Properties and Uses of Organic Titanates - Advances in Chemistry

Properties and Uses of Organic Titanates

H.H.BEACHAM

Downloaded by PENNSYLVANIA STATE UNIV on September 27, 2013 | http://pubs.acs.org Publication Date: January 1, 1959 | doi: 10.1021/ba-1959-0023.ch026

Titanium Division, National Lead Co., South Amboy, N. J.

Alkyl titanates obtained from titanium tetrachloride can undergo the following reactions among a variety of other chemical reactions: ester exchange, hydrol ysis or pyrolysis to polymers, and coordination with atoms possessing unshared electron pairs. The chelates can be tailor-made to specific purposes; the most widely known are those of the amino alcohols, polyols, and hydroxy acids.

From its position in the periodic table, titanium might be expected to exhibit a close similarity to the better known "organic" elements of group IV, such as carbon and silicon as well as germanium, tin, and lead. However, this expected similarity must be modified with the recognition that titanium, zirconium, hafnium, and thorium are also in the subgroup of group IV—that is, they are transition elements. An examina tion of the electronic configuration (Table I) shows that the 4-valence electrons of Table I.

Electronic Configuration of Group IV Elements

Principal quantum number Serial quantum number C Si Ti Ge

1 s 2 2 2 2

2 s 2 2 2 2

ρ 2

6 6 6

s 2 2 2

3 ρ 2

6 6

4 d

s

2 10

2 2

carbon, silicon, germanium, tin, and lead are all found in the same principal quantum level, while those of the transition elements are divided between two principal quantum levels, in the case of titanium, the 3d and the 4s levels. The 2s, 2p and the 3s, 3p electronic levels of carbon and silicon, respectively, are known to yield compounds having tetrahedral structures. However, the 3d, 4s electron distribution of titanium could not be expected to have such a structure. The consequences of this difference in electron configuration are readily found when one compares the chemical and physical properties of compounds of the elements of the two subgroups of group IV. Such comparisons have recently been the subject of extensive studies as reviewed by Wardlaw (9). A comparison of the boiling points of the metal chlorides of elements of group IV is sufficient to show the consequences of the different electronic configurations (Table Π)· A similar series of properties is found for the alkoxides of the metals. The fact that the boiling points of the chlorides and alkoxides of the titanium subgroup of group I V are considerably higher than might be expected is attributable to the much stronger intermolecular bonding forces in these compounds than in the compounds of other members of this group. The exact molecular structure of the titanium alkoxides is not known and therefore is given no further consideration here. 282

In METAL-ORGANIC COMPOUNDS; Advances in Chemistry; American Chemical Society: Washington, DC, 1959.

BEACHAM—ORGANIC TITANATES

283

Table II.

Boiling Points of G r o u p IV Chlorides

Element C Si

Boiling Point, ° C . 76.8 57.6

Ti

Ge

83.1

Zr

Sn

114.1

136.4 300 (subl.)


F o r p r e s e n t p u r p o s e s a n o c t a h e d r o n , as s h o w n , i s sufficient, because i n t h e presence of a n y a t o m s c o n t a i n i n g u n s h a r e d p a i r s o f electrons, s u c h a s t r u c t u r e seems r e a s o n able f o r t i t a n i u m c o m p o u n d s .

