,n), and low polydispersity of the chains. The groups of students can apply the theoretical relations of the kinetics of the free radical polymerization (5, 6) to analyze their results (see the Supplemental MaterialW). The calculated values xave,n0 (without transfer reaction), xave,n (with transfer reaction), and the measured values xave,n (with transfer reaction obtained by chromatography) for each experiment are given in Table 2. They can be compared to calculate the α ratio of termination by combination. For a constant monomer concentration, the quantity of AIBN initiator influences the termination. The higher the initiator concentration, the higher the termination by combination. Some www.JCE.DivCHED.org
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[I]/ (10᎑3 mol L᎑1)
Mave,n/ (g mol᎑1)
Yield (%)
Mave,W/ (g mol᎑1)
D
2.2
0
—
—
—
4.7
11
7230
25300
4.5
8.8
4
5050
22700
4.5
11.1
23
6620
18000
2.8
22.2
40
5390
14500
2.7
33.2
51
3150
7240
2.3
55.4
56
4250
7200
1.7
67.1
64
3360
6910
2.1
94.2
77
3400
6300
1.9
NOTE: D = Mave,W/Mave,n.
Table 2. Study of the Free Radical MMA Polymerizations
I:M/ (10᎑3)
Exp xave,n
No Transfer Rxn
With Transfer Rxn
Calc xave,n0
α
Calc xave,n
α
72.3
96.4
᎑0.66
93.4
᎑0.58
13.0
66.2
62.5
0.11
61.2
0.15
26.1
53.9
44.2
0.36
43.6
0.40
39.1
31.5
36.1
᎑0.29
35.7
᎑0.27
65.1
42.5
27.9
0.69
27.7
0.70
78.8
33.6
25.4
0.49
24.8
0.52
34
21.4
0.76
20.8
0.77
5.47
110.7
xave,n values measured by chromatography were erroneous and did not agree with the corresponding calculated values; consequently, the α ratio obtained from the measured values was near to zero or negative. The results also show that the transfer reactions have a low influence on the number of units per chain.
Anionic Polymerization After assigning the 1H NMR peaks of their PMMA samples, the groups of students measured the peak areas and calculated the ratio of isotactic, syndiotactic, and atactic segments (Table 3) (1H NMR spectra and explanations are given in the Supplemental MaterialW). The anionic polymerization of MMA monomer (without THF) gives highly isotactic stereoregular polymers. The spatial structure of the living end governs the stereochemistry of the addition of the MMA monomer. The mechanism involves the counterion (Li+) blocking the last two units of the growing chain and the monomer (by their carbonyl groups). At each step, therefore, the geometry of the living end and the way the monomer approaches remain the same. The resulting sequence leads to an isotactic structure. When a polar solvent (THF) is added to the reactive medium, the double coordination of lithium is decreased and the addition of monomer is less oriented. Syndiotactic structure is favored at low THF concentration and atactic structure is favored at higher THF concentration.
Vol. 83 No. 3 March 2006
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In the Laboratory
Sy ndiot act ic (%)
A t act ic (%)
Is ot act ic (%)
Tg / °C
Ionic
11.0
09.2
79.8
48
Ionic (2.5% TH F)
20.3
11.6
68.1
80
kinetics of both polymerizations may be studied by using a dilatometer and, in this way, orders of reaction and rate constants can be calculated (8). An organometallic initiator of the MMA anionic polymerization can be prepared in a prior experiment and examined (e.g., phenylmagnesium bromide C6H5MgBr).
Ionic (5% TH F)
12.2
22.9
64.9
56
W
Ionic (10% TH F)
09.6
28.9
61.5
42
Radical
55.5
28.6
15.9
97
Table 3. 1H NMR and DSC Analyses of PMMA T y pe o f Poly me rizat ion
The free radical polymerization of MMA monomer gives more syndiotactic polymers owing to the steric and electrostatic repulsions between the (⫺COOCH3) groups (Table 3). The Tg of PMMA significantly varies according to its tacticity (Table 3). The molecular structure of the chain and the orientation of the side groups influence Tg. In syndiotactic chains, the bulky substituents (⫺COOCH3) are distributed on both sides of the backbone. They behave as an anchor between the chains (7). Consequently, they slow down the motions of the chain segments during heating; this increases Tg. When the bulky substituents are on the same side of the polymer chain (isotactic) or highly randomly distributed (atactic), they do not have this function, so the Tg is lower. Thus, the Tg of ionic PMMA is low.
Supplemental Material
Instructions for the students, notes for the instructor, kinetics of free radical polymerization, and NMR spectra are available in this issue of JCE Online. Note 1. Polymer weights can be expressed as either the numberaverage molecular weight, Mave,n, or the weight-average molecular weight, Mave,W. An article in this Journal describes these terms and gives sample calculations (9).
Literature Cited 1. 2. 3. 4.
Conclusions
5. 6. 7.
Various modifications can be applied to this experimental procedure. For example, with minor modifications, the free radical polymerization procedure of MMA may be applied to other common vinyl monomers such as styrene, vinyl acetate, vinylidene chloride, acrylonitrile, and acrylamide. The
8.
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9.
Jefferson, A.; Phillips, D. N. J. Chem. Educ. 1999, 76, 232. Chisholm, M. S. J. Chem. Educ. 2000, 77, 841. Vallo, C. I. Polym. Int. 2000, 49, 831. Sandler, S. R.; Karo, W.; Bonesteel, J.-A.; Pearce, E. M. In Polymer Synthesis and Characterization: A Laboratory Manual; Acadamic Press: New York, 1998. Morton, M. J. Chem. Educ. 1973, 50, 740. McGrath, J. E. J. Chem. Educ. 1981, 58, 844. Macrogalleria Level 3. http://www.pslc.ws/macrog/level3.htm (accessed Jan 2006). The students should read the part concerning the glass transition. Martin, O.; Mendicuti, F.; Tarazona, M. P. J. Chem. Educ. 1998, 75, 1479. Snyder, D. M. J. Chem. Educ. 1992, 69, 422.
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