rirr I n cases w h e r e s t r o n g i n t e r m o l e c u l a r a t t r a c t i o n s a r e f o u n d a m o n g t i t a n i u m c o m p o u n d m o l e c u l e s , t h i s s t r u c t u r e r e p r e s e n t s c o o r d i n a t i o n o f t h e l i g a n d a t o m s o f one m o l e c u l e w i t h t h e t i t a n i u m a t o m of a s e c o n d m o l e c u l e a n d t h i s c a n b e r e p e a t e d t h r o u g h s t i l l m o r e m o l e c u l a r u n i t s , l e a d i n g t o a s o r t of c o o r d i n a t e l y b o n d e d p o l y m e r . O n e p r i n c i p l e of t i t a n i u m c h e m i s t r y seems t o s t a n d o u t — n a m e l y , t h e d r i v i n g force of q u a d r i v a l e n t t i t a n i u m r e a c t i o n s t e n d s t o increase t h e c o o r d i n a t i o n n u m b e r t o 6. T h a t is, molecules containing fourfold coordinated t i t a n i u m atoms are i n h e r e n t l y v e r y r e a c t i v e , w h i l e those c o n t a i n i n g s i x f o l d c o o r d i n a t e d a t o m s a r e r e l a t i v e l y s t a b l e as exemplified b y the v e r y inert rutile structure. T h e s i m p l e t i t a n i u m alcoholates a r e e x a m p l e s of f o u r f o l d c o o r d i n a t e d t i t a n i u m c o m p o u n d s , a l t h o u g h e v e n here s t r o n g i n t e r m o l e c u l a r a t t r a c t i o n s a r e f o u n d i n some cases, t h u s p e r m i t t i n g t h e t i t a n i u m a t o m s t o s a t i s f y t h e i r c o o r d i n a t i o n r e q u i r e m e n t s .

Reactions of Organic

Titanates

General Reactions. T h e alcoholates are i n general v e r y reactive, undergoing i n t e r c h a n g e w i t h a l m o s t a l l m o l e c u l e s c o n t a i n i n g a c t i v e h y d r o g e n a t o m s s u c h as a l c o hols, p h e n o l s , a c i d s , a n d e n o l i z a b l e substances. ( R O ) T i + H x t=± ( R O ) T i X + R O H 4

3

( R O ) T i + H O H ^± ( R O ) T i O H + R O H 4

3

( R O ) T i + H O R ' ^± ( R O ) T i O R ' + R O H 4

3

Ο

Ο

Il

II

( R O ) T i + H O C R ' ^± ( R O ) T i O C — R ' + R O H T h e reactions, however, are reversible, t h e e q u i l i b r i u m point being determined b y the concentration of t h e react ants a n d t h e relative r e a c t i v i t y o f t h e t w o t i t a n i u m compounds. T h e m e c h a n i s m o f t h e r e a c t i o n s c a n b e p o s t u l a t e d as i n v o l v i n g a coordination-type intermediate : 4

3

HOR

T h e l a b i l i t y o f t h e h y d r o g e n a t o m a n d t h e a c i d i c n a t u r e of t h e c o m p l e x a r e d e m o n s t r a t e d b y t h e a b i l i t y o f these c o m p o u n d s t o f o r m salts w i t h s t r o n g a l k a l i e s , as first p o i n t e d o u t b y M e e r w e i n a n d B e r s i n (6). Ti(OR)

4

· H O R + M O R ' -> M + T i ( O R ) - + H O R ' 5


284

ADVANCES IN CHEMISTRY SERIES


T h e s e were l a t e r u t i l i z e d as ester exchange c a t a l y s t s i n t h e p r e p a r a t i o n of p o l y e s t e r resins as d e s c r i b e d i n p a t e n t s b y C a l d w e l l a n d W e l l m a n (2, 3). I n O r g a n i c M e d i a . O n e o f the best s t u d i e d r e a c t i o n s o f t h e t i t a n i u m esters, l e a d i n g t o a m o r e c o m p l e t e l y c o o r d i n a t e d s t a t e , i s the r e a c t i o n w i t h m a c r o m o l e c u l e s , p a r t i c u l a r l y t h e cellulosics (1, 4, 5,7). I n the case of t h e cellulosics, s u c h as e t h y l c e l l u lose, esters, a n d n i t r o c e l l u l o s e , t h e h i g h c o n c e n t r a t i o n of o x y g e n a t o m s i n these m o l e c u l e s p e r m i t s the t i t a n i u m ester m o l e c u l e t o a t t a c h itself v e r y s t r o n g l y t o t w o o r p o s s i b l y m o r e n e i g h b o r i n g cellulosic c h a i n s , r e s u l t i n g i n a c r o s s - l i n k i n g a c t i o n a n d l e a d i n g t o gelation. I n t h i s g e l a t i o n , t h e cross l i n k i n g of h y d r o x y l g r o u p s i n t h e cellulosics a p p e a r s t o be o f p r i m e i m p o r t a n c e , b u t o t h e r cross l i n k s a p p a r e n t l y also are f o r m e d , a n d these a r e p r o b a b l y o f a c o o r d i n a t i o n n a t u r e . D i s t i n g u i s h i n g b e t w e e n the t w o t y p e s o f cross l i n k s is d i f f i c u l t , because e i t h e r c a n l e a d t o a gel state. S t u d y o f s o l u t i o n s c a p a b l e of c o n v e r s i o n t o films has b e e n m o s t u s e f u l i n s h o w i n g t h e i m p o r t a n c e o f h y d r o x y l g r o u p s i n the cellulosic a s r e l a t e d t o t h e i r a b i l i t y t o react w i t h a s i m p l e a l k y l t i t a n a t e t o f o r m stable c r o s s - l i n k e d p r o d u c t s . n - C e l l u l o s i c — O H + T i ( O R ) τ± ( R O ) T i ( O — c e l l u l o s i c ) _„ + n - R O H 4

4

I n conventional solvent systems the e q u i l i b r i u m of this reaction is well t o the right, w i t h t h e r e s u l t a n t f o r m a t i o n o f a c r o s s - l i n k e d gel s t r u c t u r e , t h e s t a b i l i t y o f t h i s g e l p r o b a b l y b e i n g a t t r i b u t a b l e t o a c o o r d i n a t e d state of the t i t a n i u m a t o m s as

where t h e covalently bonded oxygens are supplied b y h y d r o x y l o r possibly a f e w c a r b o x y l groups, while the coordinately bonded oxygens are supplied b y a n y of the o x y g e n s i n the c e l l u l o s i c , i n c l u d i n g the o x y g e n a t o m s f o u n d i n t h e e t h e r l i n k a g e s of t h e cellulose c h a i n .

Control of Cross Linking I n P r e s e n c e o f A l c o h o l s . T h e first s u c h m e t h o d consists of u s i n g a sufficiently high concentration of an alcohol i n the solvent system. T h i s takes advantage of the r e v e r s i b i l i t y o f t h e r e a c t i o n o f a n a l k y l t i t a n a t e w i t h a c e l l u l o s i c . I t also p e r m i t s choice o f a w i d e v a r i e t y of a l c o h o l s , e a c h t y p e o f a l c o h o l e x h i b i t i n g a c e r t a i n p o t e n t i a l f o r i n h i b i t i n g gel f o r m a t i o n . I n a system containing alcohol, a l k y l t i t a n a t e , a n d cellulosic a characteristic curve is u s u a l l y o b t a i n e d as t h e r a t i o o f a l k y l t i t a n a t e t o c e l l u l o s i c i s v a r i e d . F i g u r e 1 shows a t y p i c a l c u r v e o b t a i n e d o n b l e n d i n g b u t y l t i t a n a t e w i t h a m e d i u m v i s c o s i t y highly ethylated ethylcellulose (Hercules T y p e T - 5 0 ) a t 1 0 % concentration i n b e n z e n e - b u t a n o l s y s t e m s . T h e c u r v e i n g e n e r a l i s l o w e r as t h e a m o u n t o f a l c o h o l i s i n creased. I t i s m o s t s i g n i f i c a n t t h a t t h e l o w e r c o n c e n t r a t i o n s o f t h e t i t a n a t e p r o d u c e the higher viscosities. T h i s is a t t r i b u t a b l e t o the relative concentrations of the a l k y l t i t a n a t e t o t h e free h y d r o x y l g r o u p s o f the c e l l u l o s i c . A t l o w t i t a n a t e c o n c e n t r a t i o n s e a c h t i t a n a t e m o l e c u l e i s free t o r e a c t w i t h m o r e t h a n one cellulosic h y d r o x y l , u s u a l l y o n n e i g h b o r i n g c h a i n s , t h e r e b y f o r m i n g a cross l i n k . W h e n t h e r e is one t i t a n a t e m o l e cule f o r e a c h cellulosic h y d r o x y l , t h e p r e f e r r e d s t r u c t u r e i s o n e i n w h i c h o n l y o n e a l k o x y l g r o u p o f t h e a l k y l t i t a n a t e i s r e p l a c e d b y a cellulosic h y d r o x y l . N o t a l l alcohols e x h i b i t c o m p a r a b l e effectiveness i n s u p p r e s s i n g g e l a t i o n . Table I I I l i s t s t h e r e s u l t s o b t a i n e d w i t h a few c o m m o n alcohols. F r o m a s t u d y o f a l a r g e n u m b e r o f alcohols a n d a l c o h o l - l i k e m a t e r i a l s t w o g e n e r a l rules c a n b e e x p r e s s e d r e g a r d i n g t h e t e n a c i t y w i t h w h i c h a l c o h o l s c o m b i n e w i t h t i t a n i u m i n a t i t a n a t e ester.


285

BEACHAM-ORGANIC TITANATES 50


10 % Ε Τ H Y LC E LLULOSE (TYPE T-50) IN

0

10

20

30 40 50 60 7080 90 PER CENT BUTYL TITANATE

100

Figure 1. Effect of solvent on viscosity of butyl titanate-ethylcellulose blends

T h e higher the molecular weight of the alcohol, the more stable the corresponding ester. P r i m a r y alcohols f o r m m o r e stable esters t h a n s e c o n d a r y a l c o h o l s , w h i c h i n t u r n combine more strongly t h a n t e r t i a r y alcohols.

Table III. Effect of Cosolvent on Viscosity of a 50 to 5 0 Butyl Titanate-Ethylcellulose Blend a

Alcohol Methanol Ethanol 2-Propanol 1- Butanol 2- Me-propanol 2-Butanol 2-Me-2-butanol Ethyl lactate Ethylene glycol monoethyl ether

Viscosity, Cps. Gel Gel p-gel 275 300 370 400 225 250

Ethylcellulose (50 cps., 48.3% ethoxyl va riety) at 10% concentration in 80 to 20 xylenealcohol. a

B o t h effects a r e d i r e c t l y a t t r i b u t a b l e t o s h i e l d i n g o r t h e ease w i t h w h i c h t h e t i t a n a t e ester m o l e c u l e s c a n associate w i t h one a n o t h e r o r w i t h o t h e r l i g a n d s t o f o r m a f u l l y c o o r d i n a t e d state o f h i g h e r s t a b i l i t y . T h e l a s t t w o s o l v e n t s i n t h e t a b l e , t h e l a c t a t e ester a n d C e l l o s o l v e , are o f p a r t i c u l a r i n t e r e s t , because t h e y i n d i c a t e a n effect w h i c h i s n o t c o n n e c t e d e i t h e r w i t h size o r degree o f b r a n c h i n g , b u t r a t h e r w i t h t h e a b i l i t y t o f o r m c o m p l e x e s p r o b a b l y o f a w e a k chelate n a t u r e . I n Presence of C h e l a t i n g Agents. T h e second g e n e r a l m e t h o d o f s u p p r e s s i n g gel f o r m a t i o n i n a titanate-cellulosic system is b y f o r m a t i o n of true coordination c o m p o u n d s o f a t i t a n a t e ester. A t y p i c a l , y e t v e r y s i m p l e s y s t e m o f t h i s t y p e is s h o w n i n F i g u r e 2. H e r e a d d i -


286

ADVANCES IN CHEMISTRY SERIES

Ο ETHYLCELLULOSE 4- BUTYL TITANATE


Δ ETHYLCELLULOSE + BUTYL TITANATE Η 5 % TRIETHYLAMINE

Ο

10

20 30 40 50 60 PER CENT BUTYL TITANATE

Figure 2 . Effect of triethylamine on viscosity of butyl titanate-ethylcellulose blends t i o n o f a n amine c o m p o u n d t o the system permits f o r m a t i o n of a n amine complex w i t h t h e t i t a n i u m ester w h i c h t h e r e b y b l o c k s t h e ester exchange r e a c t i o n w i t h t h e cellulosic h y d r o x y l s ( F i g u r e 3 ) . HOR R0

r

7

0R

Ti RO*

RO,

0R

Ti

+ HOR'V

7

0R

RO'

RQr

7

OR

0R

Ti

HOR

RO

OR RNH,

RO/

7

0R

RO/+

RO* Figure 3 .

'OR

RNH

7

OR

Ti

0

R0

Properties and Uses of Organic Titanates - Advances in Chemistry

